REVIEWS OF TOPICAL PROBLEMS: Large-scale star formation in galaxies

NASA Astrophysics Data System (ADS)

Efremov, Yurii N.; Chernin, Artur D.

2003-01-01

A brief review is given of the history of modern ideas on the ongoing star formation process in the gaseous disks of galaxies. Recent studies demonstrate the key role of the interplay between the gas self-gravitation and its turbulent motions. The large scale supersonic gas flows create structures of enhanced density which then give rise to the gravitational condensation of gas into stars and star clusters. Formation of star clusters, associations and complexes is considered, as well as the possibility of isolated star formation. Special emphasis is placed on star formation under the action of ram pressure.

MYSST: Mapping Young Stars in Space and Time - The HII Complex N44 in the LMC

NASA Astrophysics Data System (ADS)

Gouliermis, Dimitrios

2016-10-01

The stellar initial mass function (IMF), and the timescale and lengthscale of star formation (SF) are critical issues for our understanding of how stars form. Low-mass pre-main-sequence (PMS) stars, having typical contraction times on the order of a few 10 Myr, are the live chronometers of the SF process and primary informants on the low-mass IMF of their host clusters. Our studies show that young star clusters, embedded in star-forming regions of the Large Magellanic Cloud (LMC), encompass rich samples of PMS stars, sufficient to study clustered SF in low-metallicities with optical HST photometry. Yet, the lack of a complete comprehensive stellar sample retains important questions about the universality of the IMF, and the time- and length-scale of SF across a typical molecular cloud unanswered. We propose to address these issues by employing both ACS and WFC3 with their high sensitivity and spatial resolving power to obtain deep photometry (m_555 29 mag) of the LMC star-forming complex N44. We will accomplish a detailed mapping of PMS stars that will trace the whole hierarchy of star formation springing from one giant molecular cloud. Our analysis will provide an unbiased determination of the timescale for SF and the sub-solar IMF down to the hydrogen burning limit in a variety of clustering scales for the first time. Our findings will have a significant impact on our comprehensive understanding of SF in the low-metallicity environment of the LMC. We maximize the HST observing efficiency using both ACS/WFC and WFC3/UVIS in parallel for the simultaneous observations of N44, its ensemble of HII regions and their young stellar clusters in the same F555W and F814W filters.

Mutiple Stellar Populations in Blanco DECam Bulge Survey Globular Clusters

NASA Astrophysics Data System (ADS)

Miller, Doryan; Pilachowski, C. A.; Johnson, C. I.; Rich, R. Michael; Clarkson, William I.; Young, M.; Michael, S.

2018-01-01

Preliminary SDSS ugrizY photometric observations of globular cluster stars included in the Blanco DECam Bulge Survey (BDBS) were examined to determine the suitability of these data to characterize stellar populations within clusters. The BDBS fields include around two dozen globular clusters, including the iron-complex cluster M22 and the pulsar-rich cluster Terzan 5. Many globular clusters show evidence for multiple stellar populations as a spread in the u-g color of stars in a given phase of stellar evolution, and in some clusters, the populations have different radial distributions. BDBS clusters with low and/or non-variable reddening and long dynamical mixing time scales were selected for study, and photometry for RGB and main sequence stars within two half-light radii from the center of each cluster was extracted from the BDBS preliminary catalog. Field contamination was reduced in each candidate cluster by removing all stars more than a tenth of a magnitude from the best-fit fiducial curves following the g-r vs r color-magnitude diagram. The remaining stars were split into separate populations based on u-g color, and effective cumulative distribution functions vs. half-light radius were compared to identify differences in the populations’ radial distributions.

INTERRUPTED STELLAR ENCOUNTERS IN STAR CLUSTERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Geller, Aaron M.; Leigh, Nathan W. C., E-mail: a-geller@northwestern.edu, E-mail: nleigh@amnh.org

Strong encounters between single stars and binaries play a pivotal role in the evolution of star clusters. Such encounters can also dramatically modify the orbital parameters of binaries, exchange partners in and out of binaries, and are a primary contributor to the rate of physical stellar collisions in star clusters. Often, these encounters are studied under the approximation that they happen quickly enough and within a small enough volume to be considered isolated from the rest of the cluster. In this paper, we study the validity of this assumption through the analysis of a large grid of single–binary and binary–binarymore » scattering experiments. For each encounter we evaluate the encounter duration, and compare this with the expected time until another single or binary star will join the encounter. We find that for lower-mass clusters, similar to typical open clusters in our Galaxy, the percent of encounters that will be “interrupted” by an interloping star or binary may be 20%–40% (or higher) in the core, though for typical globular clusters we expect ≲1% of encounters to be interrupted. Thus, the assumption that strong encounters occur in relative isolation breaks down for certain clusters. Instead, many strong encounters develop into more complex “mini-clusters,” which must be accounted for in studying, for example, the internal dynamics of star clusters, and the physical stellar collision rate.« less

Formation of Very Young Massive Clusters and Implications for Globular Clusters

NASA Astrophysics Data System (ADS)

Banerjee, Sambaran; Kroupa, Pavel

How Very Young Massive star Clusters (VYMCs; also known as "starburst" clusters), which typically are of ≳ 104 M ⊙ and are a few Myr old, form out of Giant Molecular Clouds is still largely an open question. Increasingly detailed observations of young star clusters and star-forming molecular clouds and computational studies provide clues about their formation scenarios and the underlying physical processes involved. This chapter is focused on reviewing the decade-long studies that attempt to computationally reproduce the well-observed nearby VYMCs, such as the Orion Nebula Cluster, R136 and NGC 3603 young cluster, thereby shedding light on birth conditions of massive star clusters, in general. On this regard, focus is given on direct N-body modelling of real-sized massive star clusters, with a monolithic structure and undergoing residual gas expulsion, which have consistently reproduced the observed characteristics of several VYMCs and also of young star clusters, in general. The connection of these relatively simplified model calculations with the structural richness of dense molecular clouds and the complexity of hydrodynamic calculations of star cluster formation is presented in detail. Furthermore, the connections of such VYMCs with globular clusters, which are nearly as old as our Universe, is discussed. The chapter is concluded by addressing long-term deeply gas-embedded (at least apparently) and substructured systems like W3 Main. While most of the results are quoted from existing and up-to-date literature, in an integrated fashion, several new insights and discussions are provided.

Unbound Young Stellar Systems: Star Formation on the Loose

NASA Astrophysics Data System (ADS)

Gouliermis, Dimitrios A.

2018-07-01

Unbound young stellar systems, the loose ensembles of physically related young bright stars, trace the typical regions of recent star formation in galaxies. Their morphologies vary from small few pc-size associations of newly formed stars to enormous few kpc-size complexes composed of stars few 100 Myr old. These stellar conglomerations are located within the disks and along the spiral arms and rings of star-forming disk galaxies, and they are the active star-forming centers of dwarf and starburst galaxies. Being associated with star-forming regions of various sizes, these stellar structures trace the regions where stars form at various length- and timescales, from compact clusters to whole galactic disks. Stellar associations, the prototypical unbound young systems, and their larger counterparts, stellar aggregates, and stellar complexes, have been the focus of several studies for quite a few decades, with special interest on their demographics, classification, and structural morphology. The compiled surveys of these loose young stellar systems demonstrate that the clear distinction of these systems into well-defined classes is not as straightforward as for stellar clusters, due to their low densities, asymmetric shapes and variety in structural parameters. These surveys also illustrate that unbound stellar structures follow a clear hierarchical pattern in the clustering of their stars across various scales. Stellar associations are characterized by significant sub-structure with bound stellar clusters being their most compact parts, while associations themselves are the brighter denser parts of larger stellar aggregates and stellar complexes, which are members of larger super-structures up to the scale of a whole star-forming galaxy. This structural pattern, which is usually characterized as self-similar or fractal, appears to be identical to that of star-forming giant molecular clouds and interstellar gas, driven mainly by turbulence cascade. In this short review, I make a concise compilation of our understanding of unbound young stellar systems across various environments in the local universe, as it is developed during the last 60 years. I present a factual assessment of the clustering behavior of star formation, as revealed from the assembling pattern of stars across loose stellar structures and its relation to the interstellar medium and the environmental conditions. I also provide a consistent account of the processes that possibly play important role in the formation of unbound stellar systems, compiled from both theoretical and observational investigations on the field.

Star Formation in Nearby Clusters (SFiNCs)

NASA Astrophysics Data System (ADS)

Getman, Konstantin

Most stars form in clusters that rapidly disperse, yet we have a poor understanding of the processes of cluster formation and early evolution. Do clusters form `top-down', rapidly in a dense molecular cloud core? Or, since clouds are turbulent, do clusters form `bottomup' by merging subclusters produced in small kinematically-distinct molecular structures? Do clusters principally form in elongated molecular structures such as Infrared Dark Clouds and Herschel filaments? One of the central reasons for slow progress in resolving these questions is the lack of homogeneous and reliable census of stellar members (both disk-bearing and disk-free) for a wide range of star forming environments. To address these issues we are now completing our major effort, called MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray). It combines the Chandra archive with UKIRT+2MASS near-infrared and Spitzer mid-infrared surveys to identify young stellar objects in a wide range of evolutionary stages, from protostars to disk-free pre-main sequence stars, in 20 star forming regions at distances from 0.4 to 3.6 kpc. Each MYStIX region was chosen to have a rich OB-dominated cluster. Started in 2009 with NASA/ADAP and NSF funding, MYStIX has emerged with 8 technical/catalog and the first 4 of a series of science papers (http://astro.psu.edu/mystix). Early MYStIX results include: demonstration of diverse morphologies of young clusters from simple ellipsoids to elongated, clumpy substructures; demonstration of spatio-age gradients across star formation regions; the discovery of core-halo age gradients within two rich nearby MYStIX clusters; and the discovery of important astrophysically empirical correlations among different subcluster properties such as age, absorption, core radius, central stellar density, and total intrinsic population. The early MYStIX result provide new observational evidence for subcluster merging and cluster expansion following gas dissipation. We propose here to extend the MYStIX effort to an archive study of 19 nearer and smaller star forming regions where the stellar clusters are dominated by a single late-OB star rather than numerous O stars as in the MYStIX fields. We call this project `Star Formation in Nearby Clusters' or SFiNCs (homophonic with `sphinx'). With a homogeneous analysis of the Chandra, 2MASS, Spitzer and Herschel archives, we expect to identify and characterize over 50 SFiNCs subclusters. The inferred empirical correlations among different cluster properties for nearly 200 SFiNCs+MYStIX subclusters with 30-3000 detected stars on scales of 0.1-20 pc will allow, for the first time, direct comparison with the results of theoretical simulations of cluster formation to seek deeper answers to the fundamental questions posed above. It is possible, for example, that smaller molecular clouds have less turbulence and thus produce small clusters in a single event rather than through subcluster mergers. Models based on meteoritic isotopes suggest that our Solar System formed in a complex of SFiNCs/MYStIX-like clusters (Gounelle & Meynet 2012, A&A, 545, 4). This project addresses NASA SMD Strategic Subgoals 3C (Advance scientific knowledge of the origin and history of the solar system) and 3D.3 (Understand how individual stars form and how those processes ultimately affect the formation of planetary systems). It lies in the `Star formation and pre-main sequence stars' Research Area of the Astrophysics Data Analysis program.

Intracluster age gradients in numerous young stellar clusters

NASA Astrophysics Data System (ADS)

Getman, K. V.; Feigelson, E. D.; Kuhn, M. A.; Bate, M. R.; Broos, P. S.; Garmire, G. P.

2018-05-01

The pace and pattern of star formation leading to rich young stellar clusters is quite uncertain. In this context, we analyse the spatial distribution of ages within 19 young (median t ≲ 3 Myr on the Siess et al. time-scale), morphologically simple, isolated, and relatively rich stellar clusters. Our analysis is based on young stellar object (YSO) samples from the Massive Young Star-Forming Complex Study in Infrared and X-ray and Star Formation in Nearby Clouds surveys, and a new estimator of pre-main sequence (PMS) stellar ages, AgeJX, derived from X-ray and near-infrared photometric data. Median cluster ages are computed within four annular subregions of the clusters. We confirm and extend the earlier result of Getman et al. (2014): 80 per cent of the clusters show age trends where stars in cluster cores are younger than in outer regions. Our cluster stacking analyses establish the existence of an age gradient to high statistical significance in several ways. Time-scales vary with the choice of PMS evolutionary model; the inferred median age gradient across the studied clusters ranges from 0.75 to 1.5 Myr pc-1. The empirical finding reported in the present study - late or continuing formation of stars in the cores of star clusters with older stars dispersed in the outer regions - has a strong foundation with other observational studies and with the astrophysical models like the global hierarchical collapse model of Vázquez-Semadeni et al.

A SPITZER VIEW OF STAR FORMATION IN THE CYGNUS X NORTH COMPLEX

DOE Office of Scientific and Technical Information (OSTI.GOV)

Beerer, I. M.; Koenig, X. P.; Hora, J. L.

2010-09-01

We present new images and photometry of the massive star-forming complex Cygnus X obtained with the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer for Spitzer (MIPS) on board the Spitzer Space Telescope. A combination of IRAC, MIPS, UKIRT Deep Infrared Sky Survey, and Two Micron All Sky Survey data are used to identify and classify young stellar objects (YSOs). Of the 8231 sources detected exhibiting infrared excess in Cygnus X North, 670 are classified as class I and 7249 are classified as class II. Using spectra from the FAST Spectrograph at the Fred L. Whipple Observatory and Hectospecmore » on the MMT, we spectrally typed 536 sources in the Cygnus X complex to identify the massive stars. We find that YSOs tend to be grouped in the neighborhoods of massive B stars (spectral types B0 to B9). We present a minimal spanning tree analysis of clusters in two regions in Cygnus X North. The fraction of infrared excess sources that belong to clusters with {>=}10 members is found to be 50%-70%. Most class II objects lie in dense clusters within blown out H II regions, while class I sources tend to reside in more filamentary structures along the bright-rimmed clouds, indicating possible triggered star formation.« less

Reconstructing galaxy histories from globular clusters.

PubMed

West, Michael J; Côté, Patrick; Marzke, Ronald O; Jordán, Andrés

2004-01-01

Nearly a century after the true nature of galaxies as distant 'island universes' was established, their origin and evolution remain great unsolved problems of modern astrophysics. One of the most promising ways to investigate galaxy formation is to study the ubiquitous globular star clusters that surround most galaxies. Globular clusters are compact groups of up to a few million stars. They generally formed early in the history of the Universe, but have survived the interactions and mergers that alter substantially their parent galaxies. Recent advances in our understanding of the globular cluster systems of the Milky Way and other galaxies point to a complex picture of galaxy genesis driven by cannibalism, collisions, bursts of star formation and other tumultuous events.

From Head to Sword: The Clustering Properties of Stars in Orion

NASA Astrophysics Data System (ADS)

Gomez, Mercedes; Lada, Charles J.

1998-04-01

We investigate the structure in the spatial distributions of optically selected samples of young stars in the Head (lambda Orionis) and in the Sword (Orion A) regions of the constellation of Orion with the aid of stellar surface density maps and the two-point angular correlation function. The distributions of young stars in both regions are found to be nonrandom and highly clustered. Stellar surface density maps reveal three distinct clusters in the lambda Ori region. The two-point correlation function displays significant features at angular scales that correspond to the radii and separations of the three clusters identified in the surface density maps. Most young stars in the lambda Ori region (~80%) are presently found within these three clusters, consistent with the idea that the majority of young stars in this region were formed in dense protostellar clusters that have significantly expanded since their formation. Over a scale of ~0.05d-0.5d the correlation function is well described by a single power law that increases smoothly with decreasing angular scale. This suggests that, within the clusters, the stars either are themselves hierarchically clustered or have a volume density distribution that falls steeply with radius. The relative lack of Hα emission-line stars in the one cluster in this region that contains OB stars suggests a timescale for emission-line activity of less than 4 Myr around late-type stars in the cluster and may indicate that the lifetimes of protoplanetary disks around young stellar objects are reduced in clusters containing O stars. The spatial distribution of young stars in the Orion A region is considerably more complex. The angular correlation function of the OB stars (which are mostly foreground to the Orion A molecular cloud) is very similar to that of the Hα stars (which are located mostly within the molecular cloud) and significantly different from that of the young stars in the lambda Ori region. This suggests that, although spatially separated, both populations in the Orion A region may have originated from a similar fragmentation process. Stellar surface density maps and modeling of the angular correlation function suggest that somewhat less than half of the OB and Hα stars in the Orion A cloud are presently within well-defined stellar clusters. Although all the OB stars could have originated in rich clusters, a significant fraction of the Hα stars appear to have formed outside such clusters in a more spatially dispersed manner. The close similarity of the angular correlation functions of the OB and Hα stars toward the molecular cloud, in conjunction with the earlier indications of a relatively high star formation rate and high gas pressure in this cloud, is consistent with the idea that older, foreground OB stars triggered the current episode of star formation in the Orion A cloud. One of the OB clusters (Upper Sword) that is foreground to the cloud does not appear to be associated with any of the clusterings of emission-line stars, again suggesting a timescale (<4 Myr) for emission-line activity and disk lifetimes around late-type stars born in OB clusters.

MYStIX: Dynamical evolution of young clusters

NASA Astrophysics Data System (ADS)

Kuhn, Michael A.

2014-08-01

The spatial structure of young stellar clusters in Galactic star-forming regions provides insight into these clusters’ dynamical evolution---a topic with implications for open questions in star-formation and cluster survival. The Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) provides a sample of >30,000 young stars in star-forming regions (d<3.6 kpc) that contain at least one O-type star. We use the finite mixture model analysis to identify subclusters of stars and determine their properties: including subcluster radii, intrinsic numbers of stars, central density, ellipticity, obscuration, and age. In 17 MYStIX regions we find 142 subclusters, with a diverse radii and densities and age spreads of up to ~1 Myr in a region. There is a strong negative correlation between subcluster radius and density, which indicates that embedded subclusters expand but also gain stars as they age. Subcluster expansion is also shown by a positive radius--age correlation, which indicates that subclusters are expanding at <1 km/s. The subcluster ellipticity distribution and number--density relation show signs of a hierarchical merger scenario, whereby young stellar clusters are built up through mergers of smaller clumps, causing evolution from a clumpy spatial distribution of stars (seen in some regions) to a simpler distribution of stars (seen in other regions). Many of the simple young stellar clusters show signs of dynamically relaxation, even though they are not old enough for this to have occurred through two-body interactions. However, this apparent contradiction might be explained if small subcluster, which have shorter dynamical relaxation times, can produce dynamically relaxed clusters through hierarchical mergers.

Hierarchical star formation across the grand-design spiral NGC 1566

NASA Astrophysics Data System (ADS)

Gouliermis, Dimitrios A.; Elmegreen, Bruce G.; Elmegreen, Debra M.; Calzetti, Daniela; Cignoni, Michele; Gallagher, John S., III; Kennicutt, Robert C.; Klessen, Ralf S.; Sabbi, Elena; Thilker, David; Ubeda, Leonardo; Aloisi, Alessandra; Adamo, Angela; Cook, David O.; Dale, Daniel; Grasha, Kathryn; Grebel, Eva K.; Johnson, Kelsey E.; Sacchi, Elena; Shabani, Fayezeh; Smith, Linda J.; Wofford, Aida

2017-06-01

We investigate how star formation is spatially organized in the grand-design spiral NGC 1566 from deep Hubble Space Telescope photometry with the Legacy ExtraGalactic UV Survey. Our contour-based clustering analysis reveals 890 distinct stellar conglomerations at various levels of significance. These star-forming complexes are organized in a hierarchical fashion with the larger congregations consisting of smaller structures, which themselves fragment into even smaller and more compact stellar groupings. Their size distribution, covering a wide range in length-scales, shows a power law as expected from scale-free processes. We explain this shape with a simple 'fragmentation and enrichment' model. The hierarchical morphology of the complexes is confirmed by their mass-size relation that can be represented by a power law with a fractional exponent, analogous to that determined for fractal molecular clouds. The surface stellar density distribution of the complexes shows a lognormal shape similar to that for supersonic non-gravitating turbulent gas. Between 50 and 65 per cent of the recently formed stars, as well as about 90 per cent of the young star clusters, are found inside the stellar complexes, located along the spiral arms. We find an age difference between young stars inside the complexes and those in their direct vicinity in the arms of at least 10 Myr. This time-scale may relate to the minimum time for stellar evaporation, although we cannot exclude the in situ formation of stars. As expected, star formation preferentially occurs in spiral arms. Our findings reveal turbulent-driven hierarchical star formation along the arms of a grand-design galaxy.

Probing the Dragonfish star-forming complex: the ionizing population of the young massive cluster Mercer 30

NASA Astrophysics Data System (ADS)

de la Fuente, D.; Najarro, F.; Borissova, J.; Ramírez Alegría, S.; Hanson, M. M.; Trombley, C.; Figer, D. F.; Davies, B.; Garcia, M.; Kurtev, R.; Urbaneja, M. A.; Smith, L. C.; Lucas, P. W.; Herrero, A.

2016-05-01

It has recently been claimed that the nebula, Dragonfish, is powered by a superluminous but elusive OB association. However, systematic searches in near-infrared photometric surveys have found many other cluster candidates in this region of the sky. Among these, the first confirmed young massive cluster was Mercer 30, where Wolf-Rayet stars were found.We perform a new characterization of Mercer 30 with unprecedented accuracy, combining NICMOS/HST and VVV photometric data with multi-epoch ISAAC/VLT H- and K-band spectra. Stellar parameters for most of spectroscopically observed cluster members are found through precise non-LTE atmosphere modeling with the CMFGEN code. Our spectrophotometric study for this cluster yields a new, revised distance of d = (12.4 ± 1.7) kpc and a total of QHMc30 ≈ 6.70 × 1050 s-1 Lyman ionizing photons. A cluster age of (4.0 ± 0.8) Myr is found through isochrone fitting, and a total mass of (1.6 ± 0.6) × 104M⊙ is estimated, thanks to our extensive knowledge of the post-main-sequence population. As a consequence, membership of Mercer 30 to the Dragonfish star-forming complex is confirmed, allowing us to use this cluster as a probe for the whole complex, which turns out to be extremely large (~400 pc across) and located at the outer edge of the Sagittarius-Carina spiral arm (~11 kpc from the Galactic center). The Dragonfish complex hosts 19 young clusters or cluster candidates (including Mercer 30 and a new candidate presented in this work) and an estimated minimum of nine field Wolf-Rayet stars. All these contributions account for, at least 73% of the ionization of the Dragonfish nebula and leaves little or no room for the alleged superluminous OB association; alternative explanations are discussed. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under programs IDs 179.B-2002, 081.D-0471, 083.D-0765, 087.D-0957, and 089.D-0989.

New detections of embedded clusters in the Galactic halo

NASA Astrophysics Data System (ADS)

Camargo, D.; Bica, E.; Bonatto, C.

2016-09-01

Context. Until recently it was thought that high Galactic latitude clouds were a non-star-forming ensemble. However, in a previous study we reported the discovery of two embedded clusters (ECs) far away from the Galactic plane (~ 5 kpc). In our recent star cluster catalogue we provided additional high and intermediate latitude cluster candidates. Aims: This work aims to clarify whether our previous detection of star clusters far away from the disc represents just an episodic event or whether star cluster formation is currently a systematic phenomenon in the Galactic halo. We analyse the nature of four clusters found in our recent catalogue and report the discovery of three new ECs each with an unusually high latitude and distance from the Galactic disc midplane. Methods: The analysis is based on 2MASS and WISE colour-magnitude diagrams (CMDs), and stellar radial density profiles (RDPs). The CMDs are built by applying a field-star decontamination procedure, which uncovers the cluster's intrinsic CMD morphology. Results: All of these clusters are younger than 5 Myr. The high-latitude ECs C 932, C 934, and C 939 appear to be related to a cloud complex about 5 kpc below the Galactic disc, under the Local arm. The other clusters are above the disc, C 1074 and C 1100 with a vertical distance of ~3 kpc, C 1099 with ~ 2 kpc, and C 1101 with ~1.8 kpc. Conclusions: According to the derived parameters ECs located below and above the disc occur, which gives evidence of widespread star cluster formation throughout the Galactic halo. This study therefore represents a paradigm shift, by demonstrating that a sterile halo must now be understood as a host for ongoing star formation. The origin and fate of these ECs remain open. There are two possibilities for their origin, Galactic fountains or infall. The discovery of ECs far from the disc suggests that the Galactic halo is more actively forming stars than previously thought. Furthermore, since most ECs do not survive the infant mortality, stars may be raining from the halo into the disc, and/or the halo may be harbouring generations of stars formed in clusters like those detected in our survey.

Star formation in the cluster merger DLSCL J0916.2+2953

DOE PAGES

Mansheim, A. S.; Lemaux, B. C.; Dawson, W. A.; ...

2017-01-13

We investigate star formation in DLSCL J0916.2+2953, a dissociative merger of two clusters at z=0.53 that has progressed 1:1 +1.3 -0.4 Gyr since rst pass-through. We attempt to reveal the effects a collision may have had on the evolution of the cluster galaxies by tracing their star formation history. We probe current and recent activity to identify a possible star formation event at the time of the merger using EW(Hδ), EW([OII]), and Dn(4000) measured from the composite spectra of 64 cluster and 153 coeval eld galaxies. We supplement Keck DEIMOS spectra with DLS and HST imaging to determine the color,more » stellar mass, and morphology of each galaxy and conduct a comprehensive study of the populations in this complex structure. Spectral results indicate the average cluster and cluster red sequence galaxies experienced no enhanced star formation relative to the surrounding eld during the merger, ruling out a predominantly merger-quenched population. We nd that the average blue galaxy in the North cluster is currently active and in the South cluster is currently post-starburst having undergone a recent star formation event. While the North activity could be latent or long- term merger effects, a young blue stellar population and irregular geometry suggest the cluster was still forming prior the collision. While the South activity coincides with the time of the merger, the blue early-type population could be a result of secular cluster processes. The evidence suggests that the dearth or surfeit of activity is indiscernible from normal cluster galaxy evolution.« less

Fibers in the NGC 1333 proto-cluster

NASA Astrophysics Data System (ADS)

Hacar, A.; Tafalla, M.; Alves, J.

2017-10-01

Are the initial conditions for clustered star formation the same as for non-clustered star formation? To investigate the initial gas properties in young proto-clusters we carried out a comprehensive and high-sensitivity study of the internal structure, density, temperature, and kinematics of the dense gas content of the NGC 1333 region in Perseus, one of the nearest and best studied embedded clusters. The analysis of the gas velocities in the position-position-velocity space reveals an intricate underlying gas organization both in space and velocity. We identified a total of 14 velocity-coherent, (tran-)sonic structures within NGC 1333, with similar physical and kinematic properties than those quiescent, star-forming (aka fertile) fibers previously identified in low-mass star-forming clouds. These fibers are arranged in a complex spatial network, build-up the observed total column density, and contain the dense cores and protostars in this cloud. Our results demonstrate that the presence of fibers is not restricted to low-mass clouds but can be extended to regions of increasing mass and complexity. We propose that the observational dichotomy between clustered and non-clustered star-forming regions might be naturally explained by the distinct spatial density of fertile fibers in these environments. Based on observations carried out under project number 169-11 with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).Based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut für Radioastronomie) at Effelsberg.Molecular line observations (spectral cubes) are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/606/A123

Formation of Compact Ellipticals in the merging star cluster scenario

NASA Astrophysics Data System (ADS)

Urrutia Zapata, Fernanda Cecilia; Theory and star formation group

2018-01-01

In the last years, extended old stellar clusters have been observed. They are like globular clusters (GCs) but with larger sizes(a limit of Re=10 pc is currently seen as reasonable). These extended objects (EOs) cover a huge range of mass. Objects at the low mass end with masses comparable to normal globular clusters are called extended clusters or faint fuzzies Larsen & Brodie (2000) and objects at the high-mass end are called ultra compact dwarf galaxies (UCDs). Ultra compact dwarf galaxies are compact object with luminositys above the brigtest known GCs. UCDs are more compact than typical dwarf galaxies but with comparable luminosities. Usually, a lower mass limit of 2 × 10^6 Solar masses is applied.Fellhauer & Kroupa (2002a,b) demostrated that object like ECs, FFs and UCDs can be the remnants of the merger of star clusters complexes, this scenario is called the Merging Star Cluster Scenario. Amore concise study was performed by Bruens et al. (2009, 2011).Our work tries to explain the formation of compact elliptical(cE). These objects are a comparatively rare class of spheroidal galaxies, possessing very small Re and high central surface brightnesses (Faber 1973). cEs have the same parameters as extended objects but they are slightly larger than 100 pc and the luminosities are in the range of -11 to -12 Mag.The standard formation sceanrio of these systems proposes a galaxy origin. CEs are the result of tidal stripping and truncation of nucleated larger systems. Or they could be a natural extension of the class of elliptical galaxies to lower luminosities and smaller sizes.We want to propose a completely new formation scenario for cEs. In our project we try to model cEs in a similar way that UCDs using the merging star cluster scenario extended to much higher masses and sizes. We think that in the early Universe we might have produced sufficiently strong star bursts to form cluster complexes which merge into cEs. So far it is observationally unknown if cEs are dark matter dominated objects. If our scenario is true, then they would be dark matter free very extended and massive "star clusters".

The MYStIX Infrared-Excess Source Catalog

NASA Astrophysics Data System (ADS)

Povich, Matthew S.; Kuhn, Michael A.; Getman, Konstantin V.; Busk, Heather A.; Feigelson, Eric D.; Broos, Patrick S.; Townsley, Leisa K.; King, Robert R.; Naylor, Tim

2013-12-01

The Massive Young Star-Forming Complex Study in Infrared and X-rays (MYStIX) project provides a comparative study of 20 Galactic massive star-forming complexes (d = 0.4-3.6 kpc). Probable stellar members in each target complex are identified using X-ray and/or infrared data via two pathways: (1) X-ray detections of young/massive stars with coronal activity/strong winds or (2) infrared excess (IRE) selection of young stellar objects (YSOs) with circumstellar disks and/or protostellar envelopes. We present the methodology for the second pathway using Spitzer/IRAC, 2MASS, and UKIRT imaging and photometry. Although IRE selection of YSOs is well-trodden territory, MYStIX presents unique challenges. The target complexes range from relatively nearby clouds in uncrowded fields located toward the outer Galaxy (e.g., NGC 2264, the Flame Nebula) to more distant, massive complexes situated along complicated, inner Galaxy sightlines (e.g., NGC 6357, M17). We combine IR spectral energy distribution (SED) fitting with IR color cuts and spatial clustering analysis to identify IRE sources and isolate probable YSO members in each MYStIX target field from the myriad types of contaminating sources that can resemble YSOs: extragalactic sources, evolved stars, nebular knots, and even unassociated foreground/background YSOs. Applying our methodology consistently across 18 of the target complexes, we produce the MYStIX IRE Source (MIRES) Catalog comprising 20,719 sources, including 8686 probable stellar members of the MYStIX target complexes. We also classify the SEDs of 9365 IR counterparts to MYStIX X-ray sources to assist the first pathway, the identification of X-ray-detected stellar members. The MIRES Catalog provides a foundation for follow-up studies of diverse phenomena related to massive star cluster formation, including protostellar outflows, circumstellar disks, and sequential star formation triggered by massive star feedback processes.

Runaway Massive Stars from R136: VFTS 682 is Very Likely a "Slow Runaway"

NASA Astrophysics Data System (ADS)

Banerjee, Sambaran; Kroupa, Pavel; Oh, Seungkyung

2012-02-01

We conduct a theoretical study on the ejection of runaway massive stars from R136—the central massive, starburst cluster in the 30 Doradus complex of the Large Magellanic Cloud. Specifically, we investigate the possibility of the very massive star (VMS) VFTS 682 being a runaway member of R136. Recent observations of the above VMS, by virtue of its isolated location and its moderate peculiar motion, have raised the fundamental question of whether isolated massive star formation is indeed possible. We perform the first realistic N-body computations of fully mass-segregated R136-type star clusters in which all the massive stars are in primordial binary systems. These calculations confirm that the dynamical ejection of a VMS from an R136-like cluster, with kinematic properties similar to those of VFTS 682, is common. Hence, the conjecture of isolated massive star formation is unnecessary to account for this VMS. Our results are also quite consistent with the ejection of 30 Dor 016, another suspected runaway VMS from R136. We further note that during the clusters' evolution, mergers of massive binaries produce a few single stars per cluster with masses significantly exceeding the canonical upper limit of 150 M ⊙. The observations of such single super-canonical stars in R136, therefore, do not imply an initial mass function with an upper limit greatly exceeding the accepted canonical 150 M ⊙ limit, as has been suggested recently, and they are consistent with the canonical upper limit.

Young star clusters in nearby molecular clouds

NASA Astrophysics Data System (ADS)

Getman, K. V.; Kuhn, M. A.; Feigelson, E. D.; Broos, P. S.; Bate, M. R.; Garmire, G. P.

2018-06-01

The SFiNCs (Star Formation in Nearby Clouds) project is an X-ray/infrared study of the young stellar populations in 22 star-forming regions with distances ≲ 1 kpc designed to extend our earlier MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray) survey of more distant clusters. Our central goal is to give empirical constraints on cluster formation mechanisms. Using parametric mixture models applied homogeneously to the catalogue of SFiNCs young stars, we identify 52 SFiNCs clusters and 19 unclustered stellar structures. The procedure gives cluster properties including location, population, morphology, association with molecular clouds, absorption, age (AgeJX), and infrared spectral energy distribution (SED) slope. Absorption, SED slope, and AgeJX are age indicators. SFiNCs clusters are examined individually, and collectively with MYStIX clusters, to give the following results. (1) SFiNCs is dominated by smaller, younger, and more heavily obscured clusters than MYStIX. (2) SFiNCs cloud-associated clusters have the high ellipticities aligned with their host molecular filaments indicating morphology inherited from their parental clouds. (3) The effect of cluster expansion is evident from the radius-age, radius-absorption, and radius-SED correlations. Core radii increase dramatically from ˜0.08 to ˜0.9 pc over the age range 1-3.5 Myr. Inferred gas removal time-scales are longer than 1 Myr. (4) Rich, spatially distributed stellar populations are present in SFiNCs clouds representing early generations of star formation. An appendix compares the performance of the mixture models and non-parametric minimum spanning tree to identify clusters. This work is a foundation for future SFiNCs/MYStIX studies including disc longevity, age gradients, and dynamical modelling.

Young stellar population and ongoing star formation in the H II complex Sh2-252

NASA Astrophysics Data System (ADS)

Jose, Jessy; Pandey, A. K.; Samal, M. R.; Ojha, D. K.; Ogura, K.; Kim, J. S.; Kobayashi, N.; Goyal, A.; Chauhan, N.; Eswaraiah, C.

2013-07-01

In this paper, an extensive survey of the star-forming complex Sh2-252 has been undertaken with an aim to explore its hidden young stellar population as well as to understand the structure and star formation history for the first time. This complex is composed of five prominent embedded clusters associated with the subregions A, C, E, NGC 2175s and Teu 136. We used Two Micron All Sky Survey-near-infrared and Spitzer-Infrared Array Camera, Multiband Imaging Photometer for Spitzer photometry to identify and classify the young stellar objects (YSOs) by their infrared (IR) excess emission. Using the IR colour-colour criteria, we identified 577 YSOs, of which, 163 are Class I, 400 are Class II and 14 are transition disc YSOs, suggesting a moderately rich number of YSOs in this complex. Spatial distribution of the candidate YSOs shows that they are mostly clustered around the subregions in the western half of the complex, suggesting enhanced star formation activity towards its west. Using the spectral energy distribution and optical colour-magnitude diagram-based age analyses, we derived probable evolutionary status of the subregions of Sh2-252. Our analysis shows that the region A is the youngest (˜0.5 Myr), the regions B, C and E are of similar evolutionary stage (˜1-2 Myr) and the clusters NGC 2175s and Teu 136 are slightly evolved (˜2-3 Myr). Morphology of the region in the 1.1 mm map shows a semicircular shaped molecular shell composed of several clumps and YSOs bordering the western ionization front of Sh2-252. Our analyses suggest that next generation star formation is currently under way along this border and that possibly fragmentation of the matter collected during the expansion of the H II region as one of the major processes is responsible for such stars. We observed the densest concentration of YSOs (mostly Class I, ˜0.5 Myr) at the western outskirts of the complex, within a molecular clump associated with water and methanol masers and we suggest that it is indeed a site of cluster formation at a very early evolutionary stage, sandwiched between the two relatively evolved CH II regions A and B.

Star Formation in the Heart of the Swan

NASA Image and Video Library

2011-06-09

Best known as a swan winging its way across the night, the constellation Cygnus is easily seen in the northern hemisphere summertime sky. NASA WISE telescope captured this image of a huge complex of star-forming clouds and stellar clusters.

RUNAWAY MASSIVE STARS FROM R136: VFTS 682 IS VERY LIKELY A 'SLOW RUNAWAY'

DOE Office of Scientific and Technical Information (OSTI.GOV)

Banerjee, Sambaran; Kroupa, Pavel; Oh, Seungkyung, E-mail: sambaran@astro.uni-bonn.de, E-mail: pavel@astro.uni-bonn.de, E-mail: skoh@astro.uni-bonn.de

2012-02-10

We conduct a theoretical study on the ejection of runaway massive stars from R136-the central massive, starburst cluster in the 30 Doradus complex of the Large Magellanic Cloud. Specifically, we investigate the possibility of the very massive star (VMS) VFTS 682 being a runaway member of R136. Recent observations of the above VMS, by virtue of its isolated location and its moderate peculiar motion, have raised the fundamental question of whether isolated massive star formation is indeed possible. We perform the first realistic N-body computations of fully mass-segregated R136-type star clusters in which all the massive stars are in primordialmore » binary systems. These calculations confirm that the dynamical ejection of a VMS from an R136-like cluster, with kinematic properties similar to those of VFTS 682, is common. Hence, the conjecture of isolated massive star formation is unnecessary to account for this VMS. Our results are also quite consistent with the ejection of 30 Dor 016, another suspected runaway VMS from R136. We further note that during the clusters' evolution, mergers of massive binaries produce a few single stars per cluster with masses significantly exceeding the canonical upper limit of 150 M{sub Sun }. The observations of such single super-canonical stars in R136, therefore, do not imply an initial mass function with an upper limit greatly exceeding the accepted canonical 150 M{sub Sun} limit, as has been suggested recently, and they are consistent with the canonical upper limit.« less

Massive Stars in the W33 Giant Molecular Complex

NASA Astrophysics Data System (ADS)

Messineo, Maria; Clark, J. Simon; Figer, Donald F.; Kudritzki, Rolf-Peter; Najarro, Francisco; Rich, R. Michael; Menten, Karl M.; Ivanov, Valentin D.; Valenti, Elena; Trombley, Christine; Chen, C.-H. Rosie; Davies, Ben

2015-06-01

Rich in H ii regions, giant molecular clouds are natural laboratories to study massive stars and sequential star formation. The Galactic star-forming complex W33 is located at l=˜ 12\\buildrel{\\circ}\\over{.} 8 and at a distance of 2.4 kpc and has a size of ≈ 10 pc and a total mass of ≈ (0.8-8.0) × {{10}5} M ⊙ . The integrated radio and IR luminosity of W33—when combined with the direct detection of methanol masers, the protostellar object W33A, and the protocluster embedded within the radio source W33 main—mark the region as a site of vigorous ongoing star formation. In order to assess the long-term star formation history, we performed an infrared spectroscopic search for massive stars, detecting for the first time 14 early-type stars, including one WN6 star and four O4-7 stars. The distribution of spectral types suggests that this population formed during the past ˜2-4 Myr, while the absence of red supergiants precludes extensive star formation at ages 6-30 Myr. This activity appears distributed throughout the region and does not appear to have yielded the dense stellar clusters that characterize other star-forming complexes such as Carina and G305. Instead, we anticipate that W33 will eventually evolve into a loose stellar aggregate, with Cyg OB2 serving as a useful, albeit richer and more massive, comparator. Given recent distance estimates, and despite a remarkably similar stellar population, the rich cluster Cl 1813-178 located on the northwest edge of W33 does not appear to be physically associated with W33.

The INfrared Survey of Young Nebulous Clusters (IN-SYNC): Surveying the Dynamics and Star Formation Histories of Young Clusters with APOGEE

NASA Astrophysics Data System (ADS)

Covey, Kevin R.; Cottaar, Michiel; Foster, Jonathan B.; Da Rio, Nicola; Tan, Jonathan; Meyer, Michael; Nidever, David L.; Flaherty, Kevin M.; Arce, Hector G.; Rebull, Luisa M.; Chojnowski, S. Drew; Frinchaboy, Peter M.; Hearty, Fred R.; Majewski, Steven R.; Skrutskie, Michael F.; Stassun, Keivan; Wilson, John C.; Zasowski, Gail

2015-01-01

Young clusters are the most prolific sites of star formation in the Milky Way, but demographic studies indicate that relatively few of the Milky Way's stellar clusters persist as bound structures for 100 Myrs or longer. Uniform & precise measurements of the stellar populations and internal dynamics of these regions are difficult to obtain, however, particularly for extremely young clusters whose optical visibility is greatly hampered by their parental molecular cloud. The INfrared Survey of Young Nebulous Clusters (IN-SYNC), an SDSS-III ancillary science program, leverages the stability and multiplex capability of the APOGEE spectrograph to obtain high resolution spectra at near-infrared wavelengths, where photospheric emission is better able to penetrate the dusty shrouds that surround sites of active star formation. We summarize our recent measurements of the kinematics and stellar populations of IC 348 and NGC 1333, two young clusters in the Perseus Molecular Cloud, and of the members of the Orion Nebula Cluster (ONC) and L1641 filament in the Orion molecular complex. These measurements highlight the dynamically 'warm' environment within these young clusters, and suggest a range of stellar radii within these quasi-single-age populations. We close with a preview of plans for continuing this work as part of the APOGEE-2 science portfolio: self-consistent measurements of the kinematics and star formation histories for clusters spanning a range of initial conditions and ages will provide a opportunity to disentangle the mechanisms that drive the formation and dissolution of sites of active star formation.

Kinematical evolution of tidally limited star clusters: rotational properties

NASA Astrophysics Data System (ADS)

Tiongco, Maria A.; Vesperini, Enrico; Varri, Anna Lisa

2017-07-01

We present the results of a set of N-body simulations following the long-term evolution of the rotational properties of star cluster models evolving in the external tidal field of their host galaxy, after an initial phase of violent relaxation. The effects of two-body relaxation and escape of stars lead to a redistribution of the ordered kinetic energy from the inner to the outer regions, ultimately determining a progressive general loss of angular momentum; these effects are reflected in the overall decline of the rotation curve as the cluster evolves and loses stars. We show that all of our models share the same dependence of the remaining fraction of the initial rotation on the fraction of the initial mass lost. As the cluster evolves and loses part of its initial angular momentum, it becomes increasingly dominated by random motions, but even after several tens of relaxation times, and losing a significant fraction of its initial mass, a cluster can still be characterized by a non-negligible ratio of the rotational velocity to the velocity dispersion. This result is in qualitative agreement with the recently observed kinematical complexity that characterizes several Galactic globular clusters.

T-ReX Spies the Stars of 30 Doradus

NASA Astrophysics Data System (ADS)

Broos, Patrick; Townsley, Leisa K.; Pollock, Andrew; Crowther, Paul

2017-08-01

30 Doradus (the Tarantula Nebula) is the Local Group's most massive young star-forming complex. At its heart is R136, the most massive resolved stellar cluster; R136 contains, in turn, the most massive stars known. The Chandra X-ray Observatory has recently observed 30 Dor for the 2-megasecond X-ray Visionary Project ``The Tarantula -- Revealed by X-rays'' (T-ReX). This deep observation exploits Chandra's fine spatial resolution to study the full complement of massive stars and the brightest pre-main sequence stars that trace 25 Myrs of star formation in this incomparable nearby starburst. Here we give preliminary results from the ongoing analyses of the data, focusing on the massive stars. While many remain undetected even in this deep ACIS-I observation, a few show dramatic X-ray lightcurves and/or high luminosities befitting this amazing starburst cluster.

A Chandra ACIS Study of 30 Doradus. II. X-Ray Point Sources in the Massive Star Cluster R136 and Beyond

NASA Astrophysics Data System (ADS)

Townsley, Leisa K.; Broos, Patrick S.; Feigelson, Eric D.; Garmire, Gordon P.; Getman, Konstantin V.

2006-04-01

We have studied the X-ray point-source population of the 30 Doradus (30 Dor) star-forming complex in the Large Magellanic Cloud using high spatial resolution X-ray images and spatially resolved spectra obtained with the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Here we describe the X-ray sources in a 17'×17' field centered on R136, the massive star cluster at the center of the main 30 Dor nebula. We detect 20 of the 32 Wolf-Rayet stars in the ACIS field. The cluster R136 is resolved at the subarcsecond level into almost 100 X-ray sources, including many typical O3-O5 stars, as well as a few bright X-ray sources previously reported. Over 2 orders of magnitude of scatter in LX is seen among R136 O stars, suggesting that X-ray emission in the most massive stars depends critically on the details of wind properties and the binarity of each system, rather than reflecting the widely reported characteristic value LX/Lbol~=10-7. Such a canonical ratio may exist for single massive stars in R136, but our data are too shallow to confirm this relationship. Through this and future X-ray studies of 30 Dor, the complete life cycle of a massive stellar cluster can be revealed.

TIDAL TAILS OF MINOR MERGERS. II. COMPARING STAR FORMATION IN THE TIDAL TAILS OF NGC 2782

DOE Office of Scientific and Technical Information (OSTI.GOV)

Knierman, Karen A.; Scowen, Paul; Veach, Todd

2013-09-10

The peculiar spiral NGC 2782 is the result of a minor merger with a mass ratio {approx}4: 1 occurring {approx}200 Myr ago. This merger produced a molecular and H I-rich, optically bright eastern tail and an H I-rich, optically faint western tail. Non-detection of CO in the western tail by Braine et al. suggested that star formation had not yet begun. However, deep UBVR and H{alpha} narrowband images show evidence of recent star formation in the western tail, though it lacks massive star clusters and cluster complexes. Using Herschel PACS spectroscopy, we discover 158 {mu}m [C II] emission at themore » location of the three most luminous H{alpha} sources in the eastern tail, but not at the location of the even brighter H{alpha} source in the western tail. The western tail is found to have a normal star formation efficiency (SFE), but the eastern tail has a low SFE. The lack of CO and [C II] emission suggests that the western tail H II region may have a low carbon abundance and be undergoing its first star formation. The western tail is more efficient at forming stars, but lacks massive clusters. We propose that the low SFE in the eastern tail may be due to its formation as a splash region where gas heating is important even though it has sufficient molecular and neutral gas to make massive star clusters. The western tail, which has lower gas surface density and does not form high-mass star clusters, is a tidally formed region where gravitational compression likely enhances star formation.« less

Hubble Watches Star Clusters on a Collision Course

NASA Image and Video Library

2012-08-16

Image release August 16, 2012 Astronomers using data from NASA's Hubble Space Telescope have caught two clusters full of massive stars that may be in the early stages of merging. The 30 Doradus Nebula is 170,000 light-years from Earth. What at first was thought to be only one cluster in the core of the massive star-forming region 30 Doradus has been found to be a composite of two clusters that differ in age by about one million years. The entire 30 Doradus complex has been an active star-forming region for 25 million years, and it is currently unknown how much longer this region can continue creating new stars. Smaller systems that merge into larger ones could help to explain the origin of some of the largest known star clusters. The Hubble observations, made with the Wide Field Camera 3, were taken Oct. 20-27, 2009. The blue color is light from the hottest, most massive stars; the green from the glow of oxygen; and the red from fluorescing hydrogen. To read more about this image go to: www.nasa.gov/mission_pages/hubble/science/cluster-collisi... Image Credit: NASA, ESA, and E. Sabbi (ESA/STScI) NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

The end of the White Dwarf Cooling Sequence of NGC 6752

NASA Astrophysics Data System (ADS)

Bedin, Luigi

2017-08-01

We propose to study the last HST-accessible white dwarf (WD) cooling sequence (CS) for a nearby globular cluster (GC), the chemically complex, extreme blue horizontal branch cluster NGC 6752. Over 97% of stars end their lives as WDs, and the WD CS provides constraints not only on the age, but also potentially the star formation history of a GC. The CS of WDs also lies in the least-explored region of the color-magnitude diagram of old stellar populations. Recent deep imaging with HST has successfully reached the end of the WD CS in only three classical old GCs, M4, NGC 6397 and 47 Tuc, and reveals an unexpectedly complex, and double-peaked, WD CS in the metal rich old open cluster NGC 6791. One more investigation is in progress on the massive globular Omega Centauri, where over 14 sub-populations are known to exist.While almost every cluster is known to host multiple populations, every single cluster is unique. NGC 6752 is a bridge between the relatively simple globular clusters, and Omega Cen, the most complex globular cluster known. NGC 6752 has an extended blue horizontal branch, a collapsed core and 3 chemically distinct populations. It is our last chance to add diversity to our very limited sample of WD CS, so far containing only 3 globular clusters, one old open cluster, and the complex Omega Cen system. We need to undertake this investigation while HST is still operational, as there is no foreseeable opportunity in the post-HST era to have one extra WD CS in the homogeneus optical photometric system of HST.

Stellar Content and Star Formation in Young Clusters Influenced by Massive Stars

NASA Astrophysics Data System (ADS)

Jose, J.

2014-09-01

Star Formation (SF) in extreme environment is always challenging and can be significantly different from that in quiet environments. This study presents the comprehensive multi-wavelength (optical, NIR, MIR and radio) observational analysis of three Galactic starforming regions associated with H II regions/young clusters and located at > 2 kpc, which are found to be evolving under the influence of massive stars within their vicinity. The candidate massive stars, young stellar objects, their mass, age, age spread, the form of K-band Luminosity Function (KLF), Initial Mass Function (IMF) and a possible formation history of each region are studied. The major results on Sh2-252, an extended H II region that appears to be undergoing multiple episodes of SF, are highlighted. Our analysis shows that all the regions are undergoing complex SF activity and the new generation of stars in each region seem to be an outcome of the influence by the presence of massive stars within them. SF process in these regions are likely to be multi-fold and the results suggest that multiple modes of triggering mechanism and hierarchial modes of SF are a common phenomena within young clusters.

HUBBLE PROBES THE VIOLENT BIRTH OF STARS IN GALAXY NGC 253 [Left

NASA Technical Reports Server (NTRS)

2002-01-01

An image of the spiral galaxy NGC 253, taken with a ground-based telescope. The galaxy is located about 8 million light-years away in the constellation Sculptor. Credit: Jay Gallagher (University of Wisconsin-Madison), Alan Watson (Lowell Observatory, Flagstaff, AZ), and NASA [Right] This NASA Hubble Space Telescope image of the core of the nearest starburst spiral galaxy, NGC 253, reveals violent star formation within a region 1,000 light-years across. A starburst galaxy has an exceptionally high rate of star birth, first identified by its excess of infrared radiation from warm dust. Hubble's high resolution allows astronomers to quantify complex structures in the starburst core of the galaxy for the first time, including luminous star clusters, dust lanes which trace regions of dense gas and filaments of glowing gas. Hubble identifies several regions of intense star formation, which include a bright, super-compact star cluster. These observations confirm that stars are often born in dense clusters within starbursts, and that dense gas coexists with and obscures the starburst core. This image was taken with Hubble's Wide Field Planetary Camera 2 (in PC mode). Credit: Carnegie Institution of Washington

Hundreds of new cluster candidates in the VISTA Variables in the Vía Láctea survey DR1

NASA Astrophysics Data System (ADS)

Barbá, R. H.; Roman-Lopes, A.; Nilo Castellón, J. L.; Firpo, V.; Minniti, D.; Lucas, P.; Emerson, J. P.; Hempel, M.; Soto, M.; Saito, R. K.

2015-09-01

Context. VISTA variables in the Vía Láctea is an ESO Public survey dedicated to scanning the bulge and an adjacent portion of the Galactic disk in the fourth quadrant using the VISTA telescope and its near-infrared camera VIRCAM. One of the leading goals of the VVV survey is to contribute to knowledge of the star cluster population of the Milky Way. Aims: To improve the census of Galactic star clusters, we performed a systematic and careful scan of the JHKs images of the Galactic plane section of the VVV survey. Methods: Our detection procedure is based on a combination of stellar density maps and visual inspection of promising features in the J-, H-, and KS-band images. The material examined are VVV JHKS color-composite images corresponding to Data Release 1 of VVV. Results: We report the discovery of 493 new infrared star cluster candidates. The analysis of the spatial distribution show that the clusters are very concentrated in the Galactic plane, presenting some local maxima around the position of large star-forming complexes, such as G305, RCW 95, and RCW 106. The vast majority of the new star cluster candidates are quite compact and generally surrounded by bright and/or dark nebulosities. IRAS point sources are associated with 59% of the sample, while 88% are associated with MSX point sources. GLIMPSE 8 μm images of the cluster candidates show a variety of morphologies, with 292 clusters dominated by knotty sources, while 361 clusters show some kind of nebulosity in this wavelength regime. Spatial cross-correlation with young stellar objects, masers, and extended green-object catalogs suggest that a large sample of the new cluster candidates are extremely young. In particular, 104 star clusters associated with methanol masers are excellent candidates for ongoing massive star formation. Also, there is a special set of sixteen cluster candidates that present clear signposts of star-forming activity having associated simultaneosly dark nebulae, young stellar objects, extended green objects, and masers. Full Tables 1-3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/581/A120

Massive stars, disks, and clustered star formation

NASA Astrophysics Data System (ADS)

Moeckel, Nickolas Barry

The formation of an isolated massive star is inherently more complex than the relatively well-understood collapse of an isolated, low-mass star. The dense, clustered environment where massive stars are predominantly found further complicates the picture, and suggests that interactions with other stars may play an important role in the early life of these objects. In this thesis we present the results of numerical hydrodynamic experiments investigating interactions between a massive protostar and its lower-mass cluster siblings. We explore the impact of these interactions on the orientation of disks and outflows, which are potentially observable indications of encounters during the formation of a star. We show that these encounters efficiently form eccentric binary systems, and in clusters similar to Orion they occur frequently enough to contribute to the high multiplicity of massive stars. We suggest that the massive protostar in Cepheus A is currently undergoing a series of interactions, and present simulations tailored to that system. We also apply the numerical techniques used in the massive star investigations to a much lower-mass regime, the formation of planetary systems around Solar- mass stars. We perform a small number of illustrative planet-planet scattering experiments, which have been used to explain the eccentricity distribution of extrasolar planets. We add the complication of a remnant gas disk, and show that this feature has the potential to stabilize the system against strong encounters between planets. We present preliminary simulations of Bondi-Hoyle accretion onto a protoplanetary disk, and consider the impact of the flow on the disk properties as well as the impact of the disk on the accretion flow.

Multiwavelength observations of two B-star nurseries - DR 15 and DR 20

NASA Technical Reports Server (NTRS)

Odenwald, S. F.; Campbell, M. F.; Shivanandan, K.; Schwartz, P.; Fazio, G. G.; Moseley, H.

1990-01-01

New observations of DR 15 and 20 are reported as part of a study of compact H II regions in the Cyg X region. The radio and FIR data for these objects, when combined with (C-12)O maps, IRAS imagery, and optical photographs, provide new insights into the structure of this complex region and the nature of the star-formation process there. The observations show that DR 15 may consist of one or two B0 ZAMS stars whose H I regions have formed a low-density cavity within a molecular cloud. DR 20 appears to be a young OB cluster. The cluster is dominated by an O5.5 ZAMS star and also contains an approximately 3500-yr-old B0 star appearing as a compact H II region, along with weak FIR sources that may be B0-star candidates.

Star-forming brightest cluster galaxies at 0.25

DOE Office of Scientific and Technical Information (OSTI.GOV)

McDonald, M.; Stalder, B.; Bayliss, M.

2016-01-22

We present a multiwavelength study of the 90 brightest cluster galaxies (BCGs) in a sample of galaxy clusters selected via the Sunyaev Zel'dovich effect by the South Pole Telescope, utilizing data from various ground- and space-based facilities. We infer the star-formation rate (SFR) for the BCG in each cluster—based on the UV and IR continuum luminosity, as well as the [O ii]λλ3726,3729 emission line luminosity in cases where spectroscopy is available—and find seven systems with SFR > 100 M⊙ yr-1. We find that the BCG SFR exceeds 10 M⊙ yr-1 in 31 of 90 (34%) cases at 0.25 < z < 1.25, compared to ~1%–5% at z ~ 0 from the literature. At z gsim 1, this fraction increases tomore » $${92}_{-31}^{+6}$$%, implying a steady decrease in the BCG SFR over the past ~9 Gyr. At low-z, we find that the specific SFR in BCGs is declining more slowly with time than for field or cluster galaxies, which is most likely due to the replenishing fuel from the cooling ICM in relaxed, cool core clusters. At z gsim 0.6, the correlation between the cluster central entropy and BCG star formation—which is well established at z ~ 0—is not present. Instead, we find that the most star-forming BCGs at high-z are found in the cores of dynamically unrelaxed clusters. We use data from the Hubble Space Telescope to investigate the rest-frame near-UV morphology of a subsample of the most star-forming BCGs, and find complex, highly asymmetric UV morphologies on scales as large as ~50–60 kpc. The high fraction of star-forming BCGs hosted in unrelaxed, non-cool core clusters at early times suggests that the dominant mode of fueling star formation in BCGs may have recently transitioned from galaxy–galaxy interactions to ICM cooling.« less

Star-forming brightest cluster galaxies at 0.25 < z < 1.25: A transitioning fuel supply

DOE PAGES

McDonald, M.; Stalder, B.; Bayliss, M.; ...

2016-01-22

In this paper, we present a multiwavelength study of the 90 brightest cluster galaxies (BCGs) in a sample of galaxy clusters selected via the Sunyaev Zel'dovich effect by the South Pole Telescope, utilizing data from various ground- and space-based facilities. We infer the star-formation rate (SFR) for the BCG in each cluster—based on the UV and IR continuum luminosity, as well as the [O ii]λλ3726,3729 emission line luminosity in cases where spectroscopy is available—and find seven systems with SFR > 100 M ⊙ yr -1. We find that the BCG SFR exceeds 10 M ⊙ yr -1 in 31 of 90 (34%) cases at 0.25 < z < 1.25, compared to ~1%–5% at z ~ 0 from the literature. At z ≳ 1, this fraction increases tomore » $${92}_{-31}^{+6}$$%, implying a steady decrease in the BCG SFR over the past ~9 Gyr. At low-z, we find that the specific SFR in BCGs is declining more slowly with time than for field or cluster galaxies, which is most likely due to the replenishing fuel from the cooling ICM in relaxed, cool core clusters. At z ≳ 0.6, the correlation between the cluster central entropy and BCG star formation—which is well established at z ~ 0—is not present. Instead, we find that the most star-forming BCGs at high-z are found in the cores of dynamically unrelaxed clusters. We use data from the Hubble Space Telescope to investigate the rest-frame near-UV morphology of a subsample of the most star-forming BCGs, and find complex, highly asymmetric UV morphologies on scales as large as ~50–60 kpc. Finally, the high fraction of star-forming BCGs hosted in unrelaxed, non-cool core clusters at early times suggests that the dominant mode of fueling star formation in BCGs may have recently transitioned from galaxy–galaxy interactions to ICM cooling.« less

THE YOUNG OPEN CLUSTERS KING 12, NGC 7788, AND NGC 7790: PRE-MAIN-SEQUENCE STARS AND EXTENDED STELLAR HALOS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Davidge, T. J.

2012-12-20

The stellar contents of the open clusters King 12, NGC 7788, and NGC 7790 are investigated using MegaCam images. Comparisons with isochrones yield an age <20 Myr for King 12, 20-40 Myr for NGC 7788, and 60-80 Myr for NGC 7790 based on the properties of stars near the main-sequence turnoff (MSTO) in each cluster. The reddening of NGC 7788 is much larger than previously estimated. The luminosity functions (LFs) of King 12 and NGC 7788 show breaks that are attributed to the onset of pre-main-sequence (PMS) objects, and comparisons with models of PMS evolution yield ages that are consistentmore » with those measured from stars near the MSTO. In contrast, the r' LF of main-sequence stars in NGC 7790 is matched to r' = 20 by a model that is based on the solar neighborhood mass function. The structural properties of all three clusters are investigated by examining the two-point angular correlation function of blue main-sequence stars. King 12 and NGC 7788 are each surrounded by a stellar halo that extends out to a radius of 5 arcmin ({approx}3.4 pc). It is suggested that these halos form in response to large-scale mass ejection early in the evolution of the clusters, as predicted by models. In contrast, blue main-sequence stars in NGC 7790 are traced out to a radius of {approx}7.5 arcmin ({approx}5.5 pc), with no evidence of a halo. It is suggested that all three clusters may have originated in the same star-forming complex, but not in the same giant molecular cloud.« less

The Not So Simple Globular Cluster ω Cen. I. Spatial Distribution of the Multiple Stellar Populations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Calamida, A.; Saha, A.; Strampelli, G.

2017-04-01

We present a multi-band photometric catalog of ≈1.7 million cluster members for a field of view of ≈2° × 2° across ω Cen. Photometry is based on images collected with the Dark Energy Camera on the 4 m Blanco telescope and the Advanced Camera for Surveys on the Hubble Space Telescope . The unprecedented photometric accuracy and field coverage allowed us, for the first time, to investigate the spatial distribution of ω Cen multiple populations from the core to the tidal radius, confirming its very complex structure. We found that the frequency of blue main-sequence stars is increasing compared to red main-sequencemore » stars starting from a distance of ≈25′ from the cluster center. Blue main-sequence stars also show a clumpy spatial distribution, with an excess in the northeast quadrant of the cluster pointing toward the direction of the Galactic center. Stars belonging to the reddest and faintest red-giant branch also show a more extended spatial distribution in the outskirts of ω Cen, a region never explored before. Both these stellar sub-populations, according to spectroscopic measurements, are more metal-rich compared to the cluster main stellar population. These findings, once confirmed, make ω Cen the only stellar system currently known where metal-rich stars have a more extended spatial distribution compared to metal-poor stars. Kinematic and chemical abundance measurements are now needed for stars in the external regions of ω Cen to better characterize the properties of these sub-populations.« less

Mapping the spatial distribution of star formation in cluster galaxies at z ~ 0.5 with the Grism Lens-Amplified Survey from Space (GLASS)

NASA Astrophysics Data System (ADS)

Vulcani, Benedetta; Vulcani

We present the first study of the spatial distribution of star formation in z ~ 0.5 cluster galaxies. The analysis is based on data taken with the Wide Field Camera 3 as part of the Grism Lens-Amplified Survey from Space (GLASS). We illustrate the methodology by focusing on two clusters (MACS0717.5+3745 and MACS1423.8+2404) with different morphologies (one relaxed and one merging) and use foreground and background galaxies as field control sample. The cluster+field sample consists of 42 galaxies with stellar masses in the range 108-1011 M ⊙, and star formation rates in the range 1-20 M⊙ yr -1. In both environments, Hα is more extended than the rest-frame UV continuum in 60% of the cases, consistent with diffuse star formation and inside out growth. The Hα emission appears more extended in cluster galaxies than in the field, pointing perhaps to ionized gas being stripped and/or star formation being enhanced at large radii. The peak of the Hα emission and that of the continuum are offset by less than 1 kpc. We investigate trends with the hot gas density as traced by the X-ray emission, and with the surface mass density as inferred from gravitational lens models and find no conclusive results. The diversity of morphologies and sizes observed in Hα illustrates the complexity of the environmental process that regulate star formation.

An x-ray study of massive star forming regions with CHANDRA

NASA Astrophysics Data System (ADS)

Wang, Junfeng

2007-08-01

Massive stars are characterized by powerful stellar winds, strong ultraviolet (UV) radiation, and consequently devastating supernovae explosions, which have a profound influence on their natal clouds and galaxy evolution. However, the formation and evolution of massive stars themselves and how their low-mass siblings are affected in the wind-swept and UV-radiation-dominated environment are not well understood. Much of the stellar populations inside of the massive star forming regions (MSFRs) are poorly studied in the optical and IR wavelengths because of observational challenges caused by large distance, high extinction, and heavy contamination from unrelated sources. Although it has long been recognized that X-rays open a new window to sample the young stellar populations residing in the MSFRs, the low angular resolution of previous generation X-ray telescopes has limited the outcome from such studies. The sensitive high spatial resolution X-ray observations enabled by the Chandra X- ray Observatory and the Advanced CCD Imaging Spectrometer (ACIS) have significantly improved our ability to study the X-ray-emitting populations in the MSFRs in the last few years. In this thesis, I analyzed seven high spatial resolution Chandra /ACIS images of two massive star forming complexes, namely the NGC 6357 region hosting the 1 Myr old Pismis 24 cluster (Chapter 3) and the Rosette Complex including the 2 Myr old NGC 2244 cluster immersed in the Rosette Nebula (Chapter 4), embedded clusters in the Rosette Molecular Cloud (RMC; Chapter 5), and a triggered cluster NGC 2237 (Chapter 6). The X-ray sampled stars were studied in great details. The unique power of X-ray selection of young stellar cluster members yielded new knowledge in the stellar populations, the cluster structures, and the star formation histories. The census of cluster members is greatly improved in each region. A large fraction of the X-ray detections have optical or near-infrared (NIR) stellar counterparts (from 2MASS, SIRIUS and FLAMINGOS JHK images), most of which are previously uncatalogued young cluster members. This provides a reliable probe of the rich intermediate-mass and low-mass young stellar populations accompanying the massive OB stars in each region. For example, In the poorly- studied NGC 6357 region, our study increased the number of known members from optical study by a factor of ~40. As a result, normal initial mass functions (IMFs) for NGC 6357 and NGC 2244 were found, inconsistent with the top-heavy IMFs suspected in previous optical studies. The observed X-ray luminosity functions (XLFs) in NGC 6357 and NGC 2244 are compared to the Orion Nebula Cluster XLF, yielding the first estimate of NGC 6357's total cluster population, a few times the known Orion population. For NGC 2244, a total population of ~2000 X-ray-emitting stars is derived, consistent with previous estimate from IR studies. The morphologies and spatial structures of the clusters are investigated with absorption-stratified stellar surface density maps. Small-scale substructures superposed on the spherical clusters are found in NGC 6357 and NGC 2244. Both of their radial stellar density profiles show a power-law cusp around the density peak surrounded by an isothermal sphere. In NGC 2244, the spatial distribution of X-ray stars is strongly concentrated around the central O5 star, HD 46150. The other O4 star HD 46223 has few companions. The X-ray sources in the RMC show three distinctive structures and substructures within them, which include previously known embedded IR clusters and a new unobscured cluster (RMC A). We do not find clear evidence of sequentially triggered formation. The concentration of X-ray identified young stars implies that [Special characters omitted.] 35% of stars could be in a distributed population throughout the RMC region and clustered star formation is the dominant mode in this cloud. The NGC 2237 cluster, similar to RMC A, may have formed from collapse of pre-existing massive molecular clumps accompanying the formation of the NGC 2244 cluster. The spatial distribution of the NIR counterparts to X-ray stars in the optical dark region northwest of NGC 2237 show little evidence of triggered star formation in the pillar objects. The observed inner disk fraction in the MSFRs as indicated by K-band excess appears lower than the IR-excess disk fractions found in the nearby low-mass star formation regions of similar age. An overall K -excess disk frequency of ~6% for X-ray selected stars in the intermediate- to high-mass range in the NGC 6357 region (Chapter 3), and ~10% for stars with mass M [Special characters omitted.] in NGC 2244 (Chapter 4) are derived, which indicates that the inner disks around higher-mass stars evolve more rapidly. The X-ray stars in these regions provide an important new sample for studies of intermediate-mass PMS stars that are not accreting, in addition to the accreting HAeBe stars. The low K -excess disk frequency for X-ray selected stars in the solar mass range in NGC 2244 is intriguing, which may be attributed to different sensitivities to disk materials, selection effects between X-ray samples and IR samples and/or faster disk dissipation due to photoevaporation in the MSFRs. X-ray properties of stars across the mass spectrum are presented. Diversities in the X-ray spectra of O stars are seen, both soft X-ray emission consistent with the microshocks in stellar winds and hard X-ray components signifying magnetically confined winds or close binarity. X-ray luminosities for a sample of stars earlier than B4 in NGC 6357, NGC 2244, and M 17 confirm the long- standing log( L x /L bol ) ~ -7 relation, although larger scatter is seen among the L x /L bol ratios of B-type stars. Low-mass PMS stars frequently show X-ray flaring, including intense flares with luminosities above L x >= 10 32 ergs s - 1 . Diffuse X-ray emission is present in the NGC 6357 region and in the NGC 2244 cluster. The derived luminosity of diffuse emission in NGC 6357 is consistent with the integrated emission from the unresolved PMS stars. The NGC 2244 diffuse emission is likely originated from the wind termination shocks, and hence is truly diffuse in nature. In summary, Chandra X-ray observations offer multifaceted approaches to study the young stellar clusters in MSFRs in depth. Future perspectives with the Spitzer Space Telescope mid-IR observations for a systematic measurement of disk frequencies in X-ray sampled massive clusters and X-ray observations of the earliest phases of massive star formation are discussed.

Near-infrared study of new embedded clusters in the Carina complex

NASA Astrophysics Data System (ADS)

Oliveira, R. A. P.; Bica, E.; Bonatto, C.

2018-05-01

We analyse the nature of a sample of stellar overdensities that we found projected on the Carina complex. This study is based on the Two Micron All Sky Survey photometry and involves the photometry decontamination of field stars, elaboration of intrinsic colour-magnitude diagrams [CMDs; J × (J - Ks)], colour-colour diagrams (J - H) × (H - Ks), and radial density profiles, in order to determine the structure and the main astrophysical parameters of the best candidates. The verification of an overdensity as an embedded cluster requires a CMD consistent with a PMS content and MS stars, if any. From these results, we are able to verify if they are, in fact, embedded clusters. The results were, in general, rewarding: in a sample of 101 overdensities, the analysis provided 15 candidates, of which three were previously catalogued as clusters (CCCP-Cl 16, Treasure Chest, and FSR 1555), and the 12 remaining are discoveries that provided significant results, with ages not above 4.5 Myr and distances compatible with the studied complex. The resulting values for the differential reddening of most candidates were relatively high, confirming that these clusters are still (partially or fully) embedded in the surrounding gas and dust, as a rule within a shell. Histograms with the distribution of the masses, ages, and distances were also produced, to give an overview of the results. We conclude that all the 12 newly found embedded clusters are related to the Carina complex.

Results from the APOGEE IN-SYNC Orion: parameters and radial velocities for thousands of young stars in the Orion Complex.

NASA Astrophysics Data System (ADS)

Da Rio, Nicola; SDSS Apogee IN-SYNC ancillary program Team

2015-01-01

I will present the results of our characterization of the dynamical status of the young stellar population in the Orion A star forming region. This is based on radial velocity measurements obtained within the SDSS-III Apogee IN-SYNC Orion Survey, which obtained high-resolution spectroscopy of ~3000 objects in the region, from the dense Orion Nebula Cluster - the prototypical nearby region of active massive star formation - to the low-density environments of the L1641 region. We find evidence for kinematic subclustering along the star forming filament, where the stellar component remains kinematically associated to the gas; in the ONC we find that the stellar population is supervirial and currently expanding. We rule out the existence of a controversial candidate foreground cluster to the south of the ONC. These results, complemented with an analysis of the spatial structure of the population, enables critical tests of theories that describe the formation and early evolution of Orion and young clusters in general.

The M 16 molecular complex under the influence of NGC 6611. Herschel's perspective of the heating effect on the Eagle Nebula

NASA Astrophysics Data System (ADS)

Hill, T.; Motte, F.; Didelon, P.; White, G. J.; Marston, A. P.; Nguyên Luong, Q.; Bontemps, S.; André, Ph.; Schneider, N.; Hennemann, M.; Sauvage, M.; Di Francesco, J.; Minier, V.; Anderson, L. D.; Bernard, J. P.; Elia, D.; Griffin, M. J.; Li, J. Z.; Peretto, N.; Pezzuto, S.; Polychroni, D.; Roussel, H.; Rygl, K. L. J.; Schisano, E.; Sousbie, T.; Testi, L.; Thompson, D. Ward; Zavagno, A.

2012-06-01

We present Herschel images from the HOBYS key program of the Eagle Nebula (M 16) in the far-infrared and sub-millimetre, using the PACS and SPIRE cameras at 70 μm, 160 μm, 250 μm, 350 μm, 500 μm. M 16, home to the Pillars of Creation, is largely under the influence of the nearby NGC 6611 high-mass star cluster. The Herschel images reveal a clear dust temperature gradient running away from the centre of the cavity carved by the OB cluster. We investigate the heating effect of NGC 6611 on the entire M 16 star-forming complex seen by Herschel including the diffuse cloud environment and the dense filamentary structures identified in this region. In addition, we interpret the three-dimensional geometry of M 16 with respect to the nebula, its surrounding environment, and the NGC 6611 cavity. The dust temperature and column density maps reveal a prominent eastern filament running north-south and away from the high-mass star-forming central region and the NGC 6611 cluster, as well as a northern filament which extends around and away from the cluster. The dust temperature in each of these filaments decreases with increasing distance from the NGC 6611 cluster, indicating a heating penetration depth of ~10 pc in each direction in 3-6 × 1022 cm-2 column density filaments. We show that in high-mass star-forming regions OB clusters impact the temperature of future star-forming sites, modifying the initialconditions for collapse and effecting the evolutionary criteria of protostars developed from spectral energy distributions. Possible scenarios for the origin of the morphology seen in this region are discussed, including a western equivalent to the eastern filament, which was destroyed by the creation of the OB cluster and its subsequent winds and radiation. Herschel is a ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.org

KEY ISSUES REVIEW: Insights from simulations of star formation

NASA Astrophysics Data System (ADS)

Larson, Richard B.

2007-03-01

Although the basic physics of star formation is classical, numerical simulations have yielded essential insights into how stars form. They show that star formation is a highly nonuniform runaway process characterized by the emergence of nearly singular peaks in density, followed by the accretional growth of embryo stars that form at these density peaks. Circumstellar discs often form from the gas being accreted by the forming stars, and accretion from these discs may be episodic, driven by gravitational instabilities or by protostellar interactions. Star-forming clouds typically develop filamentary structures, which may, along with the thermal physics, play an important role in the origin of stellar masses because of the sensitivity of filament fragmentation to temperature variations. Simulations of the formation of star clusters show that the most massive stars form by continuing accretion in the dense cluster cores, and this again is a runaway process that couples star formation and cluster formation. Star-forming clouds also tend to develop hierarchical structures, and smaller groups of forming objects tend to merge into progressively larger ones, a generic feature of self-gravitating systems that is common to star formation and galaxy formation. Because of the large range of scales and the complex dynamics involved, analytic models cannot adequately describe many aspects of star formation, and detailed numerical simulations are needed to advance our understanding of the subject. 'The purpose of computing is insight, not numbers.' Richard W Hamming, in Numerical Methods for Scientists and Engineers (1962) 'There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.' William Shakespeare, in Hamlet, Prince of Denmark (1604)

Globular cluster formation with multiple stellar populations: self-enrichment in fractal massive molecular clouds

NASA Astrophysics Data System (ADS)

Bekki, Kenji

2017-08-01

Internal chemical abundance spreads are one of fundamental properties of globular clusters (GCs) in the Galaxy. In order to understand the origin of such abundance spreads, we numerically investigate GC formation from massive molecular clouds (MCs) with fractal structures using our new hydrodynamical simulations with star formation and feedback effects of core-collapse supernovae (SNe) and asymptotic giant branch (AGB) stars. We particularly investigate star formation from gas chemically contaminated by SNe and AGB stars ('self-enrichment') in forming GCs within MCs with different initial conditions and environments. The principal results are as follows. GCs with multiple generations of stars can be formed from merging of hierarchical star cluster complexes that are developed from high-density regions of fractal MCs. Feedback effects of SNe and AGB stars can control the formation efficiencies of stars formed from original gas of MCs and from gas ejected from AGB stars. The simulated GCs have strong radial gradients of helium abundances within the central 3 pc. The original MC masses need to be as large as 107 M⊙ for a canonical initial stellar mass function (IMF) so that the final masses of stars formed from AGB ejecta can be ˜105 M⊙. Since star formation from AGB ejecta is rather prolonged (˜108 yr), their formation can be strongly suppressed by SNe of the stars themselves. This result implies that the so-called mass budget problem is much more severe than ever thought in the self-enrichment scenario of GC formation and thus that IMF for the second generation of stars should be 'top-light'.

Massive runaway stars in the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.; Kroupa, P.; Pflamm-Altenburg, J.

2010-09-01

The origin of massive field stars in the Large Magellanic Cloud (LMC) has long been an enigma. The recent measurements of large offsets (˜ 100 km s-1) between the heliocentric radial velocities of some very massive (O2-type) field stars and the systemic LMC velocity provides a possible explanation of this enigma and suggests that the field stars are runaway stars ejected from their birthplaces at the very beginning of their parent cluster's dynamical evolution. A straightforward way to prove this explanation is to measure the proper motions of the field stars and to show that they are moving away from one of the nearby star clusters or OB associations. This approach is, however, complicated by the long distance to the LMC, which makes accurate proper motion measurements difficult. We used an alternative approach for solving the problem (first applied for Galactic field stars), based on the search for bow shocks produced by runaway stars. The geometry of detected bow shocks would allow us to infer the direction of stellar motion, thereby determining their possible parent clusters. In this paper we present the results of a search for bow shocks around six massive field stars that have been proposed as candidate runaway stars. Using archival Spitzer Space Telescope data, we found a bow shock associated with one of our programme stars, the O2 V((f*)) star BI 237, which is the first-ever detection of bow shocks in the LMC. Orientation of the bow shock suggests that BI 237 was ejected from the OB association LH 82 (located at ≃ 120 pc in projection from the star). A by-product of our search is the detection of bow shocks generated by four OB stars in the field of the LMC and an arc-like structure attached to the candidate luminous blue variable R81 (HD 269128). The geometry of two of these bow shocks is consistent with the possibility that their associated stars were ejected from the 30 Doradus star-forming complex. We discuss implications of our findings for the problem of the origin of runaway stars and the early dynamical evolution of star clusters.

A Spectroscopic Analysis of the Galactic Globular Cluster NGC 6273 (M19)

NASA Astrophysics Data System (ADS)

Johnson, Christian I.; Rich, R. Michael; Pilachowski, Catherine A.; Caldwell, Nelson; Mateo, Mario; Bailey, John I., III; Crane, Jeffrey D.

2015-08-01

A combined effort utilizing spectroscopy and photometry has revealed the existence of a new globular cluster class. These “anomalous” clusters, which we refer to as “iron-complex” clusters, are differentiated from normal clusters by exhibiting large (≳0.10 dex) intrinsic metallicity dispersions, complex sub-giant branches, and correlated [Fe/H] and s-process enhancements. In order to further investigate this phenomenon, we have measured radial velocities and chemical abundances for red giant branch stars in the massive, but scarcely studied, globular cluster NGC 6273. The velocities and abundances were determined using high resolution (R ˜ 27,000) spectra obtained with the Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph on the Magellan-Clay 6.5 m telescope at Las Campanas Observatory. We find that NGC 6273 has an average heliocentric radial velocity of +144.49 km s-1 (σ = 9.64 km s-1) and an extended metallicity distribution ([Fe/H] = -1.80 to -1.30) composed of at least two distinct stellar populations. Although the two dominant populations have similar [Na/Fe], [Al/Fe], and [α/Fe] abundance patterns, the more metal-rich stars exhibit significant [La/Fe] enhancements. The [La/Eu] data indicate that the increase in [La/Fe] is due to almost pure s-process enrichment. A third more metal-rich population with low [X/Fe] ratios may also be present. Therefore, NGC 6273 joins clusters such as ω Centauri, M2, M22, and NGC 5286 as a new class of iron-complex clusters exhibiting complicated star formation histories. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

The Grism Lens-Amplified Survey from Space (GLASS). V. Extent and Spatial Distribution of Star Formation in z ~ 0.5 Cluster Galaxies

NASA Astrophysics Data System (ADS)

Vulcani, Benedetta; Treu, Tommaso; Schmidt, Kasper B.; Poggianti, Bianca M.; Dressler, Alan; Fontana, Adriano; Bradač, Marusa; Brammer, Gabriel B.; Hoag, Austin; Huang, Kuan-Han; Malkan, Matthew; Pentericci, Laura; Trenti, Michele; von der Linden, Anja; Abramson, Louis; He, Julie; Morris, Glenn

2015-12-01

We present the first study of the spatial distribution of star formation in z ˜ 0.5 cluster galaxies. The analysis is based on data taken with the Wide Field Camera 3 as part of the Grism Lens-Amplified Survey from Space (GLASS). We illustrate the methodology by focusing on two clusters (MACS 0717.5+3745 and MACS 1423.8+2404) with different morphologies (one relaxed and one merging) and use foreground and background galaxies as a field control sample. The cluster+field sample consists of 42 galaxies with stellar masses in the range 108-1011 M⊙ and star formation rates in the range 1-20 M⊙ yr-1. Both in clusters and in the field, Hα is more extended than the rest-frame UV continuum in 60% of the cases, consistent with diffuse star formation and inside-out growth. In ˜20% of the cases, the Hα emission appears more extended in cluster galaxies than in the field, pointing perhaps to ionized gas being stripped and/or star formation being enhanced at large radii. The peak of the Hα emission and that of the continuum are offset by less than 1 kpc. We investigate trends with the hot gas density as traced by the X-ray emission, and with the surface mass density as inferred from gravitational lens models, and find no conclusive results. The diversity of morphologies and sizes observed in Hα illustrates the complexity of the environmental processes that regulate star formation. Upcoming analysis of the full GLASS data set will increase our sample size by almost an order of magnitude, verifying and strengthening the inference from this initial data set.

Hubble Revisits a Globular Cluster’s Age

NASA Image and Video Library

2014-08-13

This new NASA/ESA Hubble Space Telescope image shows the globular cluster IC 4499. Globular clusters are big balls of old stars that orbit around their host galaxy. It has long been believed that all the stars within a globular cluster form at the about same time, a property which can be used to determine the cluster's age. For more massive globulars however, detailed observations have shown that this is not entirely true — there is evidence that they instead consist of multiple populations of stars born at different times. One of the driving forces behind this behavior is thought to be gravity: more massive globulars manage to grab more gas and dust, which can then be transformed into new stars. IC 4499 is a somewhat special case. Its mass lies somewhere between low-mass globulars, which show a single generation build-up, and the more complex and massive globulars which can contain more than one generation of stars. By studying objects like IC 4499 astronomers can therefore explore how mass affects a cluster's contents. Astronomers found no sign of multiple generations of stars in IC 4499 — supporting the idea that less massive clusters in general only consist of a single stellar generation. Hubble observations of IC 4499 have also helped to pinpoint the cluster's age: observations of this cluster from the 1990s suggested a puzzlingly young age when compared to other globular clusters within the Milky Way. However, since those first estimates new Hubble data have been obtained and it has been found to be much more likely that IC 4499 is actually roughly the same age as other Milky Way clusters at approximately 12 billion years old. Credit: ESA and NASA NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

Spatial and kinematic structure of Monoceros star-forming region

NASA Astrophysics Data System (ADS)

Costado, M. T.; Alfaro, E. J.

2018-05-01

The principal aim of this work is to study the velocity field in the Monoceros star-forming region using the radial velocity data available in the literature, as well as astrometric data from the Gaia first release. This region is a large star-forming complex formed by two associations named Monoceros OB1 and OB2. We have collected radial velocity data for more than 400 stars in the area of 8 × 12 deg2 and distance for more than 200 objects. We apply a clustering analysis in the subspace of the phase space formed by angular coordinates and radial velocity or distance data using the Spectrum of Kinematic Grouping methodology. We found four and three spatial groupings in radial velocity and distance variables, respectively, corresponding to the Local arm, the central clusters forming the associations and the Perseus arm, respectively.

THE FORMATION OF SECONDARY STELLAR GENERATIONS IN MASSIVE YOUNG STAR CLUSTERS FROM RAPIDLY COOLING SHOCKED STELLAR WINDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wünsch, R.; Palouš, J.; Ehlerová, S.

We study a model of rapidly cooling shocked stellar winds in young massive clusters and estimate the circumstances under which secondary star formation, out of the reinserted winds from a first stellar generation (1G), is possible. We have used two implementations of the model: a highly idealized, computationally inexpensive, spherically symmetric semi-analytic model, and a complex, three-dimensional radiation-hydrodynamic, simulation; they are in a good mutual agreement. The results confirm our previous findings that, in a cluster with 1G mass 10{sup 7} M {sub ⊙} and half-mass–radius 2.38 pc, the shocked stellar winds become thermally unstable, collapse into dense gaseous structuresmore » that partially accumulate inside the cluster, self-shield against ionizing stellar radiation, and form the second generation (2G) of stars. We have used the semi-analytic model to explore a subset of the parameter space covering a wide range of the observationally poorly constrained parameters: the heating efficiency, η {sub he}, and the mass loading, η {sub ml}. The results show that the fraction of the 1G stellar winds accumulating inside the cluster can be larger than 50% if η {sub he} ≲ 10%, which is suggested by the observations. Furthermore, for low η {sub he}, the model provides a self-consistent mechanism predicting 2G stars forming only in the central zones of the cluster. Finally, we have calculated the accumulated warm gas emission in the H30 α recombination line, analyzed its velocity profile, and estimated its intensity for super star clusters in interacting galaxies NGC4038/9 (Antennae) showing that the warm gas should be detectable with ALMA.« less

Near infrared photometric and optical spectroscopic study of 22 low mass star clusters embedded in nebulae

NASA Astrophysics Data System (ADS)

Soares, J. B.; Bica, E.; Ahumada, A. V.; Clariá, J. J.

2008-02-01

Aims:Among the star clusters in the Galaxy, those embedded in nebulae represent the youngest group, which has only recently been explored. The analysis of a sample of 22 candidate embedded stellar systems in reflection nebulae and/or HII environments is presented. Methods: We employed optical spectroscopic observations of stars in the directions of the clusters carried out at CASLEO (Argentina) together with near infrared photometry from the 2MASS catalogue. Our analysis is based on source surface density, colour-colour diagrams and on theoretical pre-main sequence isochrones. We take into account the field star contamination by carrying out a statistical subtraction. Results: The studied objects have the characteristics of low mass systems. We derive their fundamental parameters. Most of the cluster ages are younger than 2 Myr. The studied embedded stellar systems in reflection nebulae and/or HII region complexes do not have stars of spectral types earlier than B. The total stellar masses locked in the clusters are in the range 20-220 M⊙. They are found to be gravitationally unstable and are expected to dissolve in a timescale of a few Myr. Based on observations made at Complejo Astronómico El Leoncito, which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba and San Juan, Argentina.

Globular cluster chemistry in fast-rotating dwarf stars belonging to intermediate-age open clusters

NASA Astrophysics Data System (ADS)

Pancino, Elena

2018-06-01

The peculiar chemistry observed in multiple populations of Galactic globular clusters is not generally found in other systems such as dwarf galaxies and open clusters, and no model can currently fully explain it. Exploring the boundaries of the multiple-population phenomenon and the variation of its extent in the space of cluster mass, age, metallicity, and compactness has proven to be a fruitful line of investigation. In the framework of a larger project to search for multiple populations in open clusters that is based on literature and survey data, I found peculiar chemical abundance patterns in a sample of intermediate-age open clusters with publicly available data. More specifically, fast-rotating dwarf stars (v sin i ≥ 50 km s-1) that belong to four clusters (Pleiades, Ursa Major, Come Berenices, and Hyades) display a bimodality in either [Na/Fe] or [O/Fe], or both, with the low-Na and high-O peak more populated than the high-Na and low-O peak. Additionally, two clusters show a Na-O anti-correlation in the fast-rotating stars, and one cluster shows a large [Mg/Fe] variation in stars with high [Na/Fe], reaching the extreme Mg depletion observed in NGC 2808. Even considering that the sample sizes are small, these patterns call for attention in the light of a possible connection with the multiple population phenomenon of globular clusters. The specific chemistry observed in these fast-rotating dwarf stars is thought to be produced by a complex interplay of different diffusion and mixing mechanisms, such as rotational mixing and mass loss, which in turn are influenced by metallicity, binarity, mass, age, variability, and so on. However, with the sample in hand, it was not possible to identify which stellar parameters cause the observed Na and O bimodality and Na-O anti-correlation. This suggests that other stellar properties might be important in addition to stellar rotation. Stellar binarity might influence the rotational properties and enhance rotational mixing and mass loss of stars in a dense environment like that of clusters (especially globulars). In conclusion, rotation and binarity appear as a promising research avenue for better understanding multiple stellar populations in globular clusters; this is certainly worth exploring further.

THE PREVALENCE AND IMPACT OF WOLF–RAYET STARS IN EMERGING MASSIVE STAR CLUSTERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sokal, Kimberly R.; Johnson, Kelsey E.; Indebetouw, Rémy

We investigate Wolf–Rayet (WR) stars as a source of feedback contributing to the removal of natal material in the early evolution of massive star clusters. Despite previous work suggesting that massive star clusters clear out their natal material before the massive stars evolve into the WR phase, WR stars have been detected in several emerging massive star clusters. These detections suggest that the timescale for clusters to emerge can be at least as long as the time required to produce WR stars (a few million years), and could also indicate that WR stars may be providing the tipping point inmore » the combined feedback processes that drive a massive star cluster to emerge. We explore the potential overlap between the emerging phase and the WR phase with an observational survey to search for WR stars in emerging massive star clusters hosting WR stars. We select candidate emerging massive star clusters from known radio continuum sources with thermal emission and obtain optical spectra with the 4 m Mayall Telescope at Kitt Peak National Observatory and the 6.5 m MMT.{sup 4} We identify 21 sources with significantly detected WR signatures, which we term “emerging WR clusters.” WR features are detected in ∼50% of the radio-selected sample, and thus we find that WR stars are commonly present in currently emerging massive star clusters. The observed extinctions and ages suggest that clusters without WR detections remain embedded for longer periods of time, and may indicate that WR stars can aid, and therefore accelerate, the emergence process.« less

A new method to unveil embedded stellar clusters

NASA Astrophysics Data System (ADS)

Lombardi, Marco; Lada, Charles J.; Alves, João

2017-11-01

In this paper we present a novel method to identify and characterize stellar clusters deeply embedded in a dark molecular cloud. The method is based on measuring stellar surface density in wide-field infrared images using star counting techniques. It takes advantage of the differing H-band luminosity functions (HLFs) of field stars and young stellar populations and is able to statistically associate each star in an image as a member of either the background stellar population or a young stellar population projected on or near the cloud. Moreover, the technique corrects for the effects of differential extinction toward each individual star. We have tested this method against simulations as well as observations. In particular, we have applied the method to 2MASS point sources observed in the Orion A and B complexes, and the results obtained compare very well with those obtained from deep Spitzer and Chandra observations where presence of infrared excess or X-ray emission directly determines membership status for every star. Additionally, our method also identifies unobscured clusters and a low resolution version of the Orion stellar surface density map shows clearly the relatively unobscured and diffuse OB 1a and 1b sub-groups and provides useful insights on their spatial distribution.

Age gradients in the stellar populations of massive star forming regions based on a new stellar chronometer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Getman, Konstantin V.; Feigelson, Eric D.; Kuhn, Michael A.

2014-06-01

A major impediment to understanding star formation in massive star-forming regions (MSFRs) is the absence of a reliable stellar chronometer to unravel their complex star formation histories. We present a new estimation of stellar ages using a new method that employs near-infrared (NIR) and X-ray photometry, Age {sub JX} . Stellar masses are derived from X-ray luminosities using the L{sub X} -M relation from the Taurus cloud. J-band luminosities are compared to mass-dependent pre-main-sequence (PMS) evolutionary models to estimate ages. Age {sub JX} is sensitive to a wide range of evolutionary stages, from disk-bearing stars embedded in a cloud tomore » widely dispersed older PMS stars. The Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) project characterizes 20 OB-dominated MSFRs using X-ray, mid-infrared, and NIR catalogs. The Age {sub JX} method has been applied to 5525 out of 31,784 MYStIX Probable Complex Members. We provide a homogeneous set of median ages for over 100 subclusters in 15 MSFRs; median subcluster ages range between 0.5 Myr and 5 Myr. The important science result is the discovery of age gradients across MYStIX regions. The wide MSFR age distribution appears as spatially segregated structures with different ages. The Age {sub JX} ages are youngest in obscured locations in molecular clouds, intermediate in revealed stellar clusters, and oldest in distributed populations. The NIR color index J – H, a surrogate measure of extinction, can serve as an approximate age predictor for young embedded clusters.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

McDonald, M.; Stalder, B.; Bayliss, M.

In this paper, we present a multiwavelength study of the 90 brightest cluster galaxies (BCGs) in a sample of galaxy clusters selected via the Sunyaev Zel'dovich effect by the South Pole Telescope, utilizing data from various ground- and space-based facilities. We infer the star-formation rate (SFR) for the BCG in each cluster—based on the UV and IR continuum luminosity, as well as the [O ii]λλ3726,3729 emission line luminosity in cases where spectroscopy is available—and find seven systems with SFR > 100 M ⊙ yr -1. We find that the BCG SFR exceeds 10 M ⊙ yr -1 in 31 of 90 (34%) cases at 0.25 < z < 1.25, compared to ~1%–5% at z ~ 0 from the literature. At z ≳ 1, this fraction increases tomore » $${92}_{-31}^{+6}$$%, implying a steady decrease in the BCG SFR over the past ~9 Gyr. At low-z, we find that the specific SFR in BCGs is declining more slowly with time than for field or cluster galaxies, which is most likely due to the replenishing fuel from the cooling ICM in relaxed, cool core clusters. At z ≳ 0.6, the correlation between the cluster central entropy and BCG star formation—which is well established at z ~ 0—is not present. Instead, we find that the most star-forming BCGs at high-z are found in the cores of dynamically unrelaxed clusters. We use data from the Hubble Space Telescope to investigate the rest-frame near-UV morphology of a subsample of the most star-forming BCGs, and find complex, highly asymmetric UV morphologies on scales as large as ~50–60 kpc. Finally, the high fraction of star-forming BCGs hosted in unrelaxed, non-cool core clusters at early times suggests that the dominant mode of fueling star formation in BCGs may have recently transitioned from galaxy–galaxy interactions to ICM cooling.« less

An Overview of the 3C-STAR project

NASA Astrophysics Data System (ADS)

Zhang, Y.

2009-04-01

Over the past three decades, city clusters have played a leading role in the economic growth of China, owing to their collective economic capacity and interdependency. However, pollution prevention lags behind the economic boom, led to a general decline in air quality in city clusters. As a result, industrial emissions and traffic exhausts together contribute to high levels of ozone (O3) and fine particulate matter (PM2.5) pollution problems ranging from urban to regional scale. Such high levels of both primary and secondary airborne pollutants lead to the development of a (perhaps typically Chinese) "air pollution complex" concept. Air pollution complex is particularly true and significant in Beijing-Tianjin area, Pearl River Delta (PRD) and Yangtze River Delta. The concurrent high concentrations of O3 and PM2.5 in PRD as well as in other China city clusters have led to rather unique pollution characteristics due to interactions between primary emissions and photochemical processes, between gaseous compounds and aerosol phase species, and between local and regional scale processes. The knowledge and experience needed to find solutions to the unique pollution complex in China are still lacking. Starting from 2007, we launch a major project "Synthesized Prevention Techniques for Air Pollution Complex and Integrated Demonstration in Key City-Cluster Region" (3C-STAR) to address those problems scientifically and technically. The purpose of the project is to build up the capacity of regional air pollution control and to establish regional coordination mechanism for joint implementation of pollution control. The project includes a number of key components technically: regional air quality monitoring network and super-sites, regional dynamic emission inventory of multi-pollutants, regional ensemble air quality forecasting model system, and regional management system supported by decision making platform. The 3C-STAR project selected PRD as a core area to have technical demonstration, and thus provide opportunities as well as challenges for PRD to improve its regional air quality. An integrated field measurement campaign 3C-STAR2008 was organized during October 15-November 19, 2008, including 3-D regional air quality monitoring network, two super-sites, and in-site meteorological and air quality forecasting. With the efforts of more than 100 scientists and students from 12 research institutes, the 3C-STAR2008 was conducted with great success. A great amount of data with rigorous QA/QC procedures has been obtained and data analysis is underway. In this talk, an overview of the 3C-STAR project will be presented, together with major findings from previous PRD campaigns (PRD2004 and PRD2006).

VizieR Online Data Catalog: Structure of young stellar clusters. II. (Kuhn+, 2015)

NASA Astrophysics Data System (ADS)

Kuhn, M. A.; Getman, K. V.; Feigelson, E. D.

2015-07-01

We investigate the intrinsic stellar populations (estimated total numbers of OB and pre-main-sequence stars down to 0.1Mȯ) that are present in 17 massive star-forming regions (MSFRs) surveyed by the MYStIX project. The study is based on the catalog of >31000 MYStIX Probable Complex Members with both disk-bearing and disk-free populations, compensating for extinction, nebulosity, and crowding effects. Correction for observational sensitivities is made using the X-ray luminosity function and the near-infrared initial mass function --a correction that is often not made by infrared surveys of young stars. The resulting maps of the projected structure of the young stellar populations, in units of intrinsic stellar surface density, allow direct comparison between different regions. Several regions have multiple dense clumps, similar in size and density to the Orion Nebula Cluster. The highest projected density of ~34000 stars/pc2 is found in the core of the RCW 38 cluster. Histograms of surface density show different ranges of values in different regions, supporting the conclusion of Bressert et al. (B10; 2010MNRAS.409L..54B) that no universal surface-density threshold can distinguish between clustered and distributed star formation. However, a large component of the young stellar population of MSFRs resides in dense environments of 200-10000 stars/pc2 (including within the nearby Orion molecular clouds), and we find that there is no evidence for the B10 conclusion that such dense regions form an extreme "tail" of the distribution. Tables of intrinsic populations for these regions are used in our companion study of young cluster properties and evolution. (3 data files).

A clustered origin for isolated massive stars

NASA Astrophysics Data System (ADS)

Lucas, William E.; Rybak, Matus; Bonnell, Ian A.; Gieles, Mark

2018-03-01

High-mass stars are commonly found in stellar clusters promoting the idea that their formation occurs due to the physical processes linked with a young stellar cluster. It has recently been reported that isolated high-mass stars are present in the Large Magellanic Cloud. Due to their low velocities, it has been argued that these are high-mass stars which formed without a surrounding stellar cluster. In this paper, we present an alternative explanation for the origin of these stars in which they formed in a cluster environment but are subsequently dispersed into the field as their natal cluster is tidally disrupted in a merger with a higher mass cluster. They escape the merged cluster with relatively low velocities typical of the cluster interaction and thus of the larger scale velocity dispersion, similarly to the observed stars. N-body simulations of cluster mergers predict a sizeable population of low-velocity (≤20 km s-1), high-mass stars at distances of >20 pc from the cluster. High-mass clusters in which gas poor mergers are frequent would be expected to commonly have haloes of young stars, including high-mass stars, which were actually formed in a cluster environment.

Star Clusters within FIRE

NASA Astrophysics Data System (ADS)

Perez, Adrianna; Moreno, Jorge; Naiman, Jill; Ramirez-Ruiz, Enrico; Hopkins, Philip F.

2017-01-01

In this work, we analyze the environments surrounding star clusters of simulated merging galaxies. Our framework employs Feedback In Realistic Environments (FIRE) model (Hopkins et al., 2014). The FIRE project is a high resolution cosmological simulation that resolves star forming regions and incorporates stellar feedback in a physically realistic way. The project focuses on analyzing the properties of the star clusters formed in merging galaxies. The locations of these star clusters are identified with astrodendro.py, a publicly available dendrogram algorithm. Once star cluster properties are extracted, they will be used to create a sub-grid (smaller than the resolution scale of FIRE) of gas confinement in these clusters. Then, we can examine how the star clusters interact with these available gas reservoirs (either by accreting this mass or blowing it out via feedback), which will determine many properties of the cluster (star formation history, compact object accretion, etc). These simulations will further our understanding of star formation within stellar clusters during galaxy evolution. In the future, we aim to enhance sub-grid prescriptions for feedback specific to processes within star clusters; such as, interaction with stellar winds and gas accretion onto black holes and neutron stars.

A critical assessment of models for the origin of multiple populations in globular clusters

NASA Astrophysics Data System (ADS)

Bastian, Nate

2017-03-01

A number of scenarios have been put forward to explain the origin of the chemical anomalies (and resulting complex colour-magnitude diagrams) observed in globular clusters (GCs), namely the AGB, Fast Rotating Massive Star, Very Massive Star, and Early Disc Accretion scenarios. We compare the predictions of these scenarios with a range of observations (including young massive clusters (YMCs), chemical patterns, and GC population properties) and find that all models are inconsistent with observations. In particular, YMCs do not show evidence for multiple epochs of star-formation and appear to be gas free by an age of ~ 3 Myr. Also, the chemical patterns displayed in GCs vary from one to the next in such a way that cannot be reproduced by standard nucleosynthetic yields. Finally, we show that the ``mass budget problem'' for the scenarios cannot be solved by invoking heavy cluster mass loss (i.e. that clusters were 10-100 times more massive at birth) as this solution makes basic predictions about the GC population that are inconsistent with observations. We conclude that none of the proposed scenarios can explain the multiple population phenomenon, hence alternative theories are needed.

The Star Cluster System in the Local Group Starburst Galaxy IC 10

NASA Astrophysics Data System (ADS)

Lim, Sungsoon; Lee, Myung Gyoon

2015-05-01

We present a survey of star clusters in the halo of IC 10, a starburst galaxy in the Local Group, based on Subaru R-band images and NOAO Local Group Survey UBVRI images. We find five new star clusters. All of these star clusters are located far from the center of IC 10, while previously known star clusters are mostly located in the main body. Interestingly, the distribution of these star clusters shows an asymmetrical structure elongated along the east and southwest directions. We derive UBVRI photometry of 66 star clusters, including these new star clusters, as well as previously known star clusters. Ages of the star clusters are estimated from a comparison of their UBVRI spectral energy distribution with the simple stellar population models. We find that the star clusters in the halo are all older than 1 Gyr, while those in the main body have various ages, from very young (several Myr) to old (\\gt 1 Gyr). The young clusters (\\lt 10 Myr) are mostly located in the Hα emission regions and are concentrated on a small region at 2\\prime\\prime in the southeast direction from the galaxy center, while the old clusters are distributed in a wider area than the disk. Intermediate-age clusters (∼100 Myr) are found in two groups. One is close to the location of the young clusters and the other is at ∼ 4\\prime\\prime from the location of the young clusters. The latter may be related to past mergers or tidal interaction.

Ultraviolet studies of O and B stars in the LMC cluster NGC 2100, the SMC cluster NGC 330 and the Galactic cluster NGC 6530

NASA Technical Reports Server (NTRS)

Boehm-Vitense, E.; Hodge, P.

1984-01-01

High-resolution and low-resolution IUE spectra of O and B stars in the LMC cluster NGC 2100, the SMC cluster NGC 330, and the young Galactic cluster NGC 6530 are investigated. Temperatures and luminosities are determined. In the LMC and SMC clusters, the most luminous stars are evolved stars on the horizontal supergiant branch, while in NGC 6530 the stars are all still on the main sequence. Extinction laws were determined. They confirm the known differences between LMC and Galactic extinctions. No mass loss was detected for the evolved B stars in the LMC and SMC clusters, while the high-luminosity stars in NGC 6530 show P Cygni profiles.

The Young Massive Stellar Cluster Sandage-96 after the Explosion of SN 2004DJ in NGC 2403

NASA Technical Reports Server (NTRS)

Vinko, J.; Sarneczky, K.; Balog, Z.; Immler, S.; Sugerman, B.; Brown, P. J.; Misselt, K.; Szabo, Gy. M.; Klagyivik, P.; Kun, M.;

Using DOUBLE STAR and CLUSTER Synoptic Observations to Test Global MHD Simulations of the Large-scale Topology of the Dayside Merging Region

NASA Astrophysics Data System (ADS)

Berchem, J.; Marchaudon, A.; Bosqued, J.; Escoubet, C. P.; Dunlop, M.; Owen, C. J.; Reme, H.; Balogh, A.; Carr, C.; Fazakerley, A. N.; Cao, J. B.

2005-12-01

Synoptic measurements from the DOUBLE STAR and CLUSTER spacecraft offer a unique opportunity to evaluate global models in simulating the complex topology and dynamics of the dayside merging region. We compare observations from the DOUBLE STAR TC-1 and CLUSTER spacecraft on May 8, 2004 with the predictions from a three-dimensional magnetohydrodynamic (MHD) simulation that uses plasma and magnetic field parameters measured upstream of the bow shock by the WIND spacecraft. Results from the global simulation are consistent with the large-scale features observed by CLUSTER and TC-1. We discuss topological changes and plasma flows at the dayside magnetospheric boundary inferred from the simulation results. The simulation shows that the DOUBLE STAR spacecraft passed through the dawn side merging region as the IMF rotated. In particular, the simulation indicates that at times TC-1 was very close to the merging region. In addition, we found that the bifurcation of the merging region in the simulation results is consistent with predictions by the antiparallel merging model. However, because of the draping of the magnetosheath field lines over the magnetopause, the positions and shape of the merging region differ significantly from those predicted by the model.

A Study of Magnetic CP Stars in Open Clusters and Associations with the 6-m Telescope

NASA Astrophysics Data System (ADS)

Romanyuk, I. I.; Semenko, E. A.; Yakunin, I. A.; Kudryavtsev, D. O.

2017-06-01

The study of magnetic CP stars in groups of different ages allows us to obtain data on the origin and evolution of large-scale magnetic fields. We selected 17 groups for observation with the 6-m telescope. Here we draw first conclusions from the study of the Orion OB1 association. Six new magnetic stars in it are added to those seventeen that had been known earlier, ten more CP stars were suspected to have fields. A complex structure of the magnetic field in the star HD 34736 has been found, which is indicative of its fossil origin.

Seeing Red in NGC 1978, NGC 55, and NGC 3109

NASA Astrophysics Data System (ADS)

Davidge, T. J.

2018-04-01

Spectra of the intermediate-age star cluster NGC 1978 and the dwarf irregular galaxies NGC 55 and NGC 3109 are discussed. The spectra were recorded with the Gemini Multi-object Spectrograph on Gemini South and span the 0.7–1.1 μm wavelength interval. Five slit pointings were observed in NGC 1978, and these are used to examine stochastic effects on the integrated red light from an intermediate-age cluster. The removal of either the brightest M giant or the brightest C star from the co-added spectrum has minor effects on the equivalent withs of the Ca triplet. The most robust signature of C stars in the integrated cluster spectrum at these wavelengths is the CN band head near 7900 Å. The equivalent widths of Ca triplet lines in the NGC 1978 spectrum and in the spectra of individual cluster stars are larger than expected for a scaled-solar abundance system. It is suggested that these stars have a lower than expected surface gravity, which might occur if the stars in NGC 1978 have been subject to extra mixing processes, as suggested by Lederer et al. The near-infrared color profile of NGC 1978 is shown to contain a prominent red cusp in the central 10 arcsec, and the suppression of light from this cusp does not affect the depth of the Ca lines in the integrated spectrum. The NGC 55 spectra run parallel to the major axis, and a gradient is found in the strength of the Ca lines, in the sense that the Ca lines weaken with increasing distance from the disk plane. Comparisons with models suggest that the disk light is dominated by stars with ages 1–2 Gyr, in agreement with star-forming histories (SFHs) obtained from the analysis of color–magnitude diagrams (CMDs). The NGC 55 spectra also sample a large star-forming complex. The age of this complex inferred from comparisons with models is broadly consistent with that estimated from a near-infrared CMD of the same region. The CN band head at 7900 Å in this part of NGC 55 is detected, but this is likely a signature of red supergiants (RSGs) rather than C stars. The NGC 3109 observations sample three different parts of that galaxy but have a low signal-to-noise ratio. Comparisons with models suggest that the light from the NGC 3109 disk at red wavelengths is dominated by RSGs with ages of at most a few tens of Myr, in qualitative agreement with SFHs that are based on photometric measurements.

NEW MEMBERS OF THE SCORPIUS-CENTAURUS COMPLEX AND AGES OF ITS SUB-REGIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Inseok; Zuckerman, B.; Bessell, M. S.

2012-07-15

We have spectroscopically identified {approx}100 G-, K-, and M-type members of the Scorpius-Centaurus complex. To deduce the age of these young stars we compare their Li {lambda}6708 absorption line strengths against those of stars in the TW Hydrae association and {beta} Pictoris moving group. These line strengths indicate that Sco-Cen stars are younger than {beta} Pic stars whose ages of {approx}12 Myr have previously been derived from a kinematic traceback analysis. Our derived age, {approx}10 Myr, for stars in the Lower Centaurus Crux and Upper Centaurus Lupus subgroups of ScoCen is younger than previously published ages based on the movingmore » cluster method and upper main-sequence fitting. The discrepant ages are likely due to an incorrect (or lack of) cross-calibration between model-dependent and model-independent age-dating methods.« less

The interfacial character of antibody paratopes: analysis of antibody-antigen structures.

PubMed

Nguyen, Minh N; Pradhan, Mohan R; Verma, Chandra; Zhong, Pingyu

2017-10-01

In this study, computational methods are applied to investigate the general properties of antigen engaging residues of a paratope from a non-redundant dataset of 403 antibody-antigen complexes to dissect the contribution of hydrogen bonds, hydrophobic, van der Waals contacts and ionic interactions, as well as role of water molecules in the antigen-antibody interface. Consistent with previous reports using smaller datasets, we found that Tyr, Trp, Ser, Asn, Asp, Thr, Arg, Gly, His contribute substantially to the interactions between antibody and antigen. Furthermore, antibody-antigen interactions can be mediated by interfacial waters. However, there is no reported comprehensive analysis for a large number of structured waters that engage in higher ordered structures at the antibody-antigen interface. From our dataset, we have found the presence of interfacial waters in 242 complexes. We present evidence that suggests a compelling role of these interfacial waters in interactions of antibodies with a range of antigens differing in shape complementarity. Finally, we carry out 296 835 pairwise 3D structure comparisons of 771 structures of contact residues of antibodies with their interfacial water molecules from our dataset using CLICK method. A heuristic clustering algorithm is used to obtain unique structural similarities, and found to separate into 368 different clusters. These clusters are used to identify structural motifs of contact residues of antibodies for epitope binding. This clustering database of contact residues is freely accessible at http://mspc.bii.a-star.edu.sg/minhn/pclick.html. minhn@bii.a-star.edu.sg, chandra@bii.a-star.edu.sg or zhong_pingyu@immunol.a-star.edu.sg. Supplementary data are available at Bioinformatics online. © The Author (2017). Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com

The Not So Simple Globular Cluster ω Cen. I. Spatial Distribution of the Multiple Stellar Populations

NASA Astrophysics Data System (ADS)

Calamida, A.; Strampelli, G.; Rest, A.; Bono, G.; Ferraro, I.; Saha, A.; Iannicola, G.; Scolnic, D.; James, D.; Smith, C.; Zenteno, A.

2017-04-01

We present a multi-band photometric catalog of ≈1.7 million cluster members for a field of view of ≈2° × 2° across ω Cen. Photometry is based on images collected with the Dark Energy Camera on the 4 m Blanco telescope and the Advanced Camera for Surveys on the Hubble Space Telescope. The unprecedented photometric accuracy and field coverage allowed us, for the first time, to investigate the spatial distribution of ω Cen multiple populations from the core to the tidal radius, confirming its very complex structure. We found that the frequency of blue main-sequence stars is increasing compared to red main-sequence stars starting from a distance of ≈25‧ from the cluster center. Blue main-sequence stars also show a clumpy spatial distribution, with an excess in the northeast quadrant of the cluster pointing toward the direction of the Galactic center. Stars belonging to the reddest and faintest red-giant branch also show a more extended spatial distribution in the outskirts of ω Cen, a region never explored before. Both these stellar sub-populations, according to spectroscopic measurements, are more metal-rich compared to the cluster main stellar population. These findings, once confirmed, make ω Cen the only stellar system currently known where metal-rich stars have a more extended spatial distribution compared to metal-poor stars. Kinematic and chemical abundance measurements are now needed for stars in the external regions of ω Cen to better characterize the properties of these sub-populations. Based on observations made with the Dark Energy Camera (DECam) on the 4 m Blanco telescope (NOAO) under programs 2014A-0327, 2015A-0151, 2016A-0189, PIs: A. Calamida, A. Rest, and on observations made with the NASA/ESA Hubble Space Telescope, obtained by the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Star Cluster Formation in Cosmological Simulations. I. Properties of Young Clusters

NASA Astrophysics Data System (ADS)

Li, Hui; Gnedin, Oleg Y.; Gnedin, Nickolay Y.; Meng, Xi; Semenov, Vadim A.; Kravtsov, Andrey V.

2017-01-01

We present a new implementation of star formation in cosmological simulations by considering star clusters as a unit of star formation. Cluster particles grow in mass over several million years at the rate determined by local gas properties, with high time resolution. The particle growth is terminated by its own energy and momentum feedback on the interstellar medium. We test this implementation for Milky Way-sized galaxies at high redshift by comparing the properties of model clusters with observations of young star clusters. We find that the cluster initial mass function is best described by a Schechter function rather than a single power law. In agreement with observations, at low masses the logarithmic slope is α ≈ 1.8{--}2, while the cutoff at high mass scales with the star formation rate (SFR). A related trend is a positive correlation between the surface density of the SFR and fraction of stars contained in massive clusters. Both trends indicate that the formation of massive star clusters is preferred during bursts of star formation. These bursts are often associated with major-merger events. We also find that the median timescale for cluster formation ranges from 0.5 to 4 Myr and decreases systematically with increasing star formation efficiency. Local variations in the gas density and cluster accretion rate naturally lead to the scatter of the overall formation efficiency by an order of magnitude, even when the instantaneous efficiency is kept constant. Comparison of the formation timescale with the observed age spread of young star clusters provides an additional important constraint on the modeling of star formation and feedback schemes.

The Effect of Star Formation History on the Inferred Stellar Initial Mass Function

NASA Astrophysics Data System (ADS)

Elmegreen, Bruce G.; Scalo, John

2006-01-01

Peaks and lulls in the star formation rate (SFR) over the history of the Galaxy produce plateaus and declines in the present-day mass function (PDMF) where the main-sequence lifetime overlaps the age and duration of the SFR variation. These PDMF features can be misinterpreted as the form of the intrinsic stellar initial mass function (IMF) if the star formation rate is assumed to be constant or slowly varying with time. This effect applies to all regions that have formed stars for longer than the age of the most massive stars, including OB associations, star complexes, and especially galactic field stars. Related problems may apply to embedded clusters. Evidence is summarized for temporal SFR variations from parsec scales to entire galaxies, all of which should contribute to inferred IMF distortions. We give examples of various star formation histories to demonstrate the types of false IMF structures that might be seen. These include short-duration bursts, stochastic histories with lognormal amplitude distributions, and oscillating histories with various periods and phases. The inferred IMF should appear steeper than the intrinsic IMF over mass ranges where the stellar lifetimes correspond to times of decreasing SFRs; shallow portions of the inferred IMF correspond to times of increasing SFRs. If field regions are populated by dispersed clusters and defined by their low current SFRs, then they should have steeper inferred IMFs than the clusters. The SFRs required to give the steep field IMFs in the LMC and SMC are determined. Structure observed in several determinations of the Milky Way field star IMF can be accounted for by a stochastic and bursty star formation history.

HIGH-RESOLUTION STUDY OF THE CLUSTER COMPLEXES IN A LENSED SPIRAL AT REDSHIFT 1.5: CONSTRAINTS ON THE BULGE FORMATION AND DISK EVOLUTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Adamo, Angela; Oestlin, G.; Zackrisson, E.

2013-04-01

We analyze the clump population of the spiral galaxy Sp 1149 at redshift 1.5. Located behind the galaxy cluster MACS J1149.5+2223, Sp 1149 has been significantly magnified allowing us to study the galaxy on physical scales down to {approx}100 pc. The galaxy cluster frame is among the targets of the Cluster Lensing And Supernova survey with Hubble (CLASH), an ongoing Hubble Space Telescope (HST) Multi-Cycle Treasury program. We have used the publicly available multi-band imaging data set to reconstruct the spectral energy distributions of the clumps in Sp 1149, and derive, by means of stellar evolutionary models, their physical properties.more » We found that 40% of the clumps observed in Sp 1149 are older than 30 Myr and can be as old as 300 Myr. These are also the more massive (luminous) clumps in the galaxy. Among the complexes in the local reference sample, the star-forming knots in luminous blue compact galaxies could be considered progenitor analogs of these long-lived clumps. The remaining 60% of clumps have colors comparable to local cluster complexes, suggesting a similar young age. We observe that the Sp 1149 clumps follow the M{proportional_to}R {sup 2} relation similar to local cluster complexes, suggesting similar formation mechanisms although they may have different initial conditions (e.g., higher gas surface densities). We suggest that the galaxy is experiencing a slow decline in star formation rate and a likely transitional phase toward a more quiescent star formation mode. The older clumps have survived between 6 and 20 dynamical times and are all located at projected distances smaller than 4 kpc from the center. Their current location suggests migration toward the center and the possibility of being the building blocks of the bulge. On the other hand, the dynamical timescale of the younger clumps is significantly shorter, meaning that they are quite close to their birthplace. We show that the clumps of Sp 1149 may account for the expected metal-rich globular cluster population usually associated with the bulge and thick disk components of local spirals.« less

Chromospheric Activity in Cool Luminous Stars

NASA Astrophysics Data System (ADS)

Dupree, Andrea

2018-04-01

Spatially unresolved spectra of giant and supergiant stars demonstrate ubiquitous signatures of chromospheric activity, variable outflows, and winds. The advent of imaging techniques and spatially resolved spectra reveal complex structures in these extended stellar atmospheres that we do not understand. The presence and behavior of these atmospheres is wide ranging and impacts stellar activity, magnetic fields, angular momentum loss, abundance determinations, and the understanding of stellar cluster populations.

Clustering and pasta phases in nuclear density functional theory

DOE PAGES

Schuetrumpf, Bastian; Zhang, Chunli; Nazarewicz, Witold

2017-05-23

Nuclear density functional theory is the tool of choice in describing properties of complex nuclei and intricate phases of bulk nucleonic matter. It is a microscopic approach based on an energy density functional representing the nuclear interaction. An attractive feature of nuclear DFT is that it can be applied to both finite nuclei and pasta phases appearing in the inner crust of neutron stars. While nuclear pasta clusters in a neutron star can be easily characterized through their density distributions, the level of clustering of nucleons in a nucleus can often be difficult to assess. To this end, we usemore » the concept of nucleon localization. We demonstrate that the localization measure provides us with fingerprints of clusters in light and heavy nuclei, including fissioning systems. Furthermore we investigate the rod-like pasta phase using twist-averaged boundary conditions, which enable calculations in finite volumes accessible by state of the art DFT solvers.« less

Search for Carbon-Rich Asymptotic Giant Branch Stars in Milky Way Globular Clusters

NASA Astrophysics Data System (ADS)

Indahl, Briana; Pessev, P.

2014-01-01

From our current understanding of stellar evolution, it would not be expected to find carbon rich asymptotic giant branch (AGB) stars in Milky Way globular clusters. Due to the low metallicity of the population II stars making up the globular clusters and their age, stars large enough to fuse carbon should have already evolved off of the asymptotic giant branch. Recently, however, there have been serendipitous discoveries of these types of stars. Matsunaga et al. (2006) discovered a Mira variable in the globular cluster Lynga 7. It was later confirmed by Feast et al. (2012) that the star is a member of the cluster and must be a product of a stellar merger. In the same year, Sharina et al. (2012) discovered a carbon star in the low metallicity globular cluster NGC6426 and reports it to be a CH star. Five more of these types of stars have been made as serendipitous discoveries and have been reported by Harding (1962), Dickens (1972), Cote et al. (1997), and Van Loon (2007). The abundance of these types of carbon stars in Milky Way globular clusters has been unknown because the discovery of these types of objects has only ever been a serendipitous discovery. These stars could have been easily overlooked in the past as they are outside the typical parameter space of galactic globular clusters. Also advances in near-infrared instruments and observing techniques have made it possible to detect the fainter carbon stars in binary systems. Having an understanding of the abundances of carbon stars in galactic globular clusters will aid in the modeling of globular cluster and galaxy formation leading to a better understanding of these processes. To get an understanding of the abundances of these stars we conducted the first comprehensive search for AGB carbon stars into all Milky Way globular clusters listed in the Harris Catalog (expect for Pyxis). I have found 128 carbon star candidates using methods of comparing color magnitude diagrams of the clusters with the carbon stars of the Large Magellenic Clouds and picking out very red stars in the red giant branch range. Observations will need to be done of these candidates to further confirm if they are carbon stars and are members of their respective globular cluster.

Star Formation in the Eagle Nebula

NASA Astrophysics Data System (ADS)

Oliveira, J. M.

2008-12-01

M16 (the Eagle Nebula) is a striking star forming region, with a complex morphology of gas and dust sculpted by the massive stars in NGC 6611. Detailed studies of the famous ``elephant trunks'' dramatically increased our understanding of the massive star feedback into the parent molecular cloud. A rich young stellar population (2-3 Myr) has been identified, from massive O-stars down to substellar masses. Deep into the remnant molecular material, embedded protostars, Herbig-Haro objects and maser sources bear evidence of ongoing star formation in the nebula, possibly triggered by the massive cluster members. M 16 is a excellent template for the study of star formation under the hostile environment created by massive O-stars. This review aims at providing an observational overview not only of the young stellar population but also of the gas remnant of the star formation process.

Star cluster formation in cosmological simulations. I. Properties of young clusters

DOE PAGES

Li, Hui; Gnedin, Oleg Y.; Gnedin, Nickolay Y.; ...

2017-01-03

We present a new implementation of star formation in cosmological simulations by considering star clusters as a unit of star formation. Cluster particles grow in mass over several million years at the rate determined by local gas properties, with high time resolution. The particle growth is terminated by its own energy and momentum feedback on the interstellar medium. We test this implementation for Milky Way-sized galaxies at high redshift by comparing the properties of model clusters with observations of young star clusters. We find that the cluster initial mass function is best described by a Schechter function rather than a single power law. In agreement with observations, at low masses the logarithmic slope ismore » $$\\alpha \\approx 1.8\\mbox{–}2$$, while the cutoff at high mass scales with the star formation rate (SFR). A related trend is a positive correlation between the surface density of the SFR and fraction of stars contained in massive clusters. Both trends indicate that the formation of massive star clusters is preferred during bursts of star formation. These bursts are often associated with major-merger events. We also find that the median timescale for cluster formation ranges from 0.5 to 4 Myr and decreases systematically with increasing star formation efficiency. Local variations in the gas density and cluster accretion rate naturally lead to the scatter of the overall formation efficiency by an order of magnitude, even when the instantaneous efficiency is kept constant. As a result, comparison of the formation timescale with the observed age spread of young star clusters provides an additional important constraint on the modeling of star formation and feedback schemes.« less

Star cluster formation in cosmological simulations. I. Properties of young clusters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Hui; Gnedin, Oleg Y.; Gnedin, Nickolay Y.

We present a new implementation of star formation in cosmological simulations by considering star clusters as a unit of star formation. Cluster particles grow in mass over several million years at the rate determined by local gas properties, with high time resolution. The particle growth is terminated by its own energy and momentum feedback on the interstellar medium. We test this implementation for Milky Way-sized galaxies at high redshift by comparing the properties of model clusters with observations of young star clusters. We find that the cluster initial mass function is best described by a Schechter function rather than a single power law. In agreement with observations, at low masses the logarithmic slope ismore » $$\\alpha \\approx 1.8\\mbox{–}2$$, while the cutoff at high mass scales with the star formation rate (SFR). A related trend is a positive correlation between the surface density of the SFR and fraction of stars contained in massive clusters. Both trends indicate that the formation of massive star clusters is preferred during bursts of star formation. These bursts are often associated with major-merger events. We also find that the median timescale for cluster formation ranges from 0.5 to 4 Myr and decreases systematically with increasing star formation efficiency. Local variations in the gas density and cluster accretion rate naturally lead to the scatter of the overall formation efficiency by an order of magnitude, even when the instantaneous efficiency is kept constant. As a result, comparison of the formation timescale with the observed age spread of young star clusters provides an additional important constraint on the modeling of star formation and feedback schemes.« less

ALE OF TWO CLUSTERS YIELDS SECRETS OF STAR BIRTH IN THE EARLY UNIVERSE

NASA Technical Reports Server (NTRS)

2002-01-01

This NASA Hubble Space Telescope (HST) image shows rich detail, previously only seen in neighboring star birth regions, in a pair of star clusters 166,000 light-years away in the Large Magellanic Cloud (LMC), in the southern constellation Doradus. The field of view is 130 light-years across and was taken with the Wide Field Planetary Camera 2. HST's unique capabilities -- ultraviolet sensitivity, ability to see faint stars, and high resolution -- have been utilized fully to identify three separate populations in this concentration of nearly 10,000 stars down to the 25th magnitude (more that twice as many as can be seen over the entire sky with the naked eye on a clear night on Earth). The field of view is only 130 light-years across. Previous observations with ground-based telescopes resolve less than 1,000 stars in the same region. About 60 percent of the stars belong to the dominant yellow cluster called NGC 1850, which is estimated to be 50 million years old. A scattering of white stars in the image are massive stars that are only about 4 million years old and represent about 20 percent of the stars in the image. (The remainder are field stars in the LMC.) Besides being much younger, the white stars are much more loosely distributed than the yellow cluster. The significant difference between the two cluster ages suggests these are two separate star groups that lie along the same line of sight. The younger, more open cluster probably lies 200 light-years beyond the older cluster. If it were in the foreground, then dust contained in the white cluster would obscure stars in the older yellow cluster. To observe two well-defined star populations separated by such a small gap of space is unusual. This juxtaposition suggests that supernova explosions in the older cluster might have triggered the birth of the younger cluster. This color composite image is assembled from exposures taken in ultraviolet, visible, and near-infrared light. Yellow stars correspond to Main Sequence stars (like our Sun) with average surface temperatures of 6000 Kelvin; red stars are cool giants and supergiants (3500 K); white stars are hot young stars (25,000 K or more) that are bright in ultraviolet. Credit: R. Gilmozzi, Space Telescope Science Institute/European Space Agency; Shawn Ewald, JPL; and NASA

The Spots and Activity of Stars in the Beehive Cluster Observed by the Kepler Space Telescope (K2)

NASA Astrophysics Data System (ADS)

Savanov, I. S.; Kalinicheva, E. S.; Dmitrienko, E. S.

2018-05-01

The spottedness parameters S (the fraction of the visible surface of the star occupied by spots) characterizing the activity of 674 stars in the Beehive Cluster (age 650 Myr) are estimated, together with variations of this parameter as a function of the rotation period, Rossby number Ro and other characteristics of the stars. The activity of the stars in this cluster is lower than the activity of stars in the younger Pleiades (125 Myr). The average S value for the Beehive Cluster stars is 0.014, while Pleiades stars have the much higher average value 0.052. The activity parameters of 61 solar-type stars in the Beehive Cluster, similar Hyades stars (of about the same age), and stars in the younger Pleiades are compared. The average S value of such objects in the Beehive Cluster is 0.014± 0.008, nearly coincident with the estimate obtained for solar-type Hyades stars. The rotation periods of these objects are 9.1 ± 3.4 day, on average, in agreement with the average rotation period of the Hyades stars (8.6 d ). Stars with periods exceeding 3-4 d are more numerous in the Beehive Cluster than in the Pleiades, and their periods have a larger range, 3-30 d . The characteristic dependence with a kink at Ro (saturation) = 0.13 is not observed in the S-Rossby number diagram for the Beehive and Hyades stars, only a clump of objects with Rossby numbers Ro > 0.7. The spottedness data for the Beehive Cluster and Hyades stars are in good agreement with the S values for dwarfs with ages of 600-700 Myr. This provides evidence for the reliability of the results of gyrochronological calibrations. The data for the Beehive and Pleiades stars are used to analyze variations in the spot-forming activity for a large number of stars of the same age that are members of a single cluster. A joint consideration of the data for two clusters can be used to draw conclusions about the time evolution of the activity of stars of different masses (over a time interval of the order of 500 Myr).

Formation and Assembly of Massive Star Clusters

NASA Astrophysics Data System (ADS)

McMillan, Stephen

The formation of stars and star clusters is a major unresolved problem in astrophysics. It is central to modeling stellar populations and understanding galaxy luminosity distributions in cosmological models. Young massive clusters are major components of starburst galaxies, while globular clusters are cornerstones of the cosmic distance scale and represent vital laboratories for studies of stellar dynamics and stellar evolution. Yet how these clusters form and how rapidly and efficiently they expel their natal gas remain unclear, as do the consequences of this gas expulsion for cluster structure and survival. Also unclear is how the properties of low-mass clusters, which form from small-scale instabilities in galactic disks and inform much of our understanding of cluster formation and star-formation efficiency, differ from those of more massive clusters, which probably formed in starburst events driven by fast accretion at high redshift, or colliding gas flows in merging galaxies. Modeling cluster formation requires simulating many simultaneous physical processes, placing stringent demands on both software and hardware. Simulations of galaxies evolving in cosmological contexts usually lack the numerical resolution to simulate star formation in detail. They do not include detailed treatments of important physical effects such as magnetic fields, radiation pressure, ionization, and supernova feedback. Simulations of smaller clusters include these effects, but fall far short of the mass of even single young globular clusters. With major advances in computing power and software, we can now directly address this problem. We propose to model the formation of massive star clusters by integrating the FLASH adaptive mesh refinement magnetohydrodynamics (MHD) code into the Astrophysical Multi-purpose Software Environment (AMUSE) framework, to work with existing stellar-dynamical and stellar evolution modules in AMUSE. All software will be freely distributed on-line, allowing open access to state-of- the-art simulation techniques within a modern, modular software environment. We will follow the gravitational collapse of 0.1-10 million-solar mass gas clouds through star formation and coalescence into a star cluster, modeling in detail the coupling of the gas and the newborn stars. We will study the effects of star formation by detecting accreting regions of gas in self-gravitating, turbulent, MHD, FLASH models that we will translate into collisional dynamical systems of stars modeled with an N-body code, coupled together in the AMUSE framework. Our FLASH models will include treatments of radiative transfer from the newly formed stars, including heating and radiative acceleration of the surrounding gas. Specific questions to be addressed are: (1) How efficiently does the gas in a star forming region form stars, how does this depend on mass, metallicity, and other parameters, and what terminates star formation? What observational predictions can be made to constrain our models? (2) How important are different mechanisms for driving turbulence and removing gas from a cluster: accretion, radiative feedback, and mechanical feedback? (3) How does the infant mortality rate of young clusters depend on the initial properties of the parent cloud? (4) What are the characteristic formation timescales of massive star clusters, and what observable imprints does the assembly process leave on their structure at an age of 10-20 Myr, when formation is essentially complete and many clusters can be observed? These studies are directly relevant to NASA missions at many electromagnetic wavelengths, including Chandra, GALEX, Hubble, and Spitzer. Each traces different aspects of cluster formation and evolution: X-rays trace supernovae, ultraviolet traces young stars, visible colors can distinguish between young blue stars and older red stars, and the infrared directly shows young embedded star clusters.

A Chemical Composition Survey of the Iron-complex Globular Cluster NGC 6273 (M19)

NASA Astrophysics Data System (ADS)

Johnson, Christian I.; Caldwell, Nelson; Rich, R. Michael; Mateo, Mario; Bailey, John I., III; Clarkson, William I.; Olszewski, Edward W.; Walker, Matthew G.

2017-02-01

Recent observations have shown that a growing number of the most massive Galactic globular clusters contain multiple populations of stars with different [Fe/H] and neutron-capture element abundances. NGC 6273 has only recently been recognized as a member of this “iron-complex” cluster class, and we provide here a chemical and kinematic analysis of >300 red giant branch and asymptotic giant branch member stars using high-resolution spectra obtained with the Magellan-M2FS and VLT-FLAMES instruments. Multiple lines of evidence indicate that NGC 6273 possesses an intrinsic metallicity spread that ranges from about [Fe/H] = -2 to -1 dex, and may include at least three populations with different [Fe/H] values. The three populations identified here contain separate first (Na/Al-poor) and second (Na/Al-rich) generation stars, but a Mg-Al anti-correlation may only be present in stars with [Fe/H] ≳ -1.65. The strong correlation between [La/Eu] and [Fe/H] suggests that the s-process must have dominated the heavy element enrichment at higher metallicities. A small group of stars with low [α/Fe] is identified and may have been accreted from a former surrounding field star population. The cluster’s large abundance variations are coupled with a complex, extended, and multimodal blue horizontal branch (HB). The HB morphology and chemical abundances suggest that NGC 6273 may have an origin that is similar to ω Cen and M54. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-14197. This paper includes data gathered with the 6.5 m Magellan Telescopes located as Las Campanas Observatory, Chile.

The real population of star clusters in the bar of the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Piatti, Andrés E.

2017-09-01

We report results on star clusters located in the south-eastern half of the Large Magellanic (LMC) bar from Washington CT1 photometry. Using appropriate kernel density estimators, we detected 73 star cluster candidates, three of which do not show any detectable trace of star cluster sequences in their colour-magnitude diagrams (CMDs). We did not detect the other 38 previously catalogued clusters, which could not be recognized when visually inspecting the C and T1 images either; the distribution of stars in their respective fields do not resemble that of a stellar aggregate. They represent 33 per cent of all catalogued objects located within the analysed LMC bar field. From matching theoretical isochrones to the cluster CMDs cleaned from field star contamination, we derived ages in the range 7.2 < log(t yr-1) < 10.1. As far as we are aware, this is the first time that homogeneous age estimates based on resolved stellar photometry are obtained for most of the studied clusters. We built the cluster frequency (CF) for the surveyed area, and found that the main star cluster formation activity has taken place during the period log(t yr-1) 8.0-9.0. Since 100 Myr ago, clusters have been formed during a few bursting formation episodes. When comparing the observed CF to that recovered from the star formation rate, we found noticeable differences, which suggests that field star and star cluster formation histories could have been significantly different. Photometric catalogues of the studied star clusters are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/606/A21

The SUNBIRD survey: characterizing the super star cluster populations of intensely star-forming galaxies

NASA Astrophysics Data System (ADS)

Randriamanakoto, Zara; Väisänen, Petri

2017-03-01

Super star clusters (SSCs) represent the youngest and most massive form of known gravitationally bound star clusters in the Universe. They are born abundantly in environments that trigger strong and violent star formation. We investigate the properties of these massive SSCs in a sample of 42 nearby starbursts and luminous infrared galaxies. The targets form the sample of the SUperNovae and starBursts in the InfraReD (SUNBIRD) survey that were imaged using near-infrared (NIR) K-band adaptive optics mounted on the Gemini/NIRI and the VLT/NaCo instruments. Results from i) the fitted power-laws to the SSC K-band luminosity functions, ii) the NIR brightest star cluster magnitude - star formation rate (SFR) relation and iii) the star cluster age and mass distributions have shown the importance of studying SSC host galaxies with high SFR levels to determine the role of the galactic environments in the star cluster formation, evolution and disruption mechanisms.

New Wolf-Rayet stars in Galactic open clusters - Sher 1 and the giant H II region core Westerlund 2

NASA Technical Reports Server (NTRS)

Moffat, Anthony F. J.; Shara, Michael M.; Potter, Michael

1991-01-01

Two new Galactic Wolf-Rayet stars were found in open clusters: a WN4 star in the O9 cluster Sher 1 and a WN7 star in the O7 cluster Westerlund 2. This confirms a previous trend, namely that fainter, hotter WN stars tend to be older than brighter, cooler WN stars. This may be a consequence of evolution via extreme mass loss.

A method for determining the radius of an open cluster from stellar proper motions

NASA Astrophysics Data System (ADS)

Sánchez, Néstor; Alfaro, Emilio J.; López-Martínez, Fátima

2018-04-01

We propose a method for calculating the radius of an open cluster in an objective way from an astrometric catalogue containing, at least, positions and proper motions. It uses the minimum spanning tree in the proper motion space to discriminate cluster stars from field stars and it quantifies the strength of the cluster-field separation by means of a statistical parameter defined for the first time in this paper. This is done for a range of different sampling radii from where the cluster radius is obtained as the size at which the best cluster-field separation is achieved. The novelty of this strategy is that the cluster radius is obtained independently of how its stars are spatially distributed. We test the reliability and robustness of the method with both simulated and real data from a well-studied open cluster (NGC 188), and apply it to UCAC4 data for five other open clusters with different catalogued radius values. NGC 188, NGC 1647, NGC 6603, and Ruprecht 155 yielded unambiguous radius values of 15.2 ± 1.8, 29.4 ± 3.4, 4.2 ± 1.7, and 7.0 ± 0.3 arcmin, respectively. ASCC 19 and Collinder 471 showed more than one possible solution, but it is not possible to know whether this is due to the involved uncertainties or due to the presence of complex patterns in their proper motion distributions, something that could be inherent to the physical object or due to the way in which the catalogue was sampled.

Stellar complexes in spiral arms of galaxies

NASA Astrophysics Data System (ADS)

Efremov, Yu. N.

The history of the introduction and development of the star complexes conception is briefly described. These large groups of stars were picked out and named as such ones in our Galaxy with argumentation and evidence for their physical unity (using the Cepheid variables the distances and ages of which are easy determined from their periods); anyway earlier the complexes were noted along the spiral arms of the Andromeda galaxy, but were not recognized as a new kind of star group. The chains of complexes along the spiral arms are observed quite rarely; their origin is explained by magneto- gravitational or purely gravitational instability developing along the arm. It is not clear why these chains are quite a rare phenomenon - and more so why sometimes the regular chain of complexes are observed in one arm only. Probably intergalactic magnetic field participated in formation of such chains. Apart from the complexes located along the arms, there are isolated giant complexes known (up to 700 pc in diameter) which look like super-gigantic but rather rarefied globular clusters. Until now only two of these formations are studied, in NGC 6946 and M51.

RR Lyrae stars in and around NGC 6441: signatures of dissolving cluster stars

NASA Astrophysics Data System (ADS)

Kunder, Andrea

2018-06-01

Detailed elemental abundance patterns of metal-poor ([Fe/H]~ -1 dex) stars in the Galactic bulge indicate that a number of them are consistent with globular cluster (GC) stars and may be former members of dissolved GCs. This would indicate that a few per cent of the Galactic bulge was built up from destruction and/or evaporation of globular clusters. Here an attempt is made to identify such presumptive destroyed stars originating from the massive, inner Galaxy globular cluster NGC~6441 using its rich RR Lyrae variable star (RRL) population. We present radial velocities of forty RRLs centered on the globular cluster NGC~6441. All of the 13 RRLs observed within the cluster tidal radius have velocities consistent with cluster membership, with an average radial velocity of 24 +- 5~km/s and a star-to-star scatter of 11~km/s. This includes two new RRLs that were previously not associated with the cluster. Eight RRLs with radial velocities consistent with cluster membership but up to three time the distance from the tidal radius are also reported. These potential extra-tidal RRLs also have exceptionally long periods, which is a curious characteristic of the NGC~6441 RRL population that hosts RRLs with periods longer than seen anywhere else in the Milky Way. As expected of stripped cluster stars, most are inline with the cluster's orbit. Therefore, either the tidal radius of NGC~6441 is underestimated and/or we are seeing dissolving cluster stars stemming from NGC~6441 that are building up the old spheroidal bulge. Both the mean velocity of the cluster as well as the underlying field population is consistent with belonging to an old spheroidal bulge with low rotation and high velocity dispersion that formed before the bar.

Star cluster formation in a turbulent molecular cloud self-regulated by photoionization feedback

NASA Astrophysics Data System (ADS)

Gavagnin, Elena; Bleuler, Andreas; Rosdahl, Joakim; Teyssier, Romain

2017-12-01

Most stars in the Galaxy are believed to be formed within star clusters from collapsing molecular clouds. However, the complete process of star formation, from the parent cloud to a gas-free star cluster, is still poorly understood. We perform radiation-hydrodynamical simulations of the collapse of a turbulent molecular cloud using the RAMSES-RT code. Stars are modelled using sink particles, from which we self-consistently follow the propagation of the ionizing radiation. We study how different feedback models affect the gas expulsion from the cloud and how they shape the final properties of the emerging star cluster. We find that the star formation efficiency is lower for stronger feedback models. Feedback also changes the high-mass end of the stellar mass function. Stronger feedback also allows the establishment of a lower density star cluster, which can maintain a virial or sub-virial state. In the absence of feedback, the star formation efficiency is very high, as well as the final stellar density. As a result, high-energy close encounters make the cluster evaporate quickly. Other indicators, such as mass segregation, statistics of multiple systems and escaping stars confirm this picture. Observations of young star clusters are in best agreement with our strong feedback simulation.

Clustered star formation and the origin of stellar masses.

PubMed

Pudritz, Ralph E

2002-01-04

Star clusters are ubiquitous in galaxies of all types and at all stages of their evolution. We also observe them to be forming in a wide variety of environments, ranging from nearby giant molecular clouds to the supergiant molecular clouds found in starburst and merging galaxies. The typical star in our galaxy and probably in others formed as a member of a star cluster, so star formation is an intrinsically clustered and not an isolated phenomenon. The greatest challenge regarding clustered star formation is to understand why stars have a mass spectrum that appears to be universal. This review examines the observations and models that have been proposed to explain these fundamental issues in stellar formation.

Yellow supergiants in open clusters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sowell, J.R.

1986-01-01

Superluminous giant stars (SLGs) have been reported in young globular clusters in the Large Magellanic Cloud (LMC). These stars appear to be in the post-asymptotic-giant-branch phase of evolution. This program was an investigation of galactic SLG candidates in open clusters, which are more like the LMC young globular clusters. These were chosen because luminosity, mass, and age determinations can be made for members since cluster distances and interstellar reddenings are known. Color magnitude diagrams were searched for candidates, using the same selection criteria as for SLGs in the LMC. Classification spectra were obtained of 115 program stars from McGraw-Hill Observatorymore » and of 68 stars from Cerro Tololo Inter-American Observatory Chile. These stars were visually classified on the MK system using spectral scans of standard stars taken at the respective observations. Published information was combined with this program's data for 83 stars in 30 clusters. Membership probabilities were assigned to these stars, and the clusters were analyzed according to age. It was seen that the intrinsically brightest supergiants are found in the youngest clusters. With increasing cluster age, the absolute luminosities attained by the supergiants decline. Also, it appears that the evolutionary tracks of luminosity class II stars are more similar to those of class I than of class III.« less

The same frequency of planets inside and outside open clusters of stars.

PubMed

Meibom, Søren; Torres, Guillermo; Fressin, Francois; Latham, David W; Rowe, Jason F; Ciardi, David R; Bryson, Steven T; Rogers, Leslie A; Henze, Christopher E; Janes, Kenneth; Barnes, Sydney A; Marcy, Geoffrey W; Isaacson, Howard; Fischer, Debra A; Howell, Steve B; Horch, Elliott P; Jenkins, Jon M; Schuler, Simon C; Crepp, Justin

2013-07-04

Most stars and their planets form in open clusters. Over 95 per cent of such clusters have stellar densities too low (less than a hundred stars per cubic parsec) to withstand internal and external dynamical stresses and fall apart within a few hundred million years. Older open clusters have survived by virtue of being richer and denser in stars (1,000 to 10,000 per cubic parsec) when they formed. Such clusters represent a stellar environment very different from the birthplace of the Sun and other planet-hosting field stars. So far more than 800 planets have been found around Sun-like stars in the field. The field planets are usually the size of Neptune or smaller. In contrast, only four planets have been found orbiting stars in open clusters, all with masses similar to or greater than that of Jupiter. Here we report observations of the transits of two Sun-like stars by planets smaller than Neptune in the billion-year-old open cluster NGC6811. This demonstrates that small planets can form and survive in a dense cluster environment, and implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars in the Galaxy.

The Connection Between X-ray Binaries and Star Clusters in the Antennae

NASA Astrophysics Data System (ADS)

Rangelov, Blagoy; Chandar, R.; Prestwich, A.

2011-05-01

High Mass X-ray Binaries (HMXBs) are believed to form in massive, compact star clusters. However the correlation between these young binary star systems and properties of their parent clusters are still poorly known. We compare the locations of 82 X-ray binaries detected in the merging Antennae galaxies by Zezas et al. (2006) based on observations taken with the Chandra Space Telescope, with a catalog of optically selected star clusters presented recently by Whitmore et al. (2010) based on observations taken with the Hubble Space Telescope. We find 22 X-ray binaries coincident or nearly coincident with star clusters. The ages of the clusters were estimated by comparing their UBVIHα colors with predictions from stellar evolutionary models. We find that 14 of the 22 coincident sources (64%) are hosted by star clusters with ages of 6 Myr or less. At these very young ages, only stars initially more massive than M ≥ 30 Msun have evolved into compact remnants, almost certainly black holes. Therefore, these 14 sources are likely to be black hole binaries. Five of the XRBs are hosted by young clusters with ages τ 30-50 Myr, while three are hosted by intermediate age clusters with τ 100-300 Myr. We suggest that these older X-ray binaries likely have neutron stars as the compact object. We conclude that precision age-dating of star clusters, which are spatially coincident with XRBs in nearby star forming galaxies, is a powerful method of constraining the nature of the XRBs.

Deriving physical parameters of unresolved star clusters. V. M 31 PHAT star clusters

NASA Astrophysics Data System (ADS)

de Meulenaer, P.; Stonkutė, R.; Vansevičius, V.

2017-06-01

Context. This study is the fifth of a series that investigates the degeneracy and stochasticity problems present in the determination of physical parameters such as age, mass, extinction, and metallicity of partially resolved or unresolved star cluster populations in external galaxies when using HST broad-band photometry. Aims: In this work we aim to derive parameters of star clusters using models with fixed and free metallicity based on the HST WFC3+ACS photometric system. The method is applied to derive parameters of a subsample of 1363 star clusters in the Andromeda galaxy observed with the HST. Methods: Following Paper III, we derive the star cluster parameters using a large grid of stochastic models that are compared to the six observed integrated broad-band WFC3+ACS magnitudes of star clusters. Results: We show that the age, mass, and extinction of the M 31 star clusters, derived assuming fixed solar metallicity, are in agreement with previous studies. We also demonstrate the ability of the WFC3+ACS photometric system to derive metallicity of star clusters older than 1 Gyr. We show that the metallicity derived using broad-band photometry of 36 massive M 31 star clusters is in good agreement with the metallicity derived using spectroscopy. Table 1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A112

Hierarchical Star Formation in Turbulent Media: Evidence from Young Star Clusters

NASA Astrophysics Data System (ADS)

Grasha, K.; Elmegreen, B. G.; Calzetti, D.; Adamo, A.; Aloisi, A.; Bright, S. N.; Cook, D. O.; Dale, D. A.; Fumagalli, M.; Gallagher, J. S., III; Gouliermis, D. A.; Grebel, E. K.; Kahre, L.; Kim, H.; Krumholz, M. R.; Lee, J. C.; Messa, M.; Ryon, J. E.; Ubeda, L.

2017-06-01

We present an analysis of the positions and ages of young star clusters in eight local galaxies to investigate the connection between the age difference and separation of cluster pairs. We find that star clusters do not form uniformly but instead are distributed so that the age difference increases with the cluster pair separation to the 0.25-0.6 power, and that the maximum size over which star formation is physically correlated ranges from ˜200 pc to ˜1 kpc. The observed trends between age difference and separation suggest that cluster formation is hierarchical both in space and time: clusters that are close to each other are more similar in age than clusters born further apart. The temporal correlations between stellar aggregates have slopes that are consistent with predictions of turbulence acting as the primary driver of star formation. The velocity associated with the maximum size is proportional to the galaxy’s shear, suggesting that the galactic environment influences the maximum size of the star-forming structures.

Probing Massive Star Cluster Formation with ALMA

NASA Astrophysics Data System (ADS)

Johnson, Kelsey

2015-08-01

Observationally constraining the physical conditions that give rise to massive star clusters has been a long-standing challenge. Now with the ALMA Observatory coming on-line, we can finally begin to probe the birth environments of massive clusters in a variety of galaxies with sufficient angular resolution. In this talk I will give an overview of ALMA observations of galaxies in which candidate proto-super star cluster molecular clouds have been identified. These new data probe the physical conditions that give rise to super star clusters, providing information on their densities, pressures, and temperatures. In particular, the observations indicate that these clouds may be subject to external pressures of P/k > 108 K cm-3, which is consistent with the prevalence of optically observed adolescent super star clusters in interacting galaxy systems and other high pressure environments. ALMA observations also enable an assessement of the molecular cloud chemical abundances in the regions surrounding super star clusters. Molecular clouds associated with existing super star clusters are strongly correlated with HCO+ emission, but appear to have relatively low ratio of CO/HCO+ emission compared to other clouds, indicating that the super star clusters are impacting the molecular abundances in their vicinity.

MOCCA-SURVEY Database I: Is NGC 6535 a dark star cluster harbouring an IMBH?

NASA Astrophysics Data System (ADS)

Askar, Abbas; Bianchini, Paolo; de Vita, Ruggero; Giersz, Mirek; Hypki, Arkadiusz; Kamann, Sebastian

2017-01-01

We describe the dynamical evolution of a unique type of dark star cluster model in which the majority of the cluster mass at Hubble time is dominated by an intermediate-mass black hole (IMBH). We analysed results from about 2000 star cluster models (Survey Database I) simulated using the Monte Carlo code MOnte Carlo Cluster simulAtor and identified these dark star cluster models. Taking one of these models, we apply the method of simulating realistic `mock observations' by utilizing the Cluster simulatiOn Comparison with ObservAtions (COCOA) and Simulating Stellar Cluster Observation (SISCO) codes to obtain the photometric and kinematic observational properties of the dark star cluster model at 12 Gyr. We find that the perplexing Galactic globular cluster NGC 6535 closely matches the observational photometric and kinematic properties of the dark star cluster model presented in this paper. Based on our analysis and currently observed properties of NGC 6535, we suggest that this globular cluster could potentially harbour an IMBH. If it exists, the presence of this IMBH can be detected robustly with proposed kinematic observations of NGC 6535.

The Structure of the Young Star Cluster NGC 6231. II. Structure, Formation, and Fate

NASA Astrophysics Data System (ADS)

Kuhn, Michael A.; Getman, Konstantin V.; Feigelson, Eric D.; Sills, Alison; Gromadzki, Mariusz; Medina, Nicolás; Borissova, Jordanka; Kurtev, Radostin

2017-12-01

The young cluster NGC 6231 (stellar ages ˜2-7 Myr) is observed shortly after star formation activity has ceased. Using the catalog of 2148 probable cluster members obtained from Chandra, VVV, and optical surveys (Paper I), we examine the cluster’s spatial structure and dynamical state. The spatial distribution of stars is remarkably well fit by an isothermal sphere with moderate elongation, while other commonly used models like Plummer spheres, multivariate normal distributions, or power-law models are poor fits. The cluster has a core radius of 1.2 ± 0.1 pc and a central density of ˜200 stars pc-3. The distribution of stars is mildly mass segregated. However, there is no radial stratification of the stars by age. Although most of the stars belong to a single cluster, a small subcluster of stars is found superimposed on the main cluster, and there are clumpy non-isotropic distributions of stars outside ˜4 core radii. When the size, mass, and age of NGC 6231 are compared to other young star clusters and subclusters in nearby active star-forming regions, it lies at the high-mass end of the distribution but along the same trend line. This could result from similar formation processes, possibly hierarchical cluster assembly. We argue that NGC 6231 has expanded from its initial size but that it remains gravitationally bound.

Ages of Extragalactic Intermediate-Age Star Clusters

NASA Technical Reports Server (NTRS)

Flower, P. J.

1983-01-01

A dating technique for faint, distant star clusters observable in the local group of galaxies with the space telescope is discussed. Color-magnitude diagrams of Magellanic Cloud clusters are mentioned along with the metallicity of star clusters.

The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters

NASA Astrophysics Data System (ADS)

McSwain, M. Virginia; Gies, Douglas R.

2005-11-01

Be stars are a class of rapidly rotating B stars with circumstellar disks that cause Balmer and other line emission. There are three possible reasons for the rapid rotation of Be stars: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence (MS) evolution of B stars. To test the various formation scenarios, we have conducted a photometric survey of 55 open clusters in the southern sky. Of these, five clusters are probably not physically associated groups and our results for two other clusters are not reliable, but we identify 52 definite Be stars and an additional 129 Be candidates in the remaining clusters. We use our results to examine the age and evolutionary dependence of the Be phenomenon. We find an overall increase in the fraction of Be stars with age until 100 Myr, and Be stars are most common among the brightest, most massive B-type stars above the zero-age main sequence (ZAMS). We show that a spin-up phase at the terminal-age main sequence (TAMS) cannot produce the observed distribution of Be stars, but up to 73% of the Be stars detected may have been spun-up by binary mass transfer. Most of the remaining Be stars were likely rapid rotators at birth. Previous studies have suggested that low metallicity and high cluster density may also favor Be star formation. Our results indicate a possible increase in the fraction of Be stars with increasing cluster distance from the Galactic center (in environments of decreasing metallicity). However, the trend is not significant and could be ruled out due to the intrinsic scatter in our data. We also find no relationship between the fraction of Be stars and cluster density.

Star Cluster Buzzing With Pulsars

NASA Astrophysics Data System (ADS)

2005-01-01

A dense globular star cluster near the center of our Milky Way Galaxy holds a buzzing beehive of rapidly-spinning millisecond pulsars, according to astronomers who discovered 21 new pulsars in the cluster using the National Science Foundation's 100-meter Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. The cluster, called Terzan 5, now holds the record for pulsars, with 24, including three known before the GBT observations. Pulsar Diagram Pulsar Diagram: Click on image for more detail. "We hit the jackpot when we looked at this cluster," said Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, VA. "Not only does this cluster have a lot of pulsars -- and we still expect to find more in it -- but the pulsars in it are very interesting. They include at least 13 in binary systems, two of which are eclipsing, and the four fastest-rotating pulsars known in any globular cluster, with the fastest two rotating nearly 600 times per second, roughly as fast as a household blender," Ransom added. Ransom and his colleagues reported their findings to the American Astronomical Society's meeting in San Diego, CA, and in the online journal Science Express. The star cluster's numerous pulsars are expected to yield a bonanza of new information about not only the pulsars themselves, but also about the dense stellar environment in which they reside and probably even about nuclear physics, according to the scientists. For example, preliminary measurements indicate that two of the pulsars are more massive than some theoretical models would allow. "All these exotic pulsars will keep us busy for years to come," said Jason Hessels, a Ph.D student at McGill University in Montreal. Globular clusters are dense agglomerations of up to millions of stars, all of which formed at about the same time. Pulsars are spinning, superdense neutron stars that whirl "lighthouse beams" of radio waves or light around as they spin. A neutron star is what is left after a massive star explodes as a supernova at the end of its life. The pulsars in Terzan 5 are the product of a complex history. The stars in the cluster formed about 10 billion years ago, the astronomers say. Some of the most massive stars in the cluster exploded and left the neutron stars as their remnants after only a few million years. Normally, these neutron stars would no longer be seen as swiftly-rotating pulsars: their spin would have slowed because of the "drag" of their intense magnetic fields until the "lighthouse" effect is no longer observable. The Green Bank Telescope The Robert C. Byrd Green Bank Telescope CREDIT: NRAO/AUI/NSF (Click on image for GBT gallery) However, the dense concentration of stars in the cluster gave new life to the pulsars. In the core of a globular cluster, as many as a million stars may be packed into a volume that would fit easily between the Sun and our nearest neighbor star. In such close quarters, stars can pass near enough to form new binary pairs, split apart such pairs, and binary systems even can trade partners, like an elaborate cosmic square dance. When a neutron star pairs up with a "normal" companion star, its strong gravitational pull can draw material off the companion onto the neutron star. This also transfers some of the companion's spin, or angular momentum, to the neutron star, thereby "recycling" the neutron star into a rapidly-rotating millisecond pulsar. In Terzan 5, all the pulsars discovered are rotating rapidly as a result of this process. Astronomers previously had discovered three pulsars in Terzan 5, some 28,000 light-years distant in the constellation Sagittarius, but suspected there were more. On July 17, 2004, Ransom and his colleagues used the GBT, and, in a 6-hour observation, found 14 new pulsars, the most ever found in a single observation. "This was possible because of the great sensitivity of the GBT and the new capabilities of our backend processor," said Ingrid Stairs, a professor at the University of British Columbia in Vancouver. The processor, named, appropriately, the Pulsar Spigot, was built in a collaboration between the NRAO and the California Institute of Technology. The processor, which generates almost 100 GigaBytes of data per hour, allowed the astronomers to gather and analyze radio waves over a wide range of frequencies (1650-2250 MegaHertz), adding to the sensitivity of their system. Eight more observations between July and November of 2004 discovered seven additional pulsars in Terzan 5. In addition, the astronomers' data show evidence for several more pulsars that still need to be confirmed. Future studies of the pulsars in Terzan 5 will help scientists understand the nature of the cluster and the complex interactions of the stars at its dense core. Also, several of the pulsars offer a rich yield of new scientific information. The scientists suspect that one pulsar, which shows strange eclipses of its radio emission, has recently traded its original binary companion for another, and two others have white-dwarf companions that they believe may have been produced by the collision of a neutron star and a red-giant star. Subtle effects seen in these two systems can be explained by Einstein's general relativistic theory of gravity, and indicate that the neutron stars are more massive than some theories allow. The material in a neutron star is as dense as that in an atomic nucleus, so that fact has implications for nuclear physics as well as astrophysics. "Finding all these pulsars has been extremely exciting, but the excitement really has just begun," Ransom said. "Now we can start to use them as a rich and valuable cosmic laboratory," he added. In addition to Ransom, Hessels and Stairs, the research team included Paulo Freire of Arecibo Observatory in Puerto Rico, Fernando Camilo of Columbia University, Victoria Kaspi of McGill University, and David Kaplan of the Massachusetts Institute of Technology. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. The pulsar research also was supported by the Canada Foundation for Innovation, Science and Engineering Research Canada, the Quebec Foundation for Research on Nature and Technology, the Canadian Institute for Advanced Research, Canada Research Chairs Program, and the National Science Foundation.

IRAS observations of the Rho Ophiuchi infrared cluster - Spectral energy distributions and luminosity function

NASA Technical Reports Server (NTRS)

Wilking, Bruce A.; Lada, Charles J.; Young, Eric T.

1989-01-01

High-sensitivity IRAS coadded survey data, coupled with new high-sensitivity near-IR observations, are used to investigate the nature of embedded objects over an 4.3-sq-pc area comprising the central star-forming cloud of the Ophiuchi molecular complex; the area encompasses the central cloud of the Rho Ophiuchi complex and includes the core region. Seventy-eight members of the embedded cluster were identified; spectral energy distributions were constructed for 53 objects and were compared with theoretical models to gain insight into their evolutionary status. Bolometric luminosities could be estimated for nearly all of the association members, leading to a revised luminosity function for this dust-embedded cluster.

Reconstructing the Initial Relaxation Time of Young Star Clusters in the Large Magellanic Cloud: The Evolution of Star Clusters

NASA Astrophysics Data System (ADS)

Portegies Zwart, S. F.; Chen, H.-C.

2008-06-01

We reconstruct the initial two-body relaxation time at the half mass radius for a sample of young ⪉ 300 Myr star clusters in the Large Magellanic cloud. We achieve this by simulating star clusters with 12288 to 131072 stars using direct N-body integration. The equations of motion of all stars are calculated with high precision direct N-body simulations which include the effects of the evolution of single stars and binaries. We find that the initial relaxation times of the sample of observed clusters in the Large Magellanic Cloud ranges from about 200 Myr to about 2 Gyr. The reconstructed initial half-mass relaxation times for these clusters have a much narrower distribution than the currently observed distribution, which ranges over more than two orders of magnitude.

Photometry of Standard Stars and Open Star Clusters

NASA Astrophysics Data System (ADS)

Jefferies, Amanda; Frinchaboy, Peter

2010-10-01

Photometric CCD observations of open star clusters and standard stars were carried out at the McDonald Observatory in Fort Davis, Texas. This data was analyzed using aperture photometry algorithms (DAOPHOT II and ALLSTAR) and the IRAF software package. Color-magnitude diagrams of these clusters were produced, showing the evolution of each cluster along the main sequence.

Close Encounters of the Stellar Kind

NASA Astrophysics Data System (ADS)

2003-07-01

NASA's Chandra X-ray Observatory has confirmed that close encounters between stars form X-ray emitting, double-star systems in dense globular star clusters. These X-ray binaries have a different birth process than their cousins outside globular clusters, and should have a profound influence on the cluster's evolution. A team of scientists led by David Pooley of the Massachusetts Institute of Technology in Cambridge took advantage of Chandra's unique ability to precisely locate and resolve individual sources to determine the number of X-ray sources in 12 globular clusters in our Galaxy. Most of the sources are binary systems containing a collapsed star such as a neutron star or a white dwarf star that is pulling matter off a normal, Sun-like companion star. "We found that the number of X-ray binaries is closely correlated with the rate of encounters between stars in the clusters," said Pooley. "Our conclusion is that the binaries are formed as a consequence of these encounters. It is a case of nurture not nature." A similar study led by Craig Heinke of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. confirmed this conclusion, and showed that roughly 10 percent of these X-ray binary systems contain neutron stars. Most of these neutron stars are usually quiet, spending less than 10% of their time actively feeding from their companion. NGC 7099 NGC 7099 A globular cluster is a spherical collection of hundreds of thousands or even millions of stars buzzing around each other in a gravitationally-bound stellar beehive that is about a hundred light years in diameter. The stars in a globular cluster are often only about a tenth of a light year apart. For comparison, the nearest star to the Sun, Proxima Centauri, is 4.2 light years away. With so many stars moving so close together, interactions between stars occur frequently in globular clusters. The stars, while rarely colliding, do get close enough to form binary star systems or cause binary stars to exchange partners in intricate dances. The data suggest that X-ray binary systems are formed in dense clusters known as globular clusters about once a day somewhere in the universe. Observations by NASA's Uhuru X-ray satellite in the 1970's showed that globular clusters seemed to contain a disproportionately large number of X-ray binary sources compared to the Galaxy as a whole. Normally only one in a billion stars is a member of an X-ray binary system containing a neutron star, whereas in globular clusters, the fraction is more like one in a million. The present research confirms earlier suggestions that the chance of forming an X-ray binary system is dramatically increased by the congestion in a globular cluster. Under these conditions two processes, known as three-star exchange collisions, and tidal captures, can lead to a thousandfold increase in the number of X-ray sources in globular clusters. 47 Tucanae 47 Tucanae In an exchange collision, a lone neutron star encounters a pair of ordinary stars. The intense gravity of the neutron star can induce the most massive ordinary star to "change partners," and pair up with the neutron star while ejecting the lighter star. A neutron star could also make a grazing collision with a single normal star, and the intense gravity of the neutron star could distort the gravity of the normal star in the process. The energy lost in the distortion, could prevent the normal star from escaping from the neutron star, leading to what is called tidal capture. "In addition to solving a long-standing mystery, Chandra data offer an opportunity for a deeper understanding of globular cluster evolution," said Heinke. "For example, the energy released in the formation of close binary systems could keep the central parts of the cluster from collapsing to form a massive black hole." NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science, NASA Headquarters, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass. The image and additional information are available at: http://chandra.harvard.edu and http://chandra.nasa.gov

Gaia-ESO Survey: Global properties of clusters Trumpler 14 and 16 in the Carina nebula ⋆⋆

NASA Astrophysics Data System (ADS)

Damiani, F.; Klutsch, A.; Jeffries, R. D.; Randich, S.; Prisinzano, L.; Maíz Apellániz, J.; Micela, G.; Kalari, V.; Frasca, A.; Zwitter, T.; Bonito, R.; Gilmore, G.; Flaccomio, E.; Francois, P.; Koposov, S.; Lanzafame, A. C.; Sacco, G. G.; Bayo, A.; Carraro, G.; Casey, A. R.; Alfaro, E. J.; Costado, M. T.; Donati, P.; Franciosini, E.; Hourihane, A.; Jofré, P.; Lardo, C.; Lewis, J.; Magrini, L.; Monaco, L.; Morbidelli, L.; Worley, C. C.; Vink, J. S.; Zaggia, S.

2017-07-01

Aims: We present the first extensive spectroscopic study of the global population in star clusters Trumpler 16, Trumpler 14, and Collinder 232 in the Carina nebula, using data from the Gaia-ESO Survey, down to solar-mass stars. Methods: In addition to the standard homogeneous survey data reduction, a special processing was applied here because of the bright nebulosity surrounding Carina stars. Results: We find about 400 good candidate members ranging from OB types down to slightly subsolar masses. About 100 heavily reddened early-type Carina members found here were previously unrecognized or poorly classified, including two candidate O stars and several candidate Herbig Ae/Be stars. Their large brightness makes them useful tracers of the obscured Carina population. The spectroscopically derived temperatures for nearly 300 low-mass members enables the inference of individual extinction values and the study of the relative placement of stars along the line of sight. Conclusions: We find a complex spatial structure with definite clustering of low-mass members around the most massive stars and spatially variable extinction. By combining the new data with existing X-ray data, we obtain a more complete picture of the three-dimensional spatial structure of the Carina clusters and of their connection to bright and dark nebulosity and UV sources. The identification of tens of background giants also enables us to determine the total optical depth of the Carina nebula along many sightlines. We are also able to put constraints on the star formation history of the region with Trumpler 14 stars found to be systematically younger than stars in other subclusters. We find a large percentage of fast-rotating stars among Carina solar-mass members, which provide new constraints on the rotational evolution of pre-main-sequence stars in this mass range. Based on observations collected with the FLAMES spectrograph at VLT/UT2 telescope (Paranal Observatory, ESO, Chile), for the Gaia-ESO Large Public Survey (program 188.B-3002). Full Tables 1, 2, and 7 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/603/A81

Hubble Space Telescope Fine Guidance Sensor interferometric observations of the core of 30 doradus

NASA Technical Reports Server (NTRS)

Lattanzi, M. G.; Hershey, J. L.; Burg, R.; Taff, L. G.; Holfeltz, S. T.; Bucciarelli, B.; Evans, I. N.; Gilmozzi, R.; Pringle, J.; Walborn, N. R.

1994-01-01

We present the results of the first high angular resolution observations taken with a Fine Guidance Sensor (FGS) aboard the Hubble Space Telescope (HST) of a star cluster embedded in very bright background. The strong and complex background around the R136 cluster in the 30 Dor nebula does not prevent the FGS from achieving performance close to its angular resolution limit of approximately 0.015 sec per axis with reliable photometry. These FGS observations establish that the central object in R136a is a triple star with the third component delta V = 1.1 mag fainter than the primary star al approximately 0.08 sec way. We estimate from the grid of models of Maeder (1990) that the present mass of al is between 30 and 80 solar masses, with the main-sequence progenitor between 60 and 120 solar masses.

Uncovering the secrets of the Quintuplet Cluster

NASA Image and Video Library

2015-07-13

Although this cluster of stars gained its name due to its five brightest stars, it is home to hundreds more. The huge number of massive young stars in the cluster is clearly captured in this NASA/ESA Hubble Space Telescope image. The cluster is located close to the Arches Cluster and is just 100 light-years from the centre of our galaxy. The cluster’s proximity to the dust at the centre of the galaxy means that much of its visible light is blocked, which helped to keep the cluster unknown until its discovery in 1990, when it was revealed by observations in the infrared. Infrared images of the cluster, like the one shown here, allow us to see through the obscuring dust to the hot stars in the cluster. The Quintuplet Cluster hosts two extremely rare luminous blue variable stars: the Pistol Star and the lesser known V4650 Sgr. If you were to draw a line horizontally through the centre of this image from left to right, you could see the Pistol Star hovering just above the line about one third of the way along it. The Pistol Star is one of the most luminous known stars in the Milky Way and takes its name from the shape of the Pistol Nebula that it illuminates, but which is not visible in this infrared image. The exact age and future of the Pistol Star are uncertain, but it is expected to end in a supernova or even a hypernova in one to three million years. The cluster also contains a number of red supergiants. These stars are among the largest in the galaxy and are burning their fuel at an incredible speed, meaning they will have a very short lifetime. Their presence suggests an average cluster age of nearly four million years. At the moment these stars are on the verge of exploding as supernovae. During their spectacular deaths they will release vast amounts of energy which, in turn, will heat the material — dust and gas — between the other stars. This observation shows the Quintuplet Cluster in the infrared and demonstrates the leap in Hubble’s performance sinc

Open clusters. III. Fundamental parameters of B stars in NGC 6087, NGC 6250, NGC 6383, and NGC 6530 B-type stars with circumstellar envelopes

NASA Astrophysics Data System (ADS)

Aidelman, Y.; Cidale, L. S.; Zorec, J.; Panei, J. A.

2018-02-01

Context. Stellar physical properties of star clusters are poorly known and the cluster parameters are often very uncertain. Methods: Our goals are to perform a spectrophotometric study of the B star population in open clusters to derive accurate stellar parameters, search for the presence of circumstellar envelopes, and discuss the characteristics of these stars. The BCD spectrophotometric system is a powerful method to obtain stellar fundamental parameters from direct measurements of the Balmer discontinuity. To this end, we wrote the interactive code MIDE3700. The BCD parameters can also be used to infer the main properties of open clusters: distance modulus, color excess, and age. Furthermore, we inspected the Balmer discontinuity to provide evidence for the presence of circumstellar disks and identify Be star candidates. We used an additional set of high-resolution spectra in the Hα region to confirm the Be nature of these stars. Results: We provide Teff, log g, Mv, Mbol, and spectral types for a sample of 68 stars in the field of the open clusters NGC 6087, NGC 6250, NGC 6383, and NGC 6530, as well as the cluster distances, ages, and reddening. Then, based on a sample of 230 B stars in the direction of the 11 open clusters studied along this series of three papers, we report 6 new Be stars, 4 blue straggler candidates, and 15 B-type stars (called Bdd) with a double Balmer discontinuity, which indicates the presence of circumstellar envelopes. We discuss the distribution of the fraction of B, Be, and Bdd star cluster members per spectral subtype. The majority of the Be stars are dwarfs and present a maximum at the spectral type B2-B4 in young and intermediate-age open clusters (<40 Myr). Another maximum of Be stars is observed at the spectral type B6-B8 in open clusters older than 40 Myr, where the population of Bdd stars also becomes relevant. The Bdd stars seem to be in a passive emission phase. Conclusions: Our results support previous statements that the Be phenomenon is present along the whole main sequence band and occurs in very different evolutionary states. We find clear evidence of an increase of stars with circumstellar envelopes with cluster age. The Be phenomenon reaches its maximum in clusters of intermediate age (10-40 Myr) and the number of B stars with circumstellar envelopes (Be plus Bdd stars) is also high for the older clusters (40-100 Myr). Observations taken at CASLEO, operating under agreement of CONICET and the Universities of La Plata, Córdoba, and San Juan, Argentina.Tables 1, 2, 9-16 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/610/A30

Highly efficient star formation in NGC 5253 possibly from stream-fed accretion.

PubMed

Turner, J L; Beck, S C; Benford, D J; Consiglio, S M; Ho, P T P; Kovács, A; Meier, D S; Zhao, J-H

2015-03-19

Gas clouds in present-day galaxies are inefficient at forming stars. Low star-formation efficiency is a critical parameter in galaxy evolution: it is why stars are still forming nearly 14 billion years after the Big Bang and why star clusters generally do not survive their births, instead dispersing to form galactic disks or bulges. Yet the existence of ancient massive bound star clusters (globular clusters) in the Milky Way suggests that efficiencies were higher when they formed ten billion years ago. A local dwarf galaxy, NGC 5253, has a young star cluster that provides an example of highly efficient star formation. Here we report the detection of the J = 3→2 rotational transition of CO at the location of the massive cluster. The gas cloud is hot, dense, quiescent and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by the embedded star cluster. Its star-formation efficiency exceeds 50 per cent, tenfold that of clouds in the Milky Way. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy.

The VMC Survey. XXVII. Young Stellar Structures in the LMC’s Bar Star-forming Complex

NASA Astrophysics Data System (ADS)

Sun, Ning-Chen; de Grijs, Richard; Subramanian, Smitha; Bekki, Kenji; Bell, Cameron P. M.; Cioni, Maria-Rosa L.; Ivanov, Valentin D.; Marconi, Marcella; Oliveira, Joana M.; Piatti, Andrés E.; Ripepi, Vincenzo; Rubele, Stefano; Tatton, Ben L.; van Loon, Jacco Th.

2017-11-01

Star formation is a hierarchical process, forming young stellar structures of star clusters, associations, and complexes over a wide range of scales. The star-forming complex in the bar region of the Large Magellanic Cloud is investigated with upper main-sequence stars observed by the VISTA Survey of the Magellanic Clouds. The upper main-sequence stars exhibit highly nonuniform distributions. Young stellar structures inside the complex are identified from the stellar density map as density enhancements of different significance levels. We find that these structures are hierarchically organized such that larger, lower-density structures contain one or several smaller, higher-density ones. They follow power-law size and mass distributions, as well as a lognormal surface density distribution. All these results support a scenario of hierarchical star formation regulated by turbulence. The temporal evolution of young stellar structures is explored by using subsamples of upper main-sequence stars with different magnitude and age ranges. While the youngest subsample, with a median age of log(τ/yr) = 7.2, contains the most substructure, progressively older ones are less and less substructured. The oldest subsample, with a median age of log(τ/yr) = 8.0, is almost indistinguishable from a uniform distribution on spatial scales of 30-300 pc, suggesting that the young stellar structures are completely dispersed on a timescale of ˜100 Myr. These results are consistent with the characteristics of the 30 Doradus complex and the entire Large Magellanic Cloud, suggesting no significant environmental effects. We further point out that the fractal dimension may be method dependent for stellar samples with significant age spreads.

Hubble space telescope observations of young star clusters in NGC-4038/4039, 'the antennae' galaxies

NASA Technical Reports Server (NTRS)

Whitmore, Bradley C.; Schweizer, Francois

1995-01-01

New, high-resolution images of the disks of NGC 4038/4039 obtained with the Wide Field Camera of the Hubble Space Telescope (HST) are presented. NGC 4038/4039, nicknamed The Antennae, is a prototypical example of a pair of colliding galaxies believed to be at an early stage of a merger. Down to the limiting magnitude of V approximately 23 mag, the HST images reveal a population of over 700 blue pointlike objects within the disks. The mean absolute magnitude of these objects is M(sub V) = -11 mag, with the brightest objects reaching M(sub V) approximately -15. Their mean apparent color indices ar U - V = -0.7 mag and V - 1 = 0.8 mag on the Johnson UVI passband system, while their mean indices corrected for internal reddening are (u - v)(sub 0) = -1.0 mag and (V - I(sub 0) = 0.5. Their mean effective radius, determined from slightly resolved images, is 18 pc (for H(sub 0) = 50 km/s /Mpc). Based on their luminosities and resolution, most of these objects cannot be individual stars, but are likely young compact star clusters. The brighter ones are similar to the objects found in NGC 1275 and NGC 7252, which appear to be young globular clusters formed during recent galazy mergers. Based on their U - V and V - I colors, the brightest, bluest clusters of NGC 4038/4039 appear to be less than 10 Myr old. Most of these bright clusters are relatively tightly clustered themselves, with typically a dozen individual clusters belonging to a complex identified as a giant H II region from ground-based observations. The cluster luminosity function (LF) is approximately a power law, phi(L)dL proportional to L(exp -1.78+/-0.05)dL, with no hint of a turnover at fainter magnitudes. This power-law shape agrees with the LF of Magellanic Cloud clusters and Galactic open clusters, but differs from the LF of old globular cluster systems that is typically Gaussian with a Full Width at Half Maximum (FWHM) of approximately 3 mag. Besides the blue clusters, we also find about a dozen extremely red objects with V - I greater than 3.0. The highest number density of these red objects is found in the SE quadrant, where star formation appears to be most recent. We propose that these objects may be very young star clusters still embedded in their placental dust cocoons.

A Missing Link in Galaxy Evolution: The Mysteries of Dissolving Star Clusters

NASA Astrophysics Data System (ADS)

Pellerin, Anne; Meyer, Martin; Harris, Jason; Calzetti, Daniela

2007-05-01

Star-forming events in starbursts and normal galaxies have a direct impact on the global stellar content of galaxies. These events create numerous compact clusters where stars are produced in great number. These stars eventually end up in the star field background where they are smoothly distributed. However, due to instrumental limitations such as spatial resolution and sensitivity, the processes involved during the transition phase from the compact clusters to the star field background as well as the impact of the environment (spiral waves, bars, starburst) on the lifetime of clusters are still poorly constrained observationally. I will present our latest results on the physical properties of dissolving clusters directly detected in HST/ACS archival images of the three nearby galaxies IC 2574, NGC 1313, and IC 10 (D < 5 Mpc). The ACS has the capability to detect and spatially resolve individual stars in nearby galaxies within a large field-of-view. For all ACS images obtained in three filters (F435W, F555W or F606W, and F814W), we performed PSF stellar photometry in crowded field. Color-magnitude diagrams (CMD) allow us to identify the most massive stars more likely to be part of dissolving clusters (A-type and earlier), and to isolate them from the star field background. We then adapt and use a clustering algorithm on the selected stars to find groups of stars to reveal and quantify the properties of all star clusters (compactness, size, age, mass). With this algorithm, even the less compact clusters are revealed while they are being destroyed. Our sample of three galaxies covers an interesting range in gravitational potential well and explores a variety of galaxy morphological types, which allows us to discuss the dissolving cluster properties as a function of the host galaxy characteristics. The properties of the star field background will also be discussed.

Stellar Wind Retention and Expulsion in Massive Star Clusters

NASA Astrophysics Data System (ADS)

Naiman, J. P.; Ramirez-Ruiz, E.; Lin, D. N. C.

2018-05-01

Mass and energy injection throughout the lifetime of a star cluster contributes to the gas reservoir available for subsequent episodes of star formation and the feedback energy budget responsible for ejecting material from the cluster. In addition, mass processed in stellar interiors and ejected as winds has the potential to augment the abundance ratios of currently forming stars, or stars which form at a later time from a retained gas reservoir. Here we present hydrodynamical simulations that explore a wide range of cluster masses, compactnesses, metallicities and stellar population age combinations in order to determine the range of parameter space conducive to stellar wind retention or wind powered gas expulsion in star clusters. We discuss the effects of the stellar wind prescription on retention and expulsion effectiveness, using MESA stellar evolutionary models as a test bed for exploring how the amounts of wind retention/expulsion depend upon the amount of mixing between the winds from stars of different masses and ages. We conclude by summarizing some implications for gas retention and expulsion in a variety of compact (σv ≳ 20 kms-1) star clusters including young massive star clusters (105 ≲ M/M⊙ ≲ 107, age ≲ 500 Myrs), intermediate age clusters (105 ≲ M/M⊙ ≲ 107, age ≈ 1 - 4 Gyrs), and globular clusters (105 ≲ M/M⊙ ≲ 107, age ≳ 10 Gyrs).

Discovery of a loose star cluster in the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Piatti, Andrés E.

2016-06-01

We present results for an up-to-date uncatalogued star cluster projected towards the Eastern side of the Large Magellanic Cloud (LMC) outer disc. The new object was discovered from a search of loose star cluster in the Magellanic Clouds' (MCs) outskirts using kernel density estimators on Washington CT1 deep images. Contrarily to what would be commonly expected, the star cluster resulted to be a young object (log(t yr-1) = 8.45) with a slightly subsolar metal content (Z = 0.013) and a total mass of 650 M⊙. Its core, half-mass and tidal radii also are within the frequent values of LMC star clusters. However, the new star cluster is placed at the Small Magellanic Cloud distance and at 11.3 kpc from the LMC centre. We speculate with the possibility that it was born in the inner body of the LMC and soon after expelled into the intergalactic space during the recent Milky Way/MCs interaction. Nevertheless, radial velocity and chemical abundance measurements are needed to further understand its origin, as well as extensive search for loose star clusters in order to constrain the effectiveness of star cluster scattering during galaxy interactions.

The Clusters AgeS Experiment (CASE). Variable Stars in the Field of the Globular Cluster NGC 6362

NASA Astrophysics Data System (ADS)

Kaluzny, J.; Thompson, I. B.; Rozyczka, M.; Pych, W.; Narloch, W.

2014-12-01

The field of the globular cluster NGC 6362 was monitored between 1995 and 2009 in a search for variable stars. BV light curves were obtained for 69 periodic variable stars including 34 known RR Lyr stars, 10 known objects of other types and 25 newly detected variable stars. Among the latter we identified 18 proper-motion members of the cluster: seven detached eclipsing binaries (DEBs), six SX Phe stars, two W UMa binaries, two spotted red giants, and a very interesting eclipsing binary composed of two red giants - the first example of such a system found in a globular cluster. Five of the DEBs are located at the turnoff region, and the remaining two are redward of the lower main sequence. Eighty-four objects from the central 9×9 arcmin2 of the cluster were found in the region of cluster blue stragglers. Of these 70 are proper motion (PM) members of NGC 6362 (including all SX Phe and two W UMa stars), and five are field stars. The remaining nine objects lacking PM information are located at the very core of the cluster, and as such they are likely genuine blue stragglers.

Zodiacal Exoplanets in Time: Searching for Young Stars in K2

NASA Astrophysics Data System (ADS)

Morris, Nathan Ryan; Mann, Andrew; Rizzuto, Aaron

2018-01-01

Observations of planetary systems around young stars provide insight into the early stages of planetary system formation. Nearby young open clusters such as the Hyades, Pleiades, and Praesepe provide important benchmarks for the properties of stellar systems in general. These clusters are all known to be less than 1 Gyr old, making them ideal targets for a survey of young planetary systems. Few transiting planets have been detected around clusters stars, however, so this alone is too small of a sample. K2, the revived Kepler mission, has provided a vast number of light curves for young stars in clusters and elsewhere in the K2 field. This provides us with the opportunity to extend the sample of young systems to field stars while calibrating with cluster stars. We compute rotational periods from starspot patterns for ~36,000 K2 targets and use gyrochronological relationships derived from cluster stars to determine their ages. From there, we have begun searching for planets around young stars outside the clusters with the ultimate goal of shedding light on how planets and planetary systems evolve in their early, most formative years.

New insights on multiplicity and clustering in Taurus.

NASA Astrophysics Data System (ADS)

Joncour, Isabelle; Duchene, Gaspard; Moraux, Estelle; Mundy, Lee

2018-01-01

Multiplicity and clustering of young stars are critical clues to constraint star formation process. The Taurus molecular complex is the archetype of a quiescent star forming region that may retain primeval signature of star formation.Using statistical and clustering tools such as nearest neighbor statistics, correlation functions and the density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm, this work reveals new spatial substructures in Taurus.We have identified unexpected ultra wide pairs (UWPs) candidates of high order multiplicity in Taurus in the 5-60 kAU separation range (Joncour et al 2017), beyond the separation assessed for wide pairs (Kraus & Hillenbrand 2009).Our work reveals 20 local stellar substructures, the Nested Elementary Structures (NESTs). These NESTs contain nearly half the stars of Taurus and 75% of the Class 0/I objects probing that they are the preferred sites of star formation (Joncour et al, sub.). The NESTs size ranges from few kAU up to 80 kAU making a length scale bridge between wide pairs and loose group (few hundreds kAU, Kirk & Myers, 2011). The NESTs mass ranges from 0.5-10 solar mass. The balance between Class I, II and III in NESTs suggests that they may be ordered as an evolutionary temporal scheme, some of them got infertile, while other shelter stars in infancy.The UWPs and the NESTs may be pristine imprints of their spatial configuration at birth. The UWPs population may result from a cascade fragmentation scenario of the natal molecular core. They could be the older counterparts, to the 0.5 Myr prestellar cores/Class 0 multiple objects observed at radio/millimeter wavelengths (Tobin et al 2010, 2016) and the precursors of the large number of UWPs (10–100 kAU) recently identified in older moving groups (Floriano-Alonso et al, 2015 ; Elliot et al 2016). The NESTs may result from the gravitational collapse of a gas clump that fragments to give a tight collection of stars within few millions years.This project has been partly supported by the StarFormMapper project funded by the European Union's Horizon 2020 Research and Innovation Action (RIA) program under grant agreement number 687528.

Hubble Catches Stellar Exodus in Action

NASA Image and Video Library

2015-05-14

Using NASA’s Hubble Space Telescope, astronomers have captured for the first time snapshots of fledging white dwarf stars beginning their slow-paced, 40-million-year migration from the crowded center of an ancient star cluster to the less populated suburbs. White dwarfs are the burned-out relics of stars that rapidly lose mass, cool down and shut off their nuclear furnaces. As these glowing carcasses age and shed weight, their orbits begin to expand outward from the star cluster’s packed downtown. This migration is caused by a gravitational tussle among stars inside the cluster. Globular star clusters sort out stars according to their mass, governed by a gravitational billiard ball game where lower mass stars rob momentum from more massive stars. The result is that heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to the edge. This process is known as "mass segregation." Until these Hubble observations, astronomers had never definitively seen the dynamical conveyor belt in action. Astronomers used Hubble to watch the white-dwarf exodus in the globular star cluster 47 Tucanae, a dense swarm of hundreds of thousands of stars in our Milky Way galaxy. The cluster resides 16,700 light-years away in the southern constellation Tucana. Credits: NASA, ESA, and H. Richer and J. Heyl (University of British Columbia, Vancouver, Canada); acknowledgement: J. Mack (STScI) and G. Piotto (University of Padova, Italy)

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. IX. NGC 457

NASA Astrophysics Data System (ADS)

Moździerski, D.; Pigulski, A.; Kopacki, G.; Kołaczkowski, Z.; Stęślicki, M.

2014-06-01

We present results of a BVIC variability survey in the young open cluster NGC 457 based on observations obtained during three separate runs spanning almost 20 years. In total, we found 79 variable stars, of which 66 are new. The BVIC photometry was transformed to the standard system and used to derive cluster parameters by means of isochrone fitting. The cluster is about 20 Myr old, the mean reddening amounts to about 0.48 mag in terms of the color excess E(B-V). Depending on the metallicity, the isochrone fitting yields a distance between 2.3 kpc and 2.9 kpc, which locates the cluster in the Perseus arm of the Galaxy. Using the complementary Hα photometry carried out in two seasons separated by over 10 years, we find that the cluster is very rich in Be stars. In total, 15 stars in the observed field of which 14 are cluster members showed Hα in emission either during our observations or in the past. Most of the Be stars vary in brightness on different time scales including short-period variability related most likely to g-mode pulsations. A single-epoch spectrum of NGC 457-6 shows that this Be star is presently in the shell phase. The inventory of variable stars in the observed field consists of a single β Cep-type star, NGC 457-8, 13 Be stars, 21 slowly pulsating B stars, seven δ Sct stars, one γ Dor star, 16 unclassified periodic stars, 8 eclipsing systems and a dozen of stars with irregular variability, of which six are also B-type stars. As many as 45 variable stars are of spectral type B which is the largest number in all open clusters presented in this series of papers. The most interesting is the discovery of a large group of slowly pulsating B stars which occupy the cluster main sequence in the range between V=11 mag and 14.5 mag, corresponding to spectral types B3 to B8. They all have very low amplitudes and about half show pulsations with frequencies higher than 3 d-1. We argue that these are most likely fast-rotating slowly pulsating B stars, observed also in other open clusters.

WFPC2 Observations of Star Clusters in the Magellanic Clouds. Report 2; The Oldest Star Clusters in the Small Magellanic Cloud

NASA Technical Reports Server (NTRS)

Mighell, Kenneth J.; Sarajedini, Ata; French, Rica S.

1998-01-01

We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F45OW ( approximately B) and F555W (approximately V) of the intermediate-age populous star clusters NGC 121, NGC 339, NGC 361, NGC 416, and Kron 3 in the Small Magellanic Cloud. We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these seven populous star clusters in order to improve our understanding of the formation chronology of the SMC. We analyzed the V vs B-V and M(sub V) vs (B-V)(sub 0) color-magnitude diagrams of these populous Small Magellanic Cloud star clusters using a variety of techniques and determined their ages, metallicities, and reddenings. These new data enable us to improve the age-metallicity relation of star clusters in the Small Magellanic Cloud. In particular, we find that a closed-box continuous star-formation model does not reproduce the age-metallicity relation adequately. However, a theoretical model punctuated by bursts of star formation is in better agreement with the observational data presented herein.

Formation of new stellar populations from gas accreted by massive young star clusters.

PubMed

Li, Chengyuan; de Grijs, Richard; Deng, Licai; Geller, Aaron M; Xin, Yu; Hu, Yi; Faucher-Giguère, Claude-André

2016-01-28

Stars in clusters are thought to form in a single burst from a common progenitor cloud of molecular gas. However, massive, old 'globular' clusters--those with ages greater than ten billion years and masses several hundred thousand times that of the Sun--often harbour multiple stellar populations, indicating that more than one star-forming event occurred during their lifetimes. Colliding stellar winds from late-stage, asymptotic-giant-branch stars are often suggested to be triggers of second-generation star formation. For this to occur, the initial cluster masses need to be greater than a few million solar masses. Here we report observations of three massive relatively young star clusters (1-2 billion years old) in the Magellanic Clouds that show clear evidence of burst-like star formation that occurred a few hundred million years after their initial formation era. We show that such clusters could have accreted sufficient gas to form new stars if they had orbited in their host galaxies' gaseous disks throughout the period between their initial formation and the more recent bursts of star formation. This process may eventually give rise to the ubiquitous multiple stellar populations in globular clusters.

The impact of galaxy geometry and mass evolution on the survival of star clusters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Madrid, Juan P.; Hurley, Jarrod R.; Martig, Marie

2014-04-01

Direct N-body simulations of globular clusters in a realistic Milky-Way-like potential are carried out using the code NBODY6 to determine the impact of the host galaxy disk mass and geometry on the survival of star clusters. A relation between disk mass and star-cluster dissolution timescale is derived. These N-body models show that doubling the mass of the disk from 5 × 10{sup 10} M {sub ☉} to 10 × 10{sup 10} M {sub ☉} halves the dissolution time of a satellite star cluster orbiting the host galaxy at 6 kpc from the galactic center. Different geometries in a disk ofmore » identical mass can determine either the survival or dissolution of a star cluster orbiting within the inner 6 kpc of the galactic center. Furthermore, disk geometry has measurable effects on the mass loss of star clusters up to 15 kpc from the galactic center. N-body simulations performed with a fine output time step show that at each disk crossing the outer layers of star clusters experiences an increase in velocity dispersion of ∼5% of the average velocity dispersion in the outer section of star clusters. This leads to an enhancement of mass loss—a clearly discernable effect of disk shocking. By running models with different inclinations, we determine that star clusters with an orbit that is perpendicular to the Galactic plane have larger mass loss rates than do clusters that evolve in the Galactic plane or in an inclined orbit.« less

Hierarchical Star Formation in Turbulent Media: Evidence from Young Star Clusters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Grasha, K.; Calzetti, D.; Elmegreen, B. G.

We present an analysis of the positions and ages of young star clusters in eight local galaxies to investigate the connection between the age difference and separation of cluster pairs. We find that star clusters do not form uniformly but instead are distributed so that the age difference increases with the cluster pair separation to the 0.25–0.6 power, and that the maximum size over which star formation is physically correlated ranges from ∼200 pc to ∼1 kpc. The observed trends between age difference and separation suggest that cluster formation is hierarchical both in space and time: clusters that are closemore » to each other are more similar in age than clusters born further apart. The temporal correlations between stellar aggregates have slopes that are consistent with predictions of turbulence acting as the primary driver of star formation. The velocity associated with the maximum size is proportional to the galaxy’s shear, suggesting that the galactic environment influences the maximum size of the star-forming structures.« less

Do All O Stars Form in Star Clusters?

NASA Astrophysics Data System (ADS)

Weidner, C.; Gvaramadze, V. V.; Kroupa, P.; Pflamm-Altenburg, J.

The question whether or not massive stars can form in isolation or only in star clusters is of great importance for the theory of (massive) star formation as well as for the stellar initial mass function of whole galaxies (IGIMF-theory). While a seemingly easy question it is rather difficult to answer. Several physical processes (e.g. star-loss due to stellar dynamics or gas expulsion) and observational limitations (e.g. dust obscuration of young clusters, resolution) pose severe challenges to answer this question. In this contribution we will present the current arguments in favour and against the idea that all O stars form in clusters.

History of Hubble Space Telescope (HST)

NASA Image and Video Library

2001-09-06

Scientists using NASA's Hubble Space Telescope (HST) are studying the colors of star clusters to determine the age and history of starburst galaxies, a technique somewhat similar to the process of learning the age of a tree by counting its rings. One such galaxy, Galaxy NGC 3310, a hotbed of star formation showcased in this HST photograph, is forming clusters of stars at a prodigious rate. The image shows several hundred star clusters, visible as the bright blue diffuse objects tracing the galaxy's spiral arms. Each of these star clusters represents the formation of up to about a million stars, a process that takes less than 100,000 years. In addition, hundreds of individual young stars can be seen throughout the galaxy. The star clusters become redder with age as the most massive and bluest stars exhaust their fuel and burn out. Measurements in this image of the wide range of cluster colors show their ages range between about one million and more that one hundred million years. This suggests the starburst "turned on" more than 100 million years ago.

Nature vs. Nurture: The influence of OB star environments on proto-planetary disk evolution

NASA Astrophysics Data System (ADS)

Bouwman, Jeroen

2006-09-01

We propose a combined IRAC/IRS study of a large, well-defined and unbiased X-ray selected sample of pre-main-sequence stars in three OB associations: Pismis 24 in NGC 6357, NGC 2244 in the Rosette Nebula, and IC 1795 in the W3 complex. The samples are based on recent Chandra X-ray Observatory studies which reliably identify hundreds of cluster members and were carefully chosen to avoid high infrared nebular background. A new Chandra exposure of IC 1795 is requested, and an optical followup to characterise the host stars is planned.

Dynamics and stellar population of the Galactic Center (French Title: Étude de la cinématique et de la population stellaire du Centre Galactique)

NASA Astrophysics Data System (ADS)

Paumard, Thibaut

2003-09-01

The central parsec of the Galaxy has been observed using BEAR spectroimagery at high spectral resolution (up to 21 km/s) and medium spatial resolution (0.5"), in Bracket gamma (2.16 microns) and He I (2.06 microns), and high resolution imaging. These data were used to study the young, massive stars of the central parsec, and the structure and dynamics of ionized gas in Sgr A West. The stellar population has been separated into two groups: the IRS 16 complex of 6 LBVs, and at least 20 Wolf-Rayets. The IRS 13E complex has been identified as a cluster of at least 6 massive stars. All this is consistent with the young stars being born in a massive cluster a few tens of parsecs from the Galactic Centre. Providing a deep insight into the morphology of Sgr A West, our data allowed us to derive a kinematic model for the Northern Arm. Our results are in agreement with the idea that the Minispiral is made of ionisation fronts of wider neutral clouds, gravitationally stretched, coming from the CND.

EXTENDED STAR FORMATION IN THE INTERMEDIATE-AGE LARGE MAGELLANIC CLOUD STAR CLUSTER NGC 2209

DOE Office of Scientific and Technical Information (OSTI.GOV)

Keller, Stefan C.; Mackey, A. Dougal; Da Costa, Gary S.

2012-12-10

We present observations of the 1 Gyr old star cluster NGC 2209 in the Large Magellanic Cloud made with the GMOS imager on the Gemini South Telescope. These observations show that the cluster exhibits a main-sequence turnoff that spans a broader range in luminosity than can be explained by a single-aged stellar population. This places NGC 2209 amongst a growing list of intermediate-age (1-3 Gyr) clusters that show evidence for extended or multiple epochs of star formation of between 50 and 460 Myr in extent. The extended main-sequence turnoff observed in NGC 2209 is a confirmation of the prediction inmore » Keller et al. made on the basis of the cluster's large core radius. We propose that secondary star formation is a defining feature of the evolution of massive star clusters. Dissolution of lower mass clusters through evaporation results in only clusters that have experienced secondary star formation surviving for a Hubble time, thus providing a natural connection between the extended main-sequence turnoff phenomenon and the ubiquitous light-element abundance ranges seen in the ancient Galactic globular clusters.« less

Fingerprinting the Milky Way

NASA Astrophysics Data System (ADS)

2007-03-01

Using ESO's Very Large Telescope, an international team of astronomers has shown how to use the chemical composition of stars in clusters to shed light on the formation of our Milky Way. This discovery is a fundamental test for the development of a new chemical tagging technique uncovering the birth and growth of our Galactic cradle. The formation and evolution of galaxies, and in particular of the Milky Way - the 'island universe' in which we live, is one of the major puzzles of astrophysics: indeed, a detailed physical scenario is still missing and its understanding requires the joint effort of observations, theories and complex numerical simulations. ESO astronomer Gayandhi De Silva and her colleagues used the Ultraviolet and Visual Echelle Spectrograph (UVES) on ESO's VLT to find new ways to address this fundamental riddle. ESO PR Photo 15/07 ESO PR Photo 15/07 The Cluster Collinder 261 "We have analysed in great detail the chemical composition of stars in three star-clusters and shown that each cluster presents a high level of homogeneity and a very distinctive chemical signature," says De Silva, who started this research while working at the Mount Stromlo Observatory, Australia. "This paves the way to chemically tagging stars in our Galaxy to common formation sites and thus unravelling the history of the Milky Way," she adds. "Galactic star clusters are witnesses of the formation history of the Galactic disc," says Kenneth Freeman, also from Mount Stromlo and another member of the team. "The analysis of their composition is like studying ancient fossils. We are chasing pieces of galactic DNA!" Open star clusters are among the most important tools for the study of stellar and galactic evolution. They are composed of a few tens up to a few thousands of stars that are gravitationally bound, and they span a wide range of ages. The youngest date from a few million years ago, while the oldest (and more rare) can have ages up to ten billion years. The well-known Pleiades, also called the Seven Sisters, is a young bright open cluster. Conversely, Collinder 261, which was the target of the present team of astronomers, is among the oldest. It can therefore provide useful information on the early days in the existence of our Galaxy. The astronomers used UVES to observe a dozen red giants in the open cluster Collinder 261, located about 25,000 light years from the Galactic Centre. Giants are more luminous, hence they are well suited for high-precision measurements. From these observations, the abundances of a large set of chemical elements could be determined for each star, demonstrating convincingly that all stars in the cluster share the same chemical signature. "This high level of homogeneity indicates that the chemical information survived through several billion years," explains De Silva. "Thus all the stars in the cluster can be associated to the same prehistoric cloud. This corroborates what we had found for two other groups of stars." But this is not all. A comparison with the open cluster called the Hyades, and the group of stars moving with the bright star HR 1614, shows that each of them contains the same elements in different proportions. This indicates that each star cluster formed in a different primordial region, from a different cloud with a different chemical composition. "The consequences of these observations are thrilling," says Freeman. "The ages of open clusters cover the entire life of the Galaxy and each of them is expected to originate from a different patch of 'dough'. Seeing how much sodium, magnesium, calcium, iron and many other elements are present in each star cluster, we are like accurate cooks who can tell the amount of salt, sugar, eggs and flour used in different cookies. Each of them has a unique chemical signature." The astronomers will now aim to measure the chemical abundances in a larger sample of open clusters. Once the "DNA" of each star cluster is inferred, it will be possible to trace the genealogic tree of the Milky Way. This chemical mapping through time and space will be a way to test theoretical models. "The path to an extensive use of chemical tagging is still long," cautions De Silva, "but our study shows that it is possible. When the technique is tested and proven we will be able to get a detailed picture of the way our Galactic cradle formed." More Information The research presented here is discussed in a paper in the Astronomical Journal, volume 133, pages 1161-1175 ("Chemical homogeneity in Collinder 261 and implications for chemical tagging", by G.M. De Silva et al.). The team is composed of Gayandhi De Silva (ESO), Kenneth Freeman, Martin Asplund and Michael Bessell (Mount Stromlo Observatory, Australia), Joss Bland-Hawthorn (Anglo-Australian Observatory, Australia), Remo Collet (Uppsala University, Sweden).

Clustering analysis of line indices for LAMOST spectra with AstroStat

NASA Astrophysics Data System (ADS)

Chen, Shu-Xin; Sun, Wei-Min; Yan, Qi

2018-06-01

The application of data mining in astronomical surveys, such as the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey, provides an effective approach to automatically analyze a large amount of complex survey data. Unsupervised clustering could help astronomers find the associations and outliers in a big data set. In this paper, we employ the k-means method to perform clustering for the line index of LAMOST spectra with the powerful software AstroStat. Implementing the line index approach for analyzing astronomical spectra is an effective way to extract spectral features for low resolution spectra, which can represent the main spectral characteristics of stars. A total of 144 340 line indices for A type stars is analyzed through calculating their intra and inter distances between pairs of stars. For intra distance, we use the definition of Mahalanobis distance to explore the degree of clustering for each class, while for outlier detection, we define a local outlier factor for each spectrum. AstroStat furnishes a set of visualization tools for illustrating the analysis results. Checking the spectra detected as outliers, we find that most of them are problematic data and only a few correspond to rare astronomical objects. We show two examples of these outliers, a spectrum with abnormal continuumand a spectrum with emission lines. Our work demonstrates that line index clustering is a good method for examining data quality and identifying rare objects.

Chemical Complexity in the Eu-enhanced Monometallic Globular NGC 5986

NASA Astrophysics Data System (ADS)

Johnson, Christian I.; Caldwell, Nelson; Rich, R. Michael; Mateo, Mario; Bailey, John I., III; Olszewski, Edward W.; Walker, Matthew G.

2017-06-01

NGC 5986 is a poorly studied but relatively massive Galactic globular cluster that shares several physical and morphological characteristics with “iron-complex” clusters known to exhibit significant metallicity and heavy-element dispersions. In order to determine whether NGC 5986 joins the iron-complex cluster class, we investigated the chemical composition of 25 red giant branch and asymptotic giant branch cluster stars using high-resolution spectra obtained with the Magellan-M2FS instrument. Cluster membership was verified using a combination of radial velocity and [Fe/H] measurements, and we found the cluster to have a mean heliocentric radial velocity of +99.76 km s-1 (σ = 7.44 km s-1). We derived a mean metallicity of [Fe/H] = -1.54 dex (σ = 0.08 dex), but the cluster’s small dispersion in [Fe/H] and low [La/Eu] abundance preclude it from being an iron-complex cluster. NGC 5986 has < [{Eu}/{Fe}]> =+0.76 {dex} (σ = 0.08 dex), which is among the highest ratios detected in a Galactic cluster, but the small [Eu/Fe] dispersion is puzzling because such high values near [Fe/H] ˜ -1.5 are typically only found in dwarf galaxies exhibiting large [Eu/Fe] variations. NGC 5986 exhibits classical globular cluster characteristics, such as uniformly enhanced [α/Fe] ratios, a small dispersion in Fe-peak abundances, and (anti)correlated light-element variations. Similar to NGC 2808, we find evidence that NGC 5986 may host at least four to five populations with distinct light-element compositions, and the presence of a clear Mg-Al anticorrelation along with an Al-Si correlation suggests that the cluster gas experienced processing at temperatures ≳65-70 MK. However, the current data do not support burning temperatures exceeding ˜100 MK. We find some evidence that the first- and second-generation stars in NGC 5986 may be fully spatially mixed, which could indicate that the cluster has lost a significant fraction of its original mass. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Hidden Milky Way star clusters hosting Wolf-Rayet stars

NASA Astrophysics Data System (ADS)

Kurtev, R.; Borissova, J.; Ivanov, V. D.; Georgiev, L.

2009-05-01

A noticeable fraction of the hidden young star clusters contain WR and O stars providing us with unique laboratories to study the evolution of these rare objects and their maternity places. We are reporting the reddening, the distance and age of two new members of the family of massive young Galactic clusters, hosting WR stars - Glimpse 23 and Glimpse 30.

Young Star Clusters: Keys to Understanding Massive Stars

NASA Astrophysics Data System (ADS)

Davies, B.

2012-12-01

Young, coeval clusters of stars provide the perfect laboratory in which to test our understanding of how massive stars evolve. Early optical observations limited us to a handful of low-mass clusters within 1kpc. However, thanks to the recent progress in infrared astronomy, the Milky Way's population of young massive star clusters is now beginning to be revealed. Here, I will review the recent progress made in this field, what it has told us about the evolution of massive stars to supernova and beyond, the prospects for this field, and some issues that should be taken into account when interpreting the results.

The Role of Evolutionary Age and Metallicity in the Formation of Classical BE Circumstellar Disks II. Assessing the True Nature of Candidate Disk Systems

NASA Technical Reports Server (NTRS)

Wisniewski, J. P.; Bjorkman, K. S.; Magalhaes, A. M.; Bjorkman, J. E.; Meade, M. R.; Pereyra, Antonio

2007-01-01

Photometric 2-color diagram (2-CD) surveys of young cluster populations have been used to identify populations of B-type stars exhibiting excess Ha emission. The prevalence of these excess emitters, assumed to be "Be stars". has led to the establishment of links between the onset of disk formation in classical Be stars and cluster age and/or metallicity. We have obtained imaging polarization observations of six SMC and six LMC clusters whose candidate Be populations had been previously identified via 2-CDs. The interstellar polarization (ISP) associated with these data has been identified to facilitate an examination of the circumstellar environments of these candidate Be stars via their intrinsic polarization signatures, hence determine the true nature of these objects. We determined that the ISP associated with the SMC cluster NGC 330 was characterized by a modified Serkowski law with a lambda(sub max) of approx. 4500Angstroms, indicating the presence of smaller than average dust grains. The morphology of the ISP associated with the LMC cluster NGC 2100 suggests that its interstellar environment is characterized by a complex magnetic field. Our intrinsic polarization results confirm the suggestion of Wisniewski et al. that a substantial number of bona-fide classical Be stars are present in clusters of age 5-8 Myr. Hence, our data contradict recent assertions that the Be phenomenon develops in the second half of a B star's main sequence lifetime, i.e. no earlier than 10 Myr. These data imply that a significant number of B-type stars must emerge onto the zero-age-main-sequence rotating at near-critical rotation rates, although we can not rule out the possibility that these data instead reveal the presence of a sub-group of the Be phenomenon characterized by sub-critically rotating objects. Comparing the polarimetric properties of our dataset to a similar survey of Galactic classical Be stars, we find that the prevalence of polarimetric Balmer jump signatures decreases with metallicity. We speculate that these results might indicate that either it is more difficult to form large disk systems in low metallicity environments, or that the average disk temperature is higher in these low metallicity environments. We have characterized the polarimetric signatures of all candidate Be stars in our data sample and find approx. 25% are unlikely to arise from true classical Be star-disk systems. This detection of such a substantial number "contaminants" suggests one should proceed with caution when attempting to determine the role of evolutionary age and/or metallicity in the Be phenomenon purely via 2-CD results.

The NGC 281 west cluster. I. Star formation in photoevaporating clumps.

NASA Astrophysics Data System (ADS)

Megeath, S. T.; Wilson, T. L.

1997-09-01

The NGC281 West molecular cloud is an excellent test case for studying star formation in the clumpy interface between a \\hii region and a giant molecular cloud. We present here a study based on new high resolution radio and near-infrared data. Using the IRAM 30-meter telescope, we have mapped the interface in the \\cotwo, \\coone, and \\cs transitions with FWHP beamwidths <= 22''. We have imaged the same region with the VLA in the 20, 6 and 2 cm continuum bands to obtain complementary maps of the ionized gas distribution with angular resolutions <= 13''. In addition, we have obtained near-infrared J and K'-band images to detect young stars in the interface. The 30-meter data shows the molecular gas is concentrated into three clumps with masses of 570, > 210, and 300 \\msun and average volume densities of 1.4, >1, and 2 x 10(4) \\cm. We detect \\cs emission in two of the clumps, indicating peak densities in excess of 5x 10(5) \\cm are attained in the clumps. A comparison of the \\co line data with the 20 cm continuum image suggests that the molecular clumps are being photoevaporated through their direct exposure to the UV radiation from neighboring OB stars. The luminosity and extent of the observed 20 cm emission is in good agreement with theoretical predictions. We use models of photoevaporative flows to estimate the pressure exerted on the clumps by the ionized gas and find that it exceeds the internal, turbulent pressure of the clumps by a factor of a 2.5. Although a pressure equilibrium is not excluded given the uncertainties inherent in determining the pressures of the ionized and molecular gases, our best estimates of the clumps and flow parameters favor the the existence of low velocity shocks (1.5 \\kms) in the clumps. The clumps exhibit broad, non-gaussian lineshapes and complex kinematical structures suggestive of shocks. Further evidence for shocks is found in a comparison of position-velocity diagrams with published numerical simulations of imploding spherical clumps. We discuss the possibility that the knots of \\cs emission may trace gas compressed by converging shock waves. The K'-band observations show a rich cluster of primarily low mass stars in the \\hii/molecular interface, which we argue is divided into two distinct sub-clusters. We associate one sub-cluster with the two clumps nearest the OB stars, and the second sub-cluster with the third clump. The two clumps nearest the OB stars contain an embedded population, suggesting that star formation is ongoing. We discuss the impact photoevaporation is having on star formation in these two clumps. We find that photoevaporation is dispersing the molecular gas from which the cluster is forming and estimate that the molecular gas will be completely evaporated in 5 Myr. Deep K'-band imaging of the two clumps show that the stars are detected primarily on the sides of the clumps facing the OB stars and in the adjoining \\hii region. We examine three explanations for this asymmetry: displacement of the clump centers from the cluster center by the acceleration of the molecular gas through photoevaporation (i.e. the rocket effect), unveiling of young, embedded stars by ionization-shock fronts, and the triggered formation of stars by shocks advancing into the clumps. If shock compression is indeed ongoing in the clumps, then we argue that there is a good case for shock triggered star formation.

MASGOMAS project: building a bona-fide catalog of massive star cluster candidates

NASA Astrophysics Data System (ADS)

Herrero, Artemio; Rübke, Klaus; Ramírez Alegría, Sebastián; Garcia, Miriam; Marín-Franch, Antonio

2017-11-01

MASGOMAS (MAssive Stars in Galactic Obscured MAssive clusterS) is a project aiming at discovering OB stars in Galactic, dust enshrouded, star-forming massive clusters (Marín-Franch et al. 2009, A&A 502, 559). The project has gone through different phases of increasing automatization, that have allowed us to discover massive clusters like MASGOMAS-1 (Ramírez Alegría et al. 2012, A&A 541, A75) (with M~20,000 M⊙).

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schuetrumpf, Bastian; Zhang, Chunli; Nazarewicz, Witold

Nuclear density functional theory is the tool of choice in describing properties of complex nuclei and intricate phases of bulk nucleonic matter. It is a microscopic approach based on an energy density functional representing the nuclear interaction. An attractive feature of nuclear DFT is that it can be applied to both finite nuclei and pasta phases appearing in the inner crust of neutron stars. While nuclear pasta clusters in a neutron star can be easily characterized through their density distributions, the level of clustering of nucleons in a nucleus can often be difficult to assess. To this end, we usemore » the concept of nucleon localization. We demonstrate that the localization measure provides us with fingerprints of clusters in light and heavy nuclei, including fissioning systems. Furthermore we investigate the rod-like pasta phase using twist-averaged boundary conditions, which enable calculations in finite volumes accessible by state of the art DFT solvers.« less

Climbing the Ladder of Star Formation Feedback

NASA Astrophysics Data System (ADS)

Frank, Adam

2012-10-01

While much is understood about isolated star formation, the opposite is true for star formation in clusters of both low and high mass. In particular the mechanisms by which many coevally formed stars affect their parent cloud environment remains poorly characterized. Fundamental questions such as interplay between multiple outflows, ionization fronts and turbulence are just beginning to be fully articulated. Distinguishing between the nature of feedback in clusters of different mass is also critical. In high mass clusters O stars are expected to dominate energetics while in low mass clusters multiple collimated outflows may represent the dominant feedback mechanism. Thus the issue of feedback modalities in clusters of different masses represents one of the major challenges to the next generation of star formation studies. In this proposal we seek to carry forward a focused theoretical study of feedback in both low and high-mass cluster environments with direct connections to observations. Using a state-of-the-art Adaptive Mesh Refinement MHD multi-physics code {developed by our group} we propose two computational studies: {1} multiple, interacting outflows and their role in altering the properties of a parent low mass cluster {2} Poorly collimated outburst/outflows from massive star{s} and their effect on high mass cluster star forming environments. In both cases we will use initial conditions derived from high-resolution AMR MHD simulations of cloud/cluster formation. Synthetic observations derived from the simulations {in a variety of emission lines from ions to atoms to molecules} will allow for direct contact with HST and other star formation databases.

Topological analysis of long-chain branching patterns in polyolefins.

PubMed

Bonchev, D; Markel, E; Dekmezian, A

2001-01-01

Patterns in molecular topology and complexity for long-chain branching are quantitatively described. The Wiener number, the topological complexity index, and a new index of 3-starness are used to quantify polymer structure. General formulas for these indices were derived for the cases of 3-arm star, H-shaped, and B-arm comb polymers. The factors affecting complexity in monodisperse polymer systems are ranked as follows: number of arms > arm length > arm central position approximately equal to arm clustering > total molecular weight approximately equal to backbone molecular weight. Topological indices change rapidly and then plateau as the molecular weight of branches on a polyolefin backbone increases from 0 to 5 kD. Complexity calculations relate 2-arm or 3-arm comb structures to the corresponding 3-arm stars of equivalent complexity but much higher molecular weight. In a subsequent paper, we report the application of topological analysis for developing structure/property relationships for monodisperse polymers. While the focus of the present work is on the description of monodisperse, well-defined architectures, the methods may be extended to the description of polydisperse systems.

Stellar age spreads in clusters as imprints of cluster-parent clump densities

DOE Office of Scientific and Technical Information (OSTI.GOV)

Parmentier, G.; Grebel, E. K.; Pfalzner, S.

2014-08-20

It has recently been suggested that high-density star clusters have stellar age distributions much narrower than that of the Orion Nebula Cluster, indicating a possible trend of narrower age distributions for denser clusters. We show this effect to likely arise from star formation being faster in gas with a higher density. We model the star formation history of molecular clumps in equilibrium by associating a star formation efficiency per free-fall time, ε{sub ff}, to their volume density profile. We focus on the case of isothermal spheres and we obtain the evolution with time of their star formation rate. Our modelmore » predicts a steady decline of the star formation rate, which we quantify with its half-life time, namely, the time needed for the star formation rate to drop to half its initial value. Given the uncertainties affecting the star formation efficiency per free-fall time, we consider two distinct values: ε{sub ff} = 0.1 and ε{sub ff} = 0.01. When ε{sub ff} = 0.1, the half-life time is of the order of the clump free-fall time, τ{sub ff}. As a result, the age distributions of stars formed in high-density clumps have smaller full-widths at half-maximum than those of stars formed in low-density clumps. When the star formation efficiency per free-fall time is 0.01, the half-life time is 10 times longer, i.e., 10 clump free-fall times. We explore what happens if the duration of star formation is shorter than 10τ{sub ff}, that is, if the half-life time of the star formation rate cannot be defined. There, we build on the invariance of the shape of the young cluster mass function to show that an anti-correlation between the clump density and the duration of star formation is expected. We therefore conclude that, regardless of whether the duration of star formation is longer than the star formation rate half-life time, denser molecular clumps yield narrower star age distributions in clusters. Published densities and stellar age spreads of young clusters and star-forming regions actually suggest that the timescale for star formation is of order 1-4τ{sub ff}. We also discuss how the age bin size and uncertainties in stellar ages affect our results. We conclude that there is no need to invoke the existence of multiple cluster formation mechanisms to explain the observed range of stellar age spreads in clusters.« less

Multiwavelength Studies of Young OB Associations

NASA Astrophysics Data System (ADS)

Feigelson, Eric D.

We discuss how contemporary multiwavelength observations of young OB-dominated clusters address long-standing astrophysical questions: Do clusters form rapidly or slowly with an age spread? When do clusters expand and disperse to constitute the field star population? Do rich clusters form by amalgamation of smaller subclusters? What is the pattern and duration of cluster formation in massive star forming regions (MSFRs)? Past observational difficulties in obtaining good stellar censuses of MSFRs have been alleviated in recent studies that combine X-ray and infrared surveys to obtain rich, though still incomplete, censuses of young stars in MSFRs. We describe here one of these efforts, the MYStIX project, that produced a catalog of 31,784 probable members of 20 MSFRs. We find that age spread within clusters is real in the sense that the stars in the core formed after the cluster halo. This is consistent with some recent astrophysical models involving merging star-forming filaments. Cluster expansion is seen in the ensemble of (sub)clusters, and older dispersing populations are found across MSFRs. Long-lived, asynchronous star formation is pervasive across MSFRs.

GHRS observations and theoretical modeling of early type stars in R136a

NASA Astrophysics Data System (ADS)

de Koter, A.; Heap, S.; Hubeny, I.; Lanz, T.; Hutchings, J.; Lamers, H. J. G. L. M.; Maran, S.; Schmutz, W.

1994-05-01

We present the first spectroscopic observations of individual stars in R136a, the most dense part of the starburst cluster 30 Doradus in the LMC. Spectra of two stars are scheduled to be obtained with the GHRS on board the HST: R136a5, the brightest of the complex and R136a2, a Wolf-Rayet star of type WN. The 30 Doradus cluster is the only starburst region in which individual stars can be studied. Therefore, quantitative knowledge of the basic stellar parameters will yield valuable insight into the formation of massive stars in starbursts and into their subsequent evolution. Detailed modeling of the structure of the atmosphere and wind of these stars will also lead to a better understanding of the mechanism(s) that govern their dynamics. We present the first results of our detailed quantitative spectral analysis using state-of-the-art non-LTE model atmospheres for stars with extended and expanding atmospheres. The models are computed using the Improved-Sobolev Approximation wind code (ISA-WIND) of de Koter, Schmutz & Lamers (1993, A&A 277, 561), which has been extended to include C, N and Si. Our model computations are not based on the core-halo approximation, but use a unified treatment of the photosphere and wind. This approach is essential for Wolf-Rayet stars. Our synthetic spectra, dominated by the P Cygni profiles of the UV resonance lines, also account for the numerous weak metal lines of photospheric origin.

Legacy ExtraGalactic UV Survey (LEGUS): The HST View of Star Formation in Nearby Galaxies

NASA Astrophysics Data System (ADS)

Calzetti, Daniela; Lee, J. C.; Adamo, A.; Aloisi, A.; Andrews, J. E.; Brown, T. M.; Chandar, R.; Christian, C. A.; Cignoni, M.; Clayton, G. C.; Da Silva, R. L.; de Mink, S. E.; Dobbs, C.; Elmegreen, B.; Elmegreen, D. M.; Evans, A. S.; Fumagalli, M.; Gallagher, J. S.; Gouliermis, D.; Grebel, E.; Herrero-Davo`, A.; Hilbert, B.; Hunter, D. A.; Johnson, K. E.; Kennicutt, R.; Kim, H.; Krumholz, M. R.; Lennon, D. J.; Martin, C. D.; Nair, P.; Nota, A.; Pellerin, A.; Prieto, J.; Regan, M. W.; Sabbi, E.; Schaerer, D.; Schiminovich, D.; Smith, L. J.; Thilker, D. A.; Tosi, M.; Van Dyk, S. D.; Walterbos, R. A.; Whitmore, B. C.; Wofford, A.

2014-01-01

The Treasury program LEGUS (HST/GO-13364) is the first HST UV Atlas of nearby galaxies, and is aimed at the thorough investigation of star formation and its relation with galaxy environment, from the scales of individual stars to those of ~kpc clustered structures. The 154-orbits program is obtaining NUV,U,B,V,I images of 50 star-forming galaxies in the distance range 4-12 Mpc, covering the full range of morphology, star formation rate (SFR), mass, metallicity, internal structure, and interaction state found in the local Universe. The imaging survey will yield accurate recent (<50 Myr) star formation histories (SFHs) from resolved massive stars, and the extinction-corrected ages and masses of star clusters and associations. These extensive inventories of massive stars, clustered systems, and SFHs will be used to: (1) quantify how the clustering of star formation evolves both in space and in time; (2) discriminate among models of star cluster evolution; (3) investigate the effects of SFH on the UV SFR calibrations; (4) explore the impact of environment on star formation and cluster evolution across the full range of galactic and ISM properties. LEGUS observations will inform theories of star formation and galaxy evolution, and improve the understanding of the physical underpinning of the gas-star formation relation and the nature of the clumpy star formation at high redshift. LEGUS will generate the most homogeneous high-resolution, wide-field UV dataset to date, building and expanding on the GALEX legacy. Data products that will be delivered to the community include: catalogs of massive stars and star clusters, catalogs of star cluster properties (ages, masses, extinction), and a one-stop shop for all the ancillary data available for this well-studied galaxy sample. LEGUS will provide the reference survey and the foundation for future observations with JWST and with ALMA. This abstract accompanies another one from the same project, and presents the status of the project, its structure, and the data products that will be delivered to the community; the other abstract presents the science goals of LEGUS and how these will be addressed by the HST observations.

Highlights of Commission 37 Science Results

NASA Astrophysics Data System (ADS)

Carraro, Giovanni; de Grijs, Richard; Elmegreen, Bruce; Stetson, Peter; Anthony-Twarog, Barbara; Goodwin, Simon; Geisler, Douglas; Minniti, Dante

2016-04-01

It is widely accepted that stars do not form in isolation but result from the fragmentation of molecular clouds, which in turn leads to star cluster formation. Over time, clusters dissolve or are destroyed by interactions with molecular clouds or tidal stripping, and their members become part of the general field population. Star clusters are thus among the basic building blocks of galaxies. In turn, star cluster populations, from young associations and open clusters to old globulars, are powerful tracers of the formation, assembly, and evolutionary history of their parent galaxies. Although their importance (e.g., in mapping out the Milky Way) had been recognised for decades, major progress in this area has only become possible in recent years, both for Galactic and extragalactic cluster populations. Star clusters are the observational foundation for stellar astrophysics and evolution, provide essential tracers of galactic structure, and are unique stellar dynamical environments. Star formation, stellar structure, stellar evolution, and stellar nucleosynthesis continue to benefit and improve tremendously from the study of these systems. Additionally, fundamental quantities such as the initial mass function can be successfully derived from modelling either the Hertzsprung-Russell diagrams or the integrated velocity structures of, respectively, resolved and unresolved clusters and cluster populations. Star cluster studies thus span the fields of Galactic and extragalactic astrophysics, while heavily affecting our detailed understanding of the process of star formation in dense environments. This report highlights science results of the last decade in the major fields covered by IAU Commission 37: Star clusters and associations. Instead of focusing on the business meeting - the out-going president presentation can be found here: http://www.sc.eso.org/gcarraro/splinter2015.pdf - this legacy report contains highlights of the most important scientific achievements in the Commission science area, compiled by 5 well expert members.

Radiation hydrodynamics of super star cluster formation

NASA Astrophysics Data System (ADS)

Tsang, Benny Tsz Ho; Milos Milosavljevic

2018-01-01

Throughout the history of the Universe, the nuclei of super star clusters represent the most active sites for star formation. The high densities of massive stars within the clusters produce intense radiation that imparts both energy and momentum on the surrounding star-forming gas. Theoretical claims based on idealized geometries have claimed the dominant role of radiation pressure in controlling the star formation activity within the clusters. In order for cluster formation simulations to be reliable, numerical schemes have to be able to model accurately the radiation flows through the gas clumps at the cluster nuclei with high density contrasts. With a hybrid Monte Carlo radiation transport module we developed, we performed 3D radiation hydrodynamical simulations of super star cluster formation in turbulent clouds. Furthermore, our Monte Carlo radiation treatment provides a native capability to produce synthetic observations, which allows us to predict observational indicators and to inform future observations. We found that radiation pressure has definite, but minor effects on limiting the gas supply for star formation, and the final mass of the most massive cluster is about one million solar masses. The ineffective forcing was due to the density variations inside the clusters, i.e. radiation takes the paths of low densities and avoids forcing on dense clumps. Compared to a radiation-free control run, we further found that the presence of radiation amplifies the density variations. The core of the resulting cluster has a high stellar density, about the threshold required for stellar collisions and merging. The very massive star that form from the stellar merging could continue to gain mass from the surrounding gas reservoir that is gravitationally confined by the deep potential of the cluster, seeding the potential formation of a massive black hole.

Core Collapse: The Race Between Stellar Evolution and Binary Heating

NASA Astrophysics Data System (ADS)

Converse, Joseph M.; Chandar, R.

2012-01-01

The dynamical formation of binary stars can dramatically affect the evolution of their host star clusters. In relatively small clusters (M < 6000 Msun) the most massive stars rapidly form binaries, heating the cluster and preventing any significant contraction of the core. The situation in much larger globular clusters (M 105 Msun) is quite different, with many showing collapsed cores, implying that binary formation did not affect them as severely as lower mass clusters. More massive clusters, however, should take longer to form their binaries, allowing stellar evolution more time to prevent the heating by causing the larger stars to die off. Here, we simulate the evolution of clusters between those of open and globular clusters in order to find at what size a star cluster is able to experience true core collapse. Our simulations make use of a new GPU-based computing cluster recently purchased at the University of Toledo. We also present some benchmarks of this new computational resource.

The astrophysics of crowded places.

PubMed

Davies, Melvyn

2002-12-15

Today the Sun is in a relatively uncrowded place. The distance between it and the nearest other star is relatively large (about 200,000 times the Earth-Sun distance!). This is beneficial to life on Earth; a close encounter with another star is extremely unlikely. Such encounters would either remove the Earth from its orbit around the Sun or leave it on an eccentric orbit similar to a comet's. But the Sun was not formed in isolation. It was born within a more-crowded cluster of perhaps a few hundred stars. As the surrounding gas evaporated away, the cluster itself evaporated too, dispersing its stars into the Galaxy. Virtually all stars in the Galaxy share this history, and here I will describe the role of 'clusterness' in a star's life. Stars are often formed in larger stellar clusters (known as open and globular clusters), some of which are still around today. I will focus on stars in globular clusters and describe how the interactions between stars in these clusters may explain the zoo of stellar exotica which have recently been observed with instruments such as the Hubble Space Telescope and the X-ray telescopes XMM-Newton and Chandra. In recent years, myriad planets orbiting stars other than the Sun--the so-called 'extrasolar' planets--have been discovered. I will describe how a crowded environment will affect such planetary systems and may in fact explain some of their mysterious properties.

Dust-enshrouded super star-clusters

NASA Astrophysics Data System (ADS)

Sauvage, Marc; Plante, Stéphanie

2003-04-01

With the advent of either sensitive space-born infrared cameras, or their high-resolution ground-based siblings, we are uncovering a new category of star clusters: the dust-enshrouded super-star clusters. These manifest themselves only beyond a few microns, as their shroud of dust is able to block all light emitted by the stars themselves. Here we present our results on the spectacular cluster in SBS 0335-052, a very metal-poor galaxy. We also point to the growing number of galaxy analogs to SBS 0335-052, revealing the possibility that these clusters signal a major mode of star formation in starbursts. We conclude by listing a number of open points these clusters raise, in particular with respect to high-redshift counterparts.

Extended main sequence turnoffs in intermediate-age star clusters: a correlation between turnoff width and early escape velocity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goudfrooij, Paul; Kozhurina-Platais, Vera; Kalirai, Jason S.

2014-12-10

We present a color-magnitude diagram analysis of deep Hubble Space Telescope imaging of a mass-limited sample of 18 intermediate-age (1-2 Gyr old) star clusters in the Magellanic Clouds, including eight clusters for which new data were obtained. We find that all star clusters in our sample feature extended main-sequence turnoff (eMSTO) regions that are wider than can be accounted for by a simple stellar population (including unresolved binary stars). FWHM widths of the MSTOs indicate age spreads of 200-550 Myr. We evaluate the dynamical evolution of clusters with and without initial mass segregation. Our main results are (1) the fractionmore » of red clump (RC) stars in secondary RCs in eMSTO clusters scales with the fraction of MSTO stars having pseudo-ages of ≲1.35 Gyr; (2) the width of the pseudo-age distributions of eMSTO clusters is correlated with their central escape velocity v {sub esc}, both currently and at an age of 10 Myr. We find that these two results are unlikely to be reproduced by the effects of interactive binary stars or a range of stellar rotation velocities. We therefore argue that the eMSTO phenomenon is mainly caused by extended star formation within the clusters; and (3) we find that v {sub esc} ≥ 15 km s{sup –1} out to ages of at least 100 Myr for all clusters featuring eMSTOs, and v {sub esc} ≤ 12 km s{sup –1} at all ages for two lower-mass clusters in the same age range that do not show eMSTOs. We argue that eMSTOs only occur for clusters whose early escape velocities are higher than the wind velocities of stars that provide material from which second-generation stars can form. The threshold of 12-15 km s{sup –1} is consistent with wind velocities of intermediate-mass asymptotic giant branch stars and massive binary stars in the literature.« less

LoCuSS: THE SLOW QUENCHING OF STAR FORMATION IN CLUSTER GALAXIES AND THE NEED FOR PRE-PROCESSING

DOE Office of Scientific and Technical Information (OSTI.GOV)

Haines, C. P.; Pereira, M. J.; Egami, E.

2015-06-10

We present a study of the spatial distribution and kinematics of star-forming galaxies in 30 massive clusters at 0.15 < z < 0.30, combining wide-field Spitzer 24 μm and GALEX near-ultraviolet imaging with highly complete spectroscopy of cluster members. The fraction (f{sub SF}) of star-forming cluster galaxies rises steadily with cluster-centric radius, increasing fivefold by 2r{sub 200}, but remains well below field values even at 3r{sub 200}. This suppression of star formation at large radii cannot be reproduced by models in which star formation is quenched in infalling field galaxies only once they pass within r{sub 200} of the cluster,more » but is consistent with some of them being first pre-processed within galaxy groups. Despite the increasing f{sub SF}-radius trend, the surface density of star-forming galaxies actually declines steadily with radius, falling ∼15× from the core to 2r{sub 200}. This requires star formation to survive within recently accreted spirals for 2–3 Gyr to build up the apparent over-density of star-forming galaxies within clusters. The velocity dispersion profile of the star-forming galaxy population shows a sharp peak of 1.44 σ{sub ν} at 0.3r{sub 500}, and is 10%–35% higher than that of the inactive cluster members at all cluster-centric radii, while their velocity distribution shows a flat, top-hat profile within r{sub 500}. All of these results are consistent with star-forming cluster galaxies being an infalling population, but one that must also survive ∼0.5–2 Gyr beyond passing within r{sub 200}. By comparing the observed distribution of star-forming galaxies in the stacked caustic diagram with predictions from the Millennium simulation, we obtain a best-fit model in which star formation rates decline exponentially on quenching timescales of 1.73 ± 0.25 Gyr upon accretion into the cluster.« less

OGLE Collection of Star Clusters. New Objects in the Outskirts of the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Sitek, M.; Szymański, M. K.; Skowron, D. M.; Udalski, A.; Kostrzewa-Rutkowska, Z.; Skowron, J.; Karczmarek, P.; Cieślar, M.; Wyrzykowski, Ł.; Kozłowski, S.; Pietrukowicz, P.; Soszyński, I.; Mróz, P.; Pawlak, M.; Poleski, R.; Ulaczyk, K.

2016-09-01

The Magellanic System (MS), consisting of the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC) and the Magellanic Bridge (MBR), contains diverse sample of star clusters. Their spatial distribution, ages and chemical abundances may provide important information about the history of formation of the whole System. We use deep photometric maps derived from the images collected during the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV) to construct the most complete catalog of star clusters in the Large Magellanic Cloud using the homogeneous photometric data. In this paper we present the collection of star clusters found in the area of about 225 square degrees in the outer regions of the LMC. Our sample contains 679 visually identified star cluster candidates, 226 of which were not listed in any of the previously published catalogs. The new clusters are mainly young small open clusters or clusters similar to associations.

Simulating the Birth of Massive Star Clusters: Is Destruction Inevitable?

NASA Astrophysics Data System (ADS)

Rosen, Anna

2013-10-01

Very early in its operation, the Hubble Space Telescope {HST} opened an entirely new frontier: study of the demographics and properties of star clusters far beyond the Milky Way. However, interpretation of HST's observations has proven difficult, and has led to the development of two conflicting models. One view is that most massive star clusters are disrupted during their infancy by feedback from newly formed stars {i.e., "infant mortality"}, independent of cluster mass or environment. The other model is that most star clusters survive their infancy and are disrupted later by mass-dependent dynamical processes. Since observations at present have failed to discriminate between these views, we propose a theoretical investigation to provide new insight. We will perform radiation-hydrodynamic simulations of the formation of massive star clusters, including for the first time a realistic treatment of the most important stellar feedback processes. These simulations will elucidate the physics of stellar feedback, and allow us to determine whether cluster disruption is mass-dependent or -independent. We will also use our simulations to search for observational diagnostics that can distinguish bound from unbound clusters, and to predict how cluster disruption affects the cluster luminosity function in a variety of galactic environments.

Stellar family in crowded, violent neighbourhood proves to be surprisingly normal

NASA Astrophysics Data System (ADS)

2009-06-01

Using ESO's Very Large Telescope, astronomers have obtained one of the sharpest views ever of the Arches Cluster -- an extraordinary dense cluster of young stars near the supermassive black hole at the heart of the Milky Way. Despite the extreme conditions astronomers were surprised to find the same proportions of low- and high-mass young stars in the cluster as are found in more tranquil locations in our Milky Way. ESO PR Photo 21a/09 The Arches Cluster ESO PR Photo 21b/09 The Centre of the Milky Way ESO PR Photo 21c/09 Around the Arches Cluster ESO PR Video 21a/09 A voyage to the heart of the Milky Way The massive Arches Cluster is a rather peculiar star cluster. It is located 25 000 light-years away towards the constellation of Sagittarius (the Archer), and contains about a thousand young, massive stars, less than 2.5 million years old [1]. It is an ideal laboratory to study how massive stars are born in extreme conditions as it is close to the centre of our Milky Way, where it experiences huge opposing forces from the stars, gas and the supermassive black hole that reside there. The Arches Cluster is ten times heavier than typical young star clusters scattered throughout our Milky Way and is enriched with chemical elements heavier than helium. Using the NACO adaptive optics instrument on ESO's Very Large Telescope, located in Chile, astronomers scrutinised the cluster in detail. Thanks to adaptive optics, astronomers can remove most of the blurring effect of the atmosphere, and so the new NACO images of the Arches Cluster are even crisper than those obtained with telescopes in space. Observing the Arches Cluster is very challenging because of the huge quantities of absorbing dust between Earth and the Galactic Centre, which visible light cannot penetrate. This is why NACO was used to observe the region in near-infrared light. The new study confirms the Arches Cluster to be the densest cluster of massive young stars known. It is about three light-years across with more than a thousand stars packed into each cubic light-year -- an extreme density a million times greater than in the Sun's neighbourhood. Astronomers studying clusters of stars have found that higher mass stars are rarer than their less massive brethren, and their relative numbers are the same everywhere, following a universal law. For many years, the Arches Cluster seemed to be a striking exception. "With the extreme conditions in the Arches Cluster, one might indeed imagine that stars won't form in the same way as in our quiet solar neighbourhood," says Pablo Espinoza, the lead author of the paper reporting the new results. "However, our new observations showed that the masses of stars in this cluster actually do follow the same universal law". In this image the astronomers could also study the brightest stars in the cluster. "The most massive star we found has a mass of about 120 times that of the Sun," says co-author Fernando Selman. "We conclude from this that if stars more massive than 130 solar masses exist, they must live for less than 2.5 million years and end their lives without exploding as supernovae, as massive stars usually do." The total mass of the cluster seems to be about 30 000 times that of the Sun, much more than was previously thought. "That we can see so much more is due to the exquisite NACO images," says co-author Jorge Melnick. Note [1] The name "Arches" does not come from the constellation the cluster is located in (Sagittarius, i.e., the Archer), but because it is located next to arched filaments detected in radio maps of the centre of the Milky Way.

ROSAT PSPC Observations of CL0016+16

NASA Technical Reports Server (NTRS)

Hughes, John P. (Principal Investigator)

1996-01-01

Several ROSAT observations concerning with complex spatial structures in Sunyaev-Zel'dovich decrement clusters Abell 665 and CL0016+16, discovery of Be/X-ray stars in two supernova remnants in the Small Magellanic Cloud, a new transient pulsar in the Small Magellanic Cloud with an unusual x-ray spectrum, a new x-ray-discovered cluster of galaxies associated with CL0016+16, and the distance to CL0016+16 vs. the Hubble constant, are presented.

The star-forming history of the young cluster NGC 2264

NASA Technical Reports Server (NTRS)

Adams, M. T.; Strom, K. M.; Strom, S. E.

1983-01-01

UBVRI H-alpha photographic photometry was obtained for a sample of low-mass stars in the young open cluster NGC 2264 in order to investigate the star-forming history of this region. A theoretical H-R diagram was constructed for the sample of probable cluster members. Isochrones and evolutionary tracks were adopted from Cohen and Kuhi (1979). Evidence for a significant age spread in the cluster was found amounting to over ten million yr. In addition, the derived star formation rate as a function of stellar mass suggests that the principal star-forming mass range in NGC 2264 has proceeded sequentially in time from the lowest to the highest masses. The low-mass cluster stars were the first cluster members to form in significant numbers, although their present birth rate is much lower now than it was about ten million yr ago. The star-formation rate has risen to a peak at successively higher masses and then declined.

Not-so-simple stellar populations in nearby, resolved massive star clusters

NASA Astrophysics Data System (ADS)

de Grijs, Richard; Li, Chengyuan

2018-02-01

Around the turn of the last century, star clusters of all kinds were considered ‘simple’ stellar populations. Over the past decade, this situation has changed dramatically. At the same time, star clusters are among the brightest stellar population components and, as such, they are visible out to much greater distances than individual stars, even the brightest, so that understanding the intricacies of star cluster composition and their evolution is imperative for understanding stellar populations and the evolution of galaxies as a whole. In this review of where the field has moved to in recent years, we place particular emphasis on the properties and importance of binary systems, the effects of rapid stellar rotation, and the presence of multiple populations in Magellanic Cloud star clusters across the full age range. Our most recent results imply a reverse paradigm shift, back to the old simple stellar population picture for at least some intermediate-age (˜1-3 Gyr old) star clusters, opening up exciting avenues for future research efforts.

Unveiling hidden properties of young star clusters: differential reddening, star-formation spread, and binary fraction

NASA Astrophysics Data System (ADS)

Bonatto, C.; Lima, E. F.; Bica, E.

2012-04-01

Context. Usually, important parameters of young, low-mass star clusters are very difficult to obtain by means of photometry, especially when differential reddening and/or binaries occur in large amounts. Aims: We present a semi-analytical approach (ASAmin) that, when applied to the Hess diagram of a young star cluster, is able to retrieve the values of mass, age, star-formation spread, distance modulus, foreground and differential reddening, and binary fraction. Methods: The global optimisation method known as adaptive simulated annealing (ASA) is used to minimise the residuals between the observed and simulated Hess diagrams of a star cluster. The simulations are realistic and take the most relevant parameters of young clusters into account. Important features of the simulations are a normal (Gaussian) differential reddening distribution, a time-decreasing star-formation rate, the unresolved binaries, and the smearing effect produced by photometric uncertainties on Hess diagrams. Free parameters are cluster mass, age, distance modulus, star-formation spread, foreground and differential reddening, and binary fraction. Results: Tests with model clusters built with parameters spanning a broad range of values show that ASAmin retrieves the input values with a high precision for cluster mass, distance modulus, and foreground reddening, but they are somewhat lower for the remaining parameters. Given the statistical nature of the simulations, several runs should be performed to obtain significant convergence patterns. Specifically, we find that the retrieved (absolute minimum) parameters converge to mean values with a low dispersion as the Hess residuals decrease. When applied to actual young clusters, the retrieved parameters follow convergence patterns similar to the models. We show how the stochasticity associated with the early phases may affect the results, especially in low-mass clusters. This effect can be minimised by averaging out several twin clusters in the simulated Hess diagrams. Conclusions: Even for low-mass star clusters, ASAmin is sensitive to the values of cluster mass, age, distance modulus, star-formation spread, foreground and differential reddening, and to a lesser degree, binary fraction. Compared with simpler approaches, including binaries, a decaying star-formation rate, and a normally distributed differential reddening appears to yield more constrained parameters, especially the mass, age, and distance from the Sun. A robust determination of cluster parameters may have a positive impact on many fields. For instance, age, mass, and binary fraction are important for establishing the dynamical state of a cluster or for deriving a more precise star-formation rate in the Galaxy.

HOBYS and W43-HERO: Two more steps toward a Galaxy-wide understanding of high-mass star formation

NASA Astrophysics Data System (ADS)

Motte, Frédérique; Bontemps, Sylvain; Tigé, Jérémy

The Herschel/HOBYS key program allows to statistically study the formation of 10-20 M ⊙ stars. The IRAM/W43-HERO large program is itself dedicated to the much more extreme W43 molecular complex, which forms stars up to 50 M ⊙. Both reveal high-density cloud filaments of several pc3, which are forming clusters of OB-type stars. Given their activity, these so-called mini-starburst cloud ridges could be seen as ``miniature and instant models'' of starburst galaxies. Both surveys also strongly suggest that high-mass prestellar cores do not exist, in agreement with the dynamical formation of cloud ridges. The HOBYS and W43 surveys are necessary steps towards Galaxy-wide studies of high-mass star formation.

CHEMICAL DIAGNOSTICS OF THE MASSIVE STAR CLUSTER-FORMING CLOUD G33.92+0.11. I. {sup 13}CS, CH{sub 3}OH, CH{sub 3}N, OCS, H{sub 2}S, SO{sub 2}, and SiO

DOE Office of Scientific and Technical Information (OSTI.GOV)

Minh, Young Chol; Liu, Hauyu Baobab; Galvań-Madrid, Roberto

2016-06-20

Large chemical diversity was found in the gas clumps associated with the massive star cluster-forming G33.92+0.11 region with sub-arcsecond angular resolution (0.″6–0.″8) observations with ALMA. The most prominent gas clumps are associated with the dust emission peaks A1, A2, and A5. The close correlation between CH{sub 3}OH and OCS in the emission distributions strongly suggests that these species share a common origin of hot core grain mantle evaporation. The latest generation of star clusters are forming in the A5 clump, as indicated by multiple SiO outflows and its rich hot core chemistry. We also found a narrow SiO emission associatedmore » with the outflows, which may trace a cooled component of the outflows. Part of the chemical complexity may have resulted from the accreting gas from the ambient clouds, especially in the northern part of A1 and the southern part of A2. The chemical diversity found in this region is believed to mainly result from the different chemical evolutionary timescales of massive star formation. In particular, the abundance ratio between CH{sub 3}OH and CH{sub 3}CN may be a good chemical clock for the early phase of star formation.« less

Radiation pressure in super star cluster formation

NASA Astrophysics Data System (ADS)

Tsang, Benny T.-H.; Milosavljević, Miloš

2018-05-01

The physics of star formation at its extreme, in the nuclei of the densest and the most massive star clusters in the universe—potential massive black hole nurseries—has for decades eluded scrutiny. Spectroscopy of these systems has been scarce, whereas theoretical arguments suggest that radiation pressure on dust grains somehow inhibits star formation. Here, we harness an accelerated Monte Carlo radiation transport scheme to report a radiation hydrodynamical simulation of super star cluster formation in turbulent clouds. We find that radiation pressure reduces the global star formation efficiency by 30-35%, and the star formation rate by 15-50%, both relative to a radiation-free control run. Overall, radiation pressure does not terminate the gas supply for star formation and the final stellar mass of the most massive cluster is ˜1.3 × 106 M⊙. The limited impact as compared to in idealized theoretical models is attributed to a radiation-matter anti-correlation in the supersonically turbulent, gravitationally collapsing medium. In isolated regions outside massive clusters, where the gas distribution is less disturbed, radiation pressure is more effective in limiting star formation. The resulting stellar density at the cluster core is ≥108 M⊙ pc-3, with stellar velocity dispersion ≳ 70 km s-1. We conclude that the super star cluster nucleus is propitious to the formation of very massive stars via dynamical core collapse and stellar merging. We speculate that the very massive star may avoid the claimed catastrophic mass loss by continuing to accrete dense gas condensing from a gravitationally-confined ionized phase.

A home for old stars

NASA Image and Video Library

2015-12-14

This image, taken with the Wide Field Planetary Camera 2 on board the NASA/ESA Hubble Space Telescope, shows the globular cluster Terzan 1. Lying around 20 000 light-years from us in the constellation of Scorpius (The Scorpion), it is one of about 150 globular clusters belonging to our galaxy, the Milky Way. Typical globular clusters are collections of around a hundred thousand stars, held together by their mutual gravitational attraction in a spherical shape a few hundred light-years across. It is thought that every galaxy has a population of globular clusters. Some, like the Milky Way, have a few hundred, while giant elliptical galaxies can have several thousand. They contain some of the oldest stars in a galaxy, hence the reddish colours of the stars in this image — the bright blue ones are foreground stars, not part of the cluster. The ages of the stars in the globular cluster tell us that they were formed during the early stages of galaxy formation! Studying them can also help us to understand how galaxies formed. Terzan 1, like many globular clusters, is a source of X-rays. It is likely that these X-rays come from binary star systems that contain a dense neutron star and a normal star. The neutron star drags material from the companion star, causing a burst of X-ray emission. The system then enters a quiescent phase in which the neutron star cools, giving off X-ray emission with different characteristics, before enough material from the companion builds up to trigger another outburst.

Hypervelocity stars from young stellar clusters in the Galactic Centre

NASA Astrophysics Data System (ADS)

Fragione, G.; Capuzzo-Dolcetta, R.; Kroupa, P.

2017-05-01

The enormous velocities of the so-called hypervelocity stars (HVSs) derive, likely, from close interactions with massive black holes, binary stars encounters or supernova explosions. In this paper, we investigate the origin of HVSs as consequence of the close interaction between the Milky Way central massive black hole and a passing-by young stellar cluster. We found that both single and binary HVSs may be generated in a burst-like event, as the cluster passes near the orbital pericentre. High-velocity stars will move close to the initial cluster orbital plane and in the direction of the cluster orbital motion at the pericentre. The binary fraction of these HVS jets depends on the primordial binary fraction in the young cluster. The level of initial mass segregation determines the value of the average mass of the ejected stars. Some binary stars will merge, continuing their travel across and out of the Galaxy as blue stragglers.

Rotation in young massive star clusters

NASA Astrophysics Data System (ADS)

Mapelli, Michela

2017-05-01

Hydrodynamical simulations of turbulent molecular clouds show that star clusters form from the hierarchical merger of several sub-clumps. We run smoothed-particle hydrodynamics simulations of turbulence-supported molecular clouds with mass ranging from 1700 to 43 000 M⊙. We study the kinematic evolution of the main cluster that forms in each cloud. We find that the parent gas acquires significant rotation, because of large-scale torques during the process of hierarchical assembly. The stellar component of the embedded star cluster inherits the rotation signature from the parent gas. Only star clusters with final mass < few × 100 M⊙ do not show any clear indication of rotation. Our simulated star clusters have high ellipticity (˜0.4-0.5 at t = 4 Myr) and are subvirial (Qvir ≲ 0.4). The signature of rotation is stronger than radial motions due to subvirial collapse. Our results suggest that rotation is common in embedded massive (≳1000 M⊙) star clusters. This might provide a key observational test for the hierarchical assembly scenario.

Exploring the Dynamics of Exoplanetary Systems in a Young Stellar Cluster

NASA Astrophysics Data System (ADS)

Thornton, Jonathan Daniel; Glaser, Joseph Paul; Wall, Joshua Edward

2018-01-01

I describe a dynamical simulation of planetary systems in a young star cluster. One rather arbitrary aspect of cluster simulations is the choice of initial conditions. These are typically chosen from some standard model, such as Plummer or King, or from a “fractal” distribution to try to model young clumpy systems. Here I adopt the approach of realizing an initial cluster model directly from a detailed magnetohydrodynamical model of cluster formation from a 1000-solar-mass interstellar gas cloud, with magnetic fields and radiative and wind feedback from massive stars included self-consistently. The N-body simulation of the stars and planets starts once star formation is largely over and feedback has cleared much of the gas from the region where the newborn stars reside. It continues until the cluster dissolves in the galactic field. Of particular interest is what would happen to the free-floating planets created in the gas cloud simulation. Are they captured by a star or are they ejected from the cluster? This method of building a dynamical cluster simulation directly from the results of a cluster formation model allows us to better understand the evolution of young star clusters and enriches our understanding of extrasolar planet development in them. These simulations were performed within the AMUSE simulation framework, and combine N-body, multiples and background potential code.

Pulsar-irradiated stars in dense globular clusters

NASA Technical Reports Server (NTRS)

Tavani, Marco

1992-01-01

We discuss the properties of stars irradiated by millisecond pulsars in 'hard' binaries of dense globular clusters. Irradiation by a relativistic pulsar wind as in the case of the eclipsing millisecond pulsar PSR 1957+20 alter both the magnitude and color of the companion star. Some of the blue stragglers (BSs) recently discovered in dense globular clusters can be irradiated stars in binaries containing powerful millisecond pulsars. The discovery of pulsar-driven orbital modulations of BS brightness and color with periods of a few hours together with evidence for radio and/or gamma-ray emission from BS binaries would valuably contribute to the understanding of the evolution of collapsed stars in globular clusters. Pulsar-driven optical modulation of cluster stars might be the only observable effect of a new class of binary pulsars, i.e., hidden millisecond pulsars enshrouded in the evaporated material lifted off from the irradiated companion star.

The Cluster Environment of Two High-mass Protostars

NASA Astrophysics Data System (ADS)

Montes, Virginie; Hofner, Peter

2017-06-01

Characterizing the environment and stellar population in which high-mass stars form is an important step to decide between the main massive star formation theories. In the monolithic collapse model, the mass of the core will determine the final stellar mass (e.g., McKee & Tan 2003). In contrast, in the competitive accretion model (e.g., Bonnell & Bate 2006), the mass of the high-mass star is related to the properties of the cluster. As dynamical processes substantially affect the appearance of a cluster, we study early stages of high-mass star formation. These regions often show extended emission from hot dust at infrared wavelengths, which can cause difficulties to define the cluster. We use a multi-wavelength technique to study nearby high-mass star clusters, based on X-ray observations with the Chandra X-Ray Telescope, in conjunction with infrared data and VLA data. The technique relies on the fact that YSOs are particularly bright in X-ray and that contamination is relatively small. X-ray observations allow us to determine the cluster size. The cluster membership and YSOs classification is established using infrared identification of the X-ray sources, and color-color and color-magnitude diagrams.In this talk, I will present our findings on the cluster study of two high-mass star forming regions: IRAS 20126+4104 and IRAS 16562-3959. While most massive stars appear to be formed in rich a cluster environment, those two sources are candidates for the formation of massive stars in a relatively poor cluster. In contrast to what was found in previous studies (Qiu et al. 2008), the dominant B0-type protostar in IRAS 20126+4104 is associated with a small cluster of low-mass stars. I will also show our current work on IRAS 16562-3959, which contains one of the most luminous O-type protostars in the Galaxy. In the vicinity of this particularly interesting region there is a multitude of small clusters, for which I will present how their stellar population differ from the high-mass star-forming cluster IRAS 16562-3959.

Search for OB stars running away from young star clusters. II. The NGC 6357 star-forming region

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.; Kniazev, A. Y.; Kroupa, P.; Oh, S.

2011-11-01

Dynamical few-body encounters in the dense cores of young massive star clusters are responsible for the loss of a significant fraction of their massive stellar content. Some of the escaping (runaway) stars move through the ambient medium supersonically and can be revealed via detection of their bow shocks (visible in the infrared, optical or radio). In this paper, which is the second of a series of papers devoted to the search for OB stars running away from young ( ≲ several Myr) Galactic clusters and OB associations, we present the results of the search for bow shocks around the star-forming region NGC 6357. Using the archival data of the Midcourse Space Experiment (MSX) satellite and the Spitzer Space Telescope, and the preliminary data release of the Wide-Field Infrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometry is consistent with the possibility that they are generated by stars expelled from the young (~1-2 Myr) star clusters, Pismis 24 and AH03 J1725-34.4, associated with NGC 6357. Two of the seven bow shocks are driven by the already known OB stars, HD 319881 and [N78] 34. Follow-up spectroscopy of three other bow-shock-producing stars showed that they are massive (O-type) stars as well, while the 2MASS photometry of the remaining two stars suggests that they could be B0 V stars, provided that both are located at the same distance as NGC 6357. Detection of numerous massive stars ejected from the very young clusters is consistent with the theoretical expectation that star clusters can effectively lose massive stars at the very beginning of their dynamical evolution (long before the second mechanism for production of runaway stars, based on a supernova explosion in a massive tight binary system, begins to operate) and lends strong support to the idea that probably all field OB stars have been dynamically ejected from their birth clusters. A by-product of our search for bow shocks around NGC 6357 is the detection of three circular shells typical of luminous blue variable and late WN-type Wolf-Rayet stars.

The complex star cluster system of NGC 1316 (Fornax A)

NASA Astrophysics Data System (ADS)

Sesto, Leandro A.; Faifer, Favio R.; Forte, Juan C.

2016-10-01

This paper presents Gemini-gri' high-quality photometry for cluster candidates in the field of NGC 1316 (Fornax A) as part of a study that also includes GMOS spectroscopy. A preliminary discussion of the photometric data indicates the presence of four stellar cluster populations with distinctive features in terms of age, chemical abundance and spatial distribution. Two of them seem to be the usually old (metal poor and metal rich) populations typically found in elliptical galaxies. In turn, an intermediate-age (5 Gyr) globular cluster population is the dominant component of the sample (as reported by previous papers). We also find a younger cluster population with a tentative age of ≈ 1 Gyr.

Impact of a star formation efficiency profile on the evolution of open clusters

NASA Astrophysics Data System (ADS)

Shukirgaliyev, B.; Parmentier, G.; Berczik, P.; Just, A.

2017-09-01

Aims: We study the effect of the instantaneous expulsion of residual star-forming gas on star clusters in which the residual gas has a density profile that is shallower than that of the embedded cluster. This configuration is expected if star formation proceeds with a given star-formation efficiency per free-fall time in a centrally concentrated molecular gas clump. Methods: We performed direct N-body simulations whose initial conditions were generated by the program "mkhalo" from the package "falcON", adapted for our models. Our model clusters initially had a Plummer profile and are in virial equilibrium with the gravitational potential of the cluster-forming clump. The residual gas contribution was computed based on a local-density driven clustered star formation model. Our simulations included mass loss by stellar evolution and the tidal field of a host galaxy. Results: We find that a star cluster with a minimum global star formation efficiency (SFE) of 15 percent is able to survive instantaneous gas expulsion and to produce a bound cluster. Its violent relaxation lasts no longer than 20 Myr, independently of its global SFE and initial stellar mass. At the end of violent relaxation, the bound fractions of the surviving clusters with the same global SFEs are similar, regardless of their initial stellar mass. Their subsequent lifetime in the gravitational field of the Galaxy depends on their bound stellar masses. Conclusions: We therefore conclude that the critical SFE needed to produce a bound cluster is 15 percent, which is roughly half the earlier estimates of 33 percent. Thus we have improved the survival likelihood of young clusters after instantaneous gas expulsion. Young clusters can now survive instantaneous gas expulsion with a global SFEs as low as the SFEs observed for embedded clusters in the solar neighborhood (15-30 percent). The reason is that the star cluster density profile is steeper than that of the residual gas. However, in terms of the effective SFE, measured by the virial ratio of the cluster at gas expulsion, our results are in agreement with previous studies.

LoCuSS: THE STEADY DECLINE AND SLOW QUENCHING OF STAR FORMATION IN CLUSTER GALAXIES OVER THE LAST FOUR BILLION YEARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Haines, C. P.; Pereira, M. J.; Egami, E.

2013-10-01

We present an analysis of the levels and evolution of star formation activity in a representative sample of 30 massive galaxy clusters at 0.15 < z < 0.30 from the Local Cluster Substructure Survey, combining wide-field Spitzer/MIPS 24 μm data with extensive spectroscopy of cluster members. The specific SFRs of massive (M > or approx. 10{sup 10} M{sub ☉}) star-forming cluster galaxies within r{sub 200} are found to be systematically ∼28% lower than their counterparts in the field at fixed stellar mass and redshift, a difference significant at the 8.7σ level. This is the unambiguous signature of star formation inmore » most (and possibly all) massive star-forming galaxies being slowly quenched upon accretion into massive clusters, their star formation rates (SFRs) declining exponentially on quenching timescales in the range 0.7-2.0 Gyr. We measure the mid-infrared Butcher-Oemler effect over the redshift range 0.0-0.4, finding rapid evolution in the fraction (f{sub SF}) of massive (M{sub K} < – 23.1) cluster galaxies within r{sub 200} with SFRs > 3 M{sub ☉} yr{sup –1}, of the form f{sub SF}∝(1 + z){sup 7.6±1.1}. We dissect the origins of the Butcher-Oemler effect, revealing it to be due to the combination of a ∼3 × decline in the mean specific SFRs of star-forming cluster galaxies since z ∼ 0.3 with a ∼1.5 × decrease in number density. Two-thirds of this reduction in the specific SFRs of star-forming cluster galaxies is due to the steady cosmic decline in the specific SFRs among those field galaxies accreted into the clusters. The remaining one-third reflects an accelerated decline in the star formation activity of galaxies within clusters. The slow quenching of star formation in cluster galaxies is consistent with a gradual shut down of star formation in infalling spiral galaxies as they interact with the intracluster medium via ram-pressure stripping or starvation mechanisms. The observed sharp decline in star formation activity among cluster galaxies since z ∼ 0.4 likely reflects the increased susceptibility of low-redshift spiral galaxies to gas removal mechanisms as their gas surface densities decrease with time. We find no evidence for the build-up of cluster S0 bulges via major nuclear starburst episodes.« less

Star Formation in Merging Clusters of Galaxies

NASA Astrophysics Data System (ADS)

Mansheim, Alison Seiler

This thesis straddles two areas of cosmology, each of which are active, rich and plagued by controversy in their own right: merging clusters and the environmental dependence of galaxy evolution. While the greater context of this thesis is major cluster mergers, our individual subjects are galaxies, and we apply techniques traditionally used to study the differential evolution of galaxies with environment. The body of this thesis is drawn from two papers: Mansheim et al. 2016a and Mansheim et al. 2016b, one on each system. Both projects benefited from exquisite data sets assembled as part of the Merging Cluster Collaboration (MC2), and Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey, allowing us to scrutinize the evolutionary states of galaxy populations in multiple lights. Multi-band optical and near-infrared imaging was available for both systems, allowing us to calculate photometric redshifts for completeness corrections, colors (red vs. blue) and stellar masses to view the ensemble properties of the populations in and around each merger. High-resolution spectroscopy was also available for both systems, allowing us to confirm cluster members by measuring spectroscopic redshifts, which are unparalleled in accuracy, and gauge star formation rates and histories by measuring the strengths of certain spectral features. We had the luxury of HST imaging for Musket Ball, allowing us to use galaxy morphology as an additional diagnostic. For Cl J0910, 24 mum imaging allowed us to defeat a most pernicious source of uncertainty. Details on the acquisition and reduction of multi-wavelength data for each system are found within each respective chapter. It is important to note that the research presented in Chapter 3 is based on a letter which had significant space restrictions, so much of the observational details are outsourced to papers written by ORELSE collaboration members. Below is a free-standing summary of each project, drawn from the abstracts of each paper. The Chapter 1 contains an introduction to the topic and motivation to fill a vacuum in knowledge using our hypothesis. Chapter 4, following the meat of the thesis in Chapters 2 and 3, gives closure and looks to the future. In Chapter 2, we investigate star formation in DLSCL J0916.2+2953, a dissociative merger of two clusters at z = 0.53 that has progressed 1.1 +1.3-0.4 Gyr since first pass-through. We attempt to reveal the effects a collision may have had on the evolution of the cluster galaxies by tracing their star formation history. We probe current and recent activity to identify a possible star formation event at the time of the merger using EW(Hdelta), EW(OII) and Dn(4000) measured from the composite spectra of 64 cluster and 153 coeval field galaxies. We supplement Keck DEIMOS spectra with DLS and HST imaging to determine the color, stellar mass, and morphology of each galaxy and conduct a comprehensive study of the populations in this complex structure. Spectral results indicate the average cluster and cluster red sequence galaxies experienced no enhanced star formation relative to the surrounding field during the merger, ruling out a predominantly merger-quenched population. We find that the average blue galaxy in the North cluster is currently active and in the South cluster is currently post-starburst having undergone a recent star formation event. While the North activity could be latent or long-term merger effects, a young blue stellar population and irregular geometry suggest the cluster was still forming prior the collision. While the South activity coincides with the time of the merger, the blue early-type population could be a result of secular cluster processes. The evidence suggests that the dearth or surfeit of activity is indiscernible from normal cluster galaxy evolution. In Chapter 3, we examine the effects of an impending cluster merger on galaxies in the large scale structure (LSS) RX Cl J0910 at z =1.105. Using multi-wavelength data, including 102 spectral members drawn from the ORELSE survey and precise photometric redshifts, we calculate extinction-corrected star formation rates and map the specific star formation rate density of the LSS galaxies. These analyses along with an investigation of the color-magnitude properties of LSS galaxies indicate lower levels of star formation activity in the region between the merging clusters relative to the outskirts of the system. We suggest gravitational tidal forces due to the potential of merging halos may be the physical mechanisms responsible for the observed suppression of star formation in galaxies caught between the merging clusters. (Abstract shortened by ProQuest.).

Building the Galactic halo from globular clusters: evidence from chemically unusual red giants

NASA Astrophysics Data System (ADS)

Martell, S. L.; Smolinski, J. P.; Beers, T. C.; Grebel, E. K.

2011-10-01

We present a spectroscopic search for halo field stars that originally formed in globular clusters. Using moderate-resolution SDSS-III/SEGUE-2 spectra of 561 red giants with typical halo metallicities (-1.8 ≤ [Fe/H] ≤ -1.0), we identify 16 stars, 3% of the sample, with CN and CH bandstrength behavior indicating depleted carbon and enhanced nitrogen abundances relative to the rest of the data set. Since globular clusters are the only environment known in which stars form with this pattern of atypical light-element abundances, we claim that these stars are second-generation globular cluster stars that have been lost to the halo field via normal cluster mass-loss processes. Extrapolating from theoretical models of two-generation globular cluster formation, this result suggests that globular clusters contributed significant numbers of stars to the construction of the Galactic halo: we calculate that a minimum of 17% of the present-day mass of the stellar halo was originally formed in globular clusters. The ratio of CN-strong to CN-normal stars drops with Galactocentric distance, suggesting that the inner-halo population may be the primary repository of these stars. Full Tables 1 and 3 are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/534/A136

The most crowded place in the Milky Way

NASA Image and Video Library

2015-01-08

This new NASA/ESA Hubble Space Telescope image presents the Arches Cluster, the densest known star cluster in the Milky Way. It is located about 25 000 light-years from Earth in the constellation of Sagittarius (The Archer), close to the heart of our galaxy, the Milky Way. It is, like its neighbour the Quintuplet Cluster, a fairly young astronomical object at between two and four million years old. The Arches cluster is so dense that in a region with a radius equal to the distance between the Sun and its nearest star there would be over 100 000 stars! At least 150 stars within the cluster are among the brightest ever discovered in the the Milky Way. These stars are so bright and massive, that they will burn their fuel within a short time, on a cosmological scale, just a few million years, and die in spectacular supernova explosions. Due to the short lifetime of the stars in the cluster, the gas between the stars contains an unusually high amount of heavier elements, which were produced by earlier generations of stars. Despite its brightness the Arches Cluster cannot be seen with the naked eye. The visible light from the cluster is completely obscured by gigantic clouds of dust in this region. To make the cluster visible astronomers have to use detectors which can collect light from the X-ray, infrared, and radio bands, as these wavelengths can pass through the dust clouds. This observation shows the Arches Cluster in the infrared and demonstrates the leap in Hubble’s performance since its 1999 image of same object.

Accurate age determinations of several nearby open clusters containing magnetic Ap stars

NASA Astrophysics Data System (ADS)

Silaj, J.; Landstreet, J. D.

2014-06-01

Context. To study the time evolution of magnetic fields, chemical abundance peculiarities, and other characteristics of magnetic Ap and Bp stars during their main sequence lives, a sample of these stars in open clusters has been obtained, as such stars can be assumed to have the same ages as the clusters to which they belong. However, in exploring age determinations in the literature, we find a large dispersion among different age determinations, even for bright, nearby clusters. Aims: Our aim is to obtain ages that are as accurate as possible for the seven nearby open clusters α Per, Coma Ber, IC 2602, NGC 2232, NGC 2451A, NGC 2516, and NGC 6475, each of which contains at least one magnetic Ap or Bp star. Simultaneously, we test the current calibrations of Te and luminosity for the Ap/Bp star members, and identify clearly blue stragglers in the clusters studied. Methods: We explore the possibility that isochrone fitting in the theoretical Hertzsprung-Russell diagram (i.e. log (L/L⊙) vs. log Te), rather than in the conventional colour-magnitude diagram, can provide more precise and accurate cluster ages, with well-defined uncertainties. Results: Well-defined ages are found for all the clusters studied. For the nearby clusters studied, the derived ages are not very sensitive to the small uncertainties in distance, reddening, membership, metallicity, or choice of isochrones. Our age determinations are all within the range of previously determined values, but the associated uncertainties are considerably smaller than the spread in recent age determinations from the literature. Furthermore, examination of proper motions and HR diagrams confirms that the Ap stars identified in these clusters are members, and that the presently accepted temperature scale and bolometric corrections for Ap stars are approximately correct. We show that in these theoretical HR diagrams blue stragglers are particularly easy to identify. Conclusions: Constructing the theoretical HR diagram of a nearby open cluster makes possible an accurate age determination, with well defined uncertainty. This diagnostic of a cluster also provides a useful tool for studying unusual stars such as Ap stars and blue stragglers. Table 3 is available in electronic form at http://www.aanda.org

Probing Globular Cluster Formation in Low Metallicity Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Johnson, Kelsey E.; Hunt, Leslie K.; Reines, Amy E.

2008-12-01

The ubiquitous presence of globular clusters around massive galaxies today suggests that these extreme star clusters must have been formed prolifically in the earlier universe in low-metallicity galaxies. Numerous adolescent and massive star clusters are already known to be present in a variety of galaxies in the local universe; however most of these systems have metallicities of 12 + log(O/H) > 8, and are thus not representative of the galaxies in which today's ancient globular clusters were formed. In order to better understand the formation and evolution of these massive clusters in environments with few heavy elements, we have targeted several low-metallicity dwarf galaxies with radio observations, searching for newly-formed massive star clusters still embedded in their birth material. The galaxies in this initial study are HS 0822+3542, UGC 4483, Pox 186, and SBS 0335-052, all of which have metallicities of 12 + log(O/H) < 7.75. While no thermal radio sources, indicative of natal massive star clusters, are found in three of the four galaxies, SBS 0335-052 hosts two such objects, which are incredibly luminous. The radio spectral energy distributions of these intense star-forming regions in SBS 0335-052 suggest the presence of ~12,000 equivalent O-type stars, and the implied star formation rate is nearing the maximum starburst intensity limit.

uvbyβ photometry of early type open cluster and field stars

NASA Astrophysics Data System (ADS)

Handler, G.

2011-04-01

Context. The β Cephei stars and slowly pulsating B (SPB) stars are massive main sequence variables. The strength of their pulsational driving strongly depends on the opacity of iron-group elements. As many of those stars naturally occur in young open clusters, whose metallicities can be determined in several fundamental ways, it is logical to study the incidence of pulsation in several young open clusters. Aims: To provide the foundation for such an investigation, Strömgren-Crawford uvbyβ photometry of open cluster target stars was carried out to determine effective temperatures, luminosities, and therefore cluster memberships. Methods: In the course of three observing runs, uvbyβ photometry for 168 target stars was acquired and transformed into the standard system by measurements of 117 standard stars. The list of target stars also included some known cluster and field β Cephei stars, as well as β Cephei and SPB candidates that are targets of the asteroseismic part of the Kepler satellite mission. Results: The uvbyβ photometric results are presented. The data are shown to be on the standard system, and the properties of the target stars are discussed: 140 of these are indeed OB stars, a total of 101 targets lie within the β Cephei and/or SPB star instability strips, and each investigated cluster contains such potential pulsators. Conclusions: These measurements will be taken advantage of in a number of subsequent publications. Based on measurements obtained at McDonald Observatory of the University of Texas at Austin.Tables 3-6 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/528/A148

Hubble Uncovering the Secrets of the Quintuplet Cluster

NASA Image and Video Library

2017-12-08

Although this cluster of stars gained its name due to its five brightest stars, it is home to hundreds more. The huge number of massive young stars in the cluster is clearly captured in this NASA/ESA Hubble Space Telescope image. The cluster is located close to the Arches Cluster and is just 100 light-years from the center of our galaxy. The cluster’s proximity to the dust at the center of the galaxy means that much of its visible light is blocked, which helped to keep the cluster unknown until its discovery in 1990, when it was revealed by infrared observations. Infrared images of the cluster, like the one shown here, allow us to see through the obscuring dust to the hot stars in the cluster. The Quintuplet Cluster hosts two extremely rare luminous blue variable stars: the Pistol Star and the lesser known V4650 Sgr. If you were to draw a line horizontally through the center of this image from left to right, you could see the Pistol Star hovering just above the line about one third of the way along it. The Pistol Star is one of the most luminous known stars in the Milky Way and takes its name from the shape of the Pistol Nebula that it illuminates, but which is not visible in this infrared image. The exact age and future of the Pistol Star are uncertain, but it is expected to end in a supernova or even a hypernova in one to three million years. The cluster also contains a number of red supergiants. These stars are among the largest in the galaxy and are burning their fuel at an incredible speed, meaning they will have a very short lifetime. Their presence suggests an average cluster age of nearly four million years. At the moment these stars are on the verge of exploding as supernovae. During their spectacular deaths they will release vast amounts of energy which, in turn, will heat the material — dust and gas — between the other stars. This observation shows the Quintuplet Cluster in the infrared and demonstrates the leap in Hubble’s performance since its 1999 image of same object. Credit: ESA/NASA NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

Dynamical evolution of stars and gas of young embedded stellar sub-clusters

NASA Astrophysics Data System (ADS)

Sills, Alison; Rieder, Steven; Scora, Jennifer; McCloskey, Jessica; Jaffa, Sarah

2018-06-01

We present simulations of the dynamical evolution of young embedded star clusters. Our initial conditions are directly derived from X-ray, infrared, and radio observations of local systems, and our models evolve both gas and stars simultaneously. Our regions begin with both clustered and extended distributions of stars, and a gas distribution that can include a filamentary structure in addition to gas surrounding the stellar sub-clusters. We find that the regions become spherical, monolithic, and smooth quite quickly, and that the dynamical evolution is dominated by the gravitational interactions between the stars. In the absence of stellar feedback, the gas moves gently out of the centre of our regions but does not have a significant impact on the motions of the stars at the earliest stages of cluster formation. Our models at later times are consistent with observations of similar regions in the local neighbourhood. We conclude that the evolution of young protostar clusters is relatively insensitive to reasonable choices of initial conditions. Models with more realism, such as an initial population of binary and multiple stars and ongoing star formation, are the next step needed to confirm these findings.

Blue Stragglers in Clusters and Integrated Spectral Properties of Stellar Populations

NASA Astrophysics Data System (ADS)

Xin, Yu; Deng, Licai

Blue straggler stars are the most prominent bright objects in the colour-magnitude diagram of a star cluster that challenges the theory of stellar evolution. Star clusters are the closest counterparts of the theoretical concept of simple stellar populations (SSPs) in the Universe. SSPs are widely used as the basic building blocks to interpret stellar contents in galaxies. The concept of an SSP is a group of coeval stars which follows a given distribution in mass, and has the same chemical property and age. In practice, SSPs are more conveniently made by the latest stellar evolutionary models of single stars. In reality, however, stars can be more complicated than just single either at birth time or during the course of evolution in a typical environment. Observations of star clusters show that there are always exotic objects which do not follow the predictions of standard theory of stellar evolution. Blue straggler stars (BSSs), as discussed intensively in this book both observationally and theoretically, are very important in our context when considering the integrated spectral properties of a cluster, or a simple stellar population. In this chapter, we are going to describe how important the contribution of BSSs is to the total light of a cluster.

High-resolution spectroscopic observations of binary stars and yellow stragglers in three open clusters: NGC 2360, NGC 3680, and NGC 5822

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sales Silva, J. V.; Peña Suárez, V. J.; Katime Santrich, O. J.

2014-11-01

Binary stars in open clusters are very useful targets in constraining the nucleosynthesis process. The luminosities of the stars are known because the distances of the clusters are also known, so chemical peculiarities can be linked directly to the evolutionary status of a star. In addition, binary stars offer the opportunity to verify a relationship between them and the straggler population in both globular and open clusters. We carried out a detailed spectroscopic analysis to derive the atmospheric parameters for 16 red giants in binary systems and the chemical composition of 11 of them in the open clusters NGC 2360,more » NGC 3680, and NGC 5822. We obtained abundances of C, N, O, Na, Mg, Al, Ca, Si, Ti, Ni, Cr, Y, Zr, La, Ce, and Nd. The atmospheric parameters of the studied stars and their chemical abundances were determined using high-resolution optical spectroscopy. We employ the local thermodynamic equilibrium model atmospheres of Kurucz and the spectral analysis code MOOG. The abundances of the light elements were derived using the spectral synthesis technique. We found that the stars NGC 2360-92 and 96, NGC 3680-34, and NGC 5822-4 and 312 are yellow straggler stars. We show that the spectra of NGC 5822-4 and 312 present evidence of contamination by an A-type star as a secondary star. For the other yellow stragglers, evidence of contamination is given by the broad wings of the Hα. Detection of yellow straggler stars is important because the observed number can be compared with the number predicted by simulations of binary stellar evolution in open clusters. We also found that the other binary stars are not s-process enriched, which may suggest that in these binaries the secondary star is probably a faint main-sequence object. The lack of any s-process enrichment is very useful in setting constraints for the number of white dwarfs in the open cluster, a subject that is related to the birthrate of these kinds of stars in open clusters and also to the age of a cluster. Finally, rotational velocities were also determined and their values were compared with those already determined for field giant stars.« less

A photometric study of globular clusters observed by the APOGEE survey

NASA Astrophysics Data System (ADS)

Mészáros, Szabolcs; García-Hernández, D. A.; Cassisi, Santi; Monelli, Matteo; Szigeti, László; Dell'Agli, Flavia; Derekas, Alíz; Masseron, Thomas; Shetrone, Matthew; Stetson, Peter; Zamora, Olga

2018-04-01

In this paper, we describe the photometric and spectroscopic properties of multiple populations in seven northern globular clusters. In this study, we employ precise ground-based photometry from the private collection of Stetson, space photometry from the Hubble Space Telescope (HST), literature abundances of Na and O, and Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey abundances for Mg, Al, C, and N. Multiple populations are identified by their position in the CU, B, I -Vpseudo colour-magnitude diagram (pseudo-CMD) and confirmed with their chemical composition determined using abundances. We confirm the expectation from previous studies that the red giant branches (RGBs) in all seven clusters are split and the different branches have different chemical compositions. The Mg-Al anticorrelations were well explored by the APOGEE and Gaia-ESO surveys for most globular clusters, some clusters showing bimodal distributions, while others continuous distributions. Even though the structure (i.e. bimodal versus continuous) of Mg-Al can greatly vary, the Al-rich and Al-poor populations do not seem to have very different photometric properties, agreeing with theoretical calculations. There is no one-to-one correspondence between the Mg-Al anticorrelation shape (bimodal versus continuous) and the structure of the RGB seen in the HST pseudo-CMDs, with the HST photometric information usually implying more complex formation/evolution histories than the spectroscopic ones. We report on finding two second-generation horizontal branch (HB) stars in M5, and five second-generation asymptotic giant branch (AGB) stars in M92, which is the most metal-poor cluster to date in which second-generation AGB stars have been observed.

Observing Globular Cluster RR Lyrae Variables with the BYU West Mountain Observatory

NASA Astrophysics Data System (ADS)

Jeffery, E. J.; Joner, M. D.

2016-06-01

We have utilized the 0.9-meter telescope of the Brigham Young University West Mountain Observatory to secure data on six northern hemisphere globular clusters. Here we present representative observations of RR Lyrae stars located in these clusters, including light curves. We compare light curves produced using both DAOPHOT and ISIS software packages. Light curve fitting is done with FITLC. We find that for well-separated stars, DAOPHOT and ISIS provide comparable results. However, for stars within the cluster core, ISIS provides superior results. These improved techniques will allow us to better measure the properties of cluster variable stars.

A Database of Young Star Clusters for Five Hundred Galaxies

NASA Astrophysics Data System (ADS)

Whitmore, Brad

2009-07-01

We propose to use the source lists developed as part of the Hubble Legacy Archive {HLA: Data Release 1 - February 8, 2008} to obtain a large {N 50 galaxies for multi-wavelength, N 500 galaxies for ACS F814W}, uniform {ACS + WFPC2 + NICMOS: DAOphot used for object detection} database of super star clusters in nearby star-forming galaxies in order to address two fundamental astronomical questions: 1} To what degree is the cluster luminosity {and mass} function of star clusters universal ? 2} What fraction of super star clusters are "missing" in optical studies {i.e., are hidden by dust}? This database will also support comparisons with new Monte-Carlo simulations that have independently been developed in the past few years by co-I Larsen and PI Whitmore, and will be used to test the Whitmore, Chandar, Fall {2007} framework designed to understand the demographics of star clusters in all star forming galaxies. The catalogs will increase the number of galaxies with measured mass and luminosity functions by an order of magnitude, and will provide a powerful new tool for comparative studies, both ours and the community's.

Analysis of Spectral-type A/B Stars in Five Open Clusters

NASA Astrophysics Data System (ADS)

Wilhelm, Ronald J.; Rafuil Islam, M.

2014-01-01

We have obtained low resolution (R = 1000) spectroscopy of N=68, spectral-type A/B stars in five nearby open star clusters using the McDonald Observatory, 2.1m telescope. The sample of blue stars in various clusters were selected to test our new technique for determining interstellar reddening and distances in areas where interstellar reddening is high. We use a Bayesian approach to find the posterior distribution for Teff, Logg and [Fe/H] from a combination of reddened, photometric colors and spectroscopic line strengths. We will present calibration results for this technique using open cluster star data with known reddening and distances. Preliminary results suggest our technique can produce both reddening and distance determinations to within 10% of cluster values. Our technique opens the possibility of determining distances for blue stars at low Galactic latitudes where extinction can be large and differential. We will also compare our stellar parameter determinations to previously reported MK spectral classifications and discuss the probability that some of our stars are not members of their reported clusters.

Formation of intermediate-mass black holes through runaway collisions in the first star clusters

NASA Astrophysics Data System (ADS)

Sakurai, Yuya; Yoshida, Naoki; Fujii, Michiko S.; Hirano, Shingo

2017-12-01

We study the formation of massive black holes in the first star clusters. We first locate star-forming gas clouds in protogalactic haloes of ≳107 M⊙ in cosmological hydrodynamics simulations and use them to generate the initial conditions for star clusters with masses of ∼105 M⊙. We then perform a series of direct-tree hybrid N-body simulations to follow runaway stellar collisions in the dense star clusters. In all the cluster models except one, runaway collisions occur within a few million years, and the mass of the central, most massive star reaches ∼400-1900 M⊙. Such very massive stars collapse to leave intermediate-mass black holes (IMBHs). The diversity of the final masses may be attributed to the differences in a few basic properties of the host haloes such as mass, central gas velocity dispersion and mean gas density of the central core. Finally, we derive the IMBH mass to cluster mass ratios, and compare them with the observed black hole to bulge mass ratios in the present-day Universe.

Star formation and galaxy evolution in different environments, from the field to massive clusters

NASA Astrophysics Data System (ADS)

Tyler, Krystal

This thesis focuses on how a galaxy's environment affects its star formation, from the galactic environment of the most luminous IR galaxies in the universe to groups and massive clusters of galaxies. Initially, we studied a class of high-redshift galaxies with extremely red optical-to-mid-IR colors. We used Spitzer spectra and photometry to identify whether the IR outputs of these objects are dominated by AGNs or star formation. In accordance with the expectation that the AGN contribution should increase with IR luminosity, we find most of our very red IR-luminous galaxies to be dominated by an AGN, though a few appear to be star-formation dominated. We then observed how the density of the extraglactic environment plays a role in galaxy evolution. We begin with Spitzer and HST observations of intermediate-redshift groups. Although the environment has clearly changed some properties of its members, group galaxies at a given mass and morphology have comparable amounts of star formation as field galaxies. We conclude the main difference between the two environments is the higher fraction of massive early-type galaxies in groups. Clusters show even more distinct trends. Using three different star-formation indicators, we found the mass-SFR relation for cluster galaxies can look similar to the field (A2029) or have a population of low-star-forming galaxies in addition to the field-like galaxies (Coma). We contribute this to differing merger histories: recently-accreted galaxies would not have time for their star formation to be quenched by the cluster environment (A2029), while an accretion event in the past few Gyr would give galaxies enough time to have their star formation suppressed by the cluster environment. Since these two main quenching mechanisms depend on the density of the intracluster gas, we turn to a group of X-ray underluminous clusters to study how star-forming galaxies have been affected in clusters with lower than expected X-ray emission. We find the distribution of star-forming galaxies with respect to stellar mass varies from cluster to cluster, echoing what we found for Coma and A2029. In other words, while some preprocessing occurs in groups, the cluster environment still contributes to the quenching of star formation.

Chandra Detection of an Evolved Population of Young Stars in Serpens South

NASA Astrophysics Data System (ADS)

Winston, E.; Wolk, S. J.; Gutermuth, R.; Bourke, T. L.

2018-06-01

We present a Chandra study of the deeply embedded Serpens South star-forming region, examining cluster structure and disk properties at the earliest stages. In total, 152 X-ray sources are detected. Combined with Spitzer and 2MASS photometry, 66 X-ray sources are reliably matched to an IR counterpart. We identify 21 class I, 6 flat spectrum, 16 class II, and 18 class III young stars; 5 were unclassified. Eighteen sources were variable in X-rays, 8 exhibiting flare-like emission and one source being periodic. The cluster’s X-ray luminosity distance was estimated: the best match was to the nearer distance of 260 pc for the front of the Aquila Rift complex. The ratio of N H to A K is found to be ∼0.68 × 1022, similar to that measured in other young low-mass regions, but lower than that measured in the interstellar medium and high-mass clusters (∼(1.6–2) × 1022). We find that the spatial distribution closely follows that of the dense filament from which the stars have formed, with the class II population still strongly associated with the filament. There are four subclusters in the field, with three forming knots in the filament, and a fourth to the west, which may not be associated but may be contributing to the distributed class III population. A high percentage of diskless class IIIs (upper limit 30% of classified X-ray sources) in such a young cluster could indicate that processing of disks is influenced by the cluster environment and is not solely dependent on timescale.

Starburst clusters in the Galactic center

NASA Astrophysics Data System (ADS)

Habibi, Maryam

2014-09-01

The central region of the Galaxy is the most active site of star formation in the Milky Way, where massive stars have formed very recently and are still forming today. The rich population of massive stars in the Galactic center provide a unique opportunity to study massive stars in their birth environment and probe their initial mass function, which is the spectrum of stellar masses at their birth. The Arches cluster is the youngest among the three massive clusters in the Galactic center, providing a collection of high-mass stars and a very dense core which makes this cluster an excellent site to address questions about massive star formation, the stellar mass function and the dynamical evolution of massive clusters in the Galactic center. In this thesis, I perform an observational study of the Arches cluster using K_{s}-band imaging obtained with NAOS/CONICA at the VLT combined with Subaru/Cisco J-band data to gain a full understanding of the cluster mass distribution out to its tidal radius for the first time. Since the light from the Galactic center reaches us through the Galactic disc, the extinction correction is crucial when studying this region. I use a Bayesian method to construct a realistic extinction map of the cluster. It is shown in this study that the determination of the mass of the most massive star in the Arches cluster, which had been used in previous studies to establish an upper mass limit for the star formation process in the Milky Way, strongly depends on the assumed slope of the extinction law. Assuming the two regimes of widely used infrared extinction laws, I show that the difference can reach up to 30% for individually derived stellar masses and Δ A_{Ks}˜ 1 magnitude in acquired K_{s}-band extinction, while the present-day mass function slope changes by ˜ 0.17 dex. The present-day mass function slope derived assuming the more recent extinction law, which suggests a steeper wavelength dependence for the infrared extinction law, reveals an overpopulation of massive stars in the core (r<0.2 pc) with a flat slope of α_{Nishi}=-1.50 ±0.35 in comparison to the Salpeter slope of α=-2.3. The slope of the mass function increases to α_{Nishi}=-2.21 ±0.27 in the intermediate annulus (0.2

The E-MOSAICS project: simulating the formation and co-evolution of galaxies and their star cluster populations

NASA Astrophysics Data System (ADS)

Pfeffer, Joel; Kruijssen, J. M. Diederik; Crain, Robert A.; Bastian, Nate

2018-04-01

We introduce the MOdelling Star cluster population Assembly In Cosmological Simulations within EAGLE (E-MOSAICS) project. E-MOSAICS incorporates models describing the formation, evolution, and disruption of star clusters into the EAGLE galaxy formation simulations, enabling the examination of the co-evolution of star clusters and their host galaxies in a fully cosmological context. A fraction of the star formation rate of dense gas is assumed to yield a cluster population; this fraction and the population's initial properties are governed by the physical properties of the natal gas. The subsequent evolution and disruption of the entire cluster population are followed accounting for two-body relaxation, stellar evolution, and gravitational shocks induced by the local tidal field. This introductory paper presents a detailed description of the model and initial results from a suite of 10 simulations of ˜L⋆ galaxies with disc-like morphologies at z = 0. The simulations broadly reproduce key observed characteristics of young star clusters and globular clusters (GCs), without invoking separate formation mechanisms for each population. The simulated GCs are the surviving population of massive clusters formed at early epochs (z ≳ 1-2), when the characteristic pressures and surface densities of star-forming gas were significantly higher than observed in local galaxies. We examine the influence of the star formation and assembly histories of galaxies on their cluster populations, finding that (at similar present-day mass) earlier-forming galaxies foster a more massive and disruption-resilient cluster population, while galaxies with late mergers are capable of forming massive clusters even at late cosmic epochs. We find that the phenomenological treatment of interstellar gas in EAGLE precludes the accurate modelling of cluster disruption in low-density environments, but infer that simulations incorporating an explicitly modelled cold interstellar gas phase will overcome this shortcoming.

Massive open star clusters using the VVV survey. II. Discovery of six clusters with Wolf-Rayet stars

NASA Astrophysics Data System (ADS)

Chené, A.-N.; Borissova, J.; Bonatto, C.; Majaess, D. J.; Baume, G.; Clarke, J. R. A.; Kurtev, R.; Schnurr, O.; Bouret, J.-C.; Catelan, M.; Emerson, J. P.; Feinstein, C.; Geisler, D.; de Grijs, R.; Hervé, A.; Ivanov, V. D.; Kumar, M. S. N.; Lucas, P.; Mahy, L.; Martins, F.; Mauro, F.; Minniti, D.; Moni Bidin, C.

2013-01-01

Context. The ESO Public Survey "VISTA Variables in the Vía Láctea" (VVV) provides deep multi-epoch infrared observations for an unprecedented 562 sq. degrees of the Galactic bulge, and adjacent regions of the disk. Nearly 150 new open clusters and cluster candidates have been discovered in this survey. Aims: This is the second in a series of papers about young, massive open clusters observed using the VVV survey. We present the first study of six recently discovered clusters. These clusters contain at least one newly discovered Wolf-Rayet (WR) star. Methods: Following the methodology presented in the first paper of the series, wide-field, deep JHKs VVV observations, combined with new infrared spectroscopy, are employed to constrain fundamental parameters for a subset of clusters. Results: We find that the six studied stellar groups are real young (2-7 Myr) and massive (between 0.8 and 2.2 × 103 M⊙) clusters. They are highly obscured (AV ~ 5-24 mag) and compact (1-2 pc). In addition to WR stars, two of the six clusters also contain at least one red supergiant star, and one of these two clusters also contains a blue supergiant. We claim the discovery of 8 new WR stars, and 3 stars showing WR-like emission lines which could be classified WR or OIf. Preliminary analysis provides initial masses of ~30-50 M⊙ for the WR stars. Finally, we discuss the spiral structure of the Galaxy using the six new clusters as tracers, together with the previously studied VVV clusters. Based on observations with ISAAC, VLT, ESO (programme 087.D-0341A), New Technology Telescope at ESO's La Silla Observatory (programme 087.D-0490A) and with the Clay telescope at the Las Campanas Observatory (programme CN2011A-086). Also based on data from the VVV survey (programme 172.B-2002).

A Constraint on the Formation Timescale of the Young Open Cluster NGC 2264: Lithium Abundance of Pre-main Sequence Stars

NASA Astrophysics Data System (ADS)

Lim, Beomdu; Sung, Hwankyung; Kim, Jinyoung S.; Bessell, Michael S.; Hwang, Narae; Park, Byeong-Gon

2016-11-01

The timescale of cluster formation is an essential parameter in order to understand the formation process of star clusters. Pre-main sequence (PMS) stars in nearby young open clusters reveal a large spread in brightness. If the spread were considered to be a result of a real spread in age, the corresponding cluster formation timescale would be about 5-20 Myr. Hence it could be interpreted that star formation in an open cluster is prolonged for up to a few tens of Myr. However, difficulties in reddening correction, observational errors, and systematic uncertainties introduced by imperfect evolutionary models for PMS stars can result in an artificial age spread. Alternatively, we can utilize Li abundance as a relative age indicator of PMS star to determine the cluster formation timescale. The optical spectra of 134 PMS stars in NGC 2264 have been obtained with MMT/Hectochelle. The equivalent widths have been measured for 86 PMS stars with a detectable Li line (3500\\lt {T}{eff}[{{K}}]≤slant 6500). Li abundance under the condition of local thermodynamic equilibrium (LTE) was derived using the conventional curve of growth method. After correction for non-LTE effects, we find that the initial Li abundance of NGC 2264 is A({Li})=3.2+/- 0.2. From the distribution of the Li abundances, the underlying age spread of the visible PMS stars is estimated to be about 3-4 Myr and this, together with the presence of embedded populations in NGC 2264, suggests that the cluster formed on a timescale shorter than 5 Myr.

DOE Office of Scientific and Technical Information (OSTI.GOV)

McDonald, Michael; Bautz, Marshall W.; Benson, Bradford

We present Hubble Space Telescope Wide Field Camera 3 observations of the core of the Phoenix cluster (SPT-CLJ2344-4243) in five broadband filters spanning rest-frame 1000-5500 A. These observations reveal complex, filamentary blue emission, extending for >40 kpc from the brightest cluster galaxy. We observe an underlying, diffuse population of old stars, following an r {sup 1/4} distribution, confirming that this system is somewhat relaxed. The spectral energy distribution in the inner part of the galaxy, as well as along the extended filaments, is a smooth continuum and is consistent with that of a star-forming galaxy, suggesting that the extended, filamentarymore » emission is not due to the central active galactic nucleus, either from a large-scale ionized outflow or scattered polarized UV emission, but rather a massive population of young stars. We estimate an extinction-corrected star formation rate of 798 {+-} 42 M{sub Sun} yr{sup -1}, consistent with our earlier work based on low spatial resolution ultraviolet, optical, and infrared imaging. The lack of tidal features and multiple bulges, combine with the need for an exceptionally massive (>10{sup 11} M{sub Sun }) cold gas reservoir, suggest that this star formation is not the result of a merger of gas-rich galaxies. Instead, we propose that the high X-ray cooling rate of {approx}2700 M{sub Sun} yr{sup -1} is the origin of the cold gas reservoir. The combination of such a high cooling rate and the relatively weak radio source in the cluster core suggests that feedback has been unable to halt cooling in this system, leading to this tremendous burst of star formation.« less

Spots and the Activity of Stars in the Hyades Cluster from Observations with the Kepler Space Telescope (K2)

NASA Astrophysics Data System (ADS)

Savanov, I. S.; Dmitrienko, E. S.

2018-03-01

Observations of the K2 mission (continuing the program of the Kepler Space Telescope) are used to estimate the spot coverage S (the fractional area of spots on the surface of an active star) for stars of the Hyades cluster. The analysis is based on data on the photometric variations of 47 confirmed single cluster members, together with their atmospheric parameters, masses, and rotation periods. The resulting values of S for these Hyades objects are lower than those stars of the Pleiades cluster (on average, by Δ S 0.05-0.06). A comparison of the results of studies of cool, low-mass dwarfs in the Hyades and Pleiades clusters, as well as the results of a study of 1570 M stars from the main field observed in the Kepler SpaceMission, indicates that the Hyades stars are more evolved than the Pleiades stars, and demonstrate lower activity. The activity of seven solar-type Hyades stars ( S = 0.013 ± 0.006) almost approaches the activity level of the present-day Sun, and is lower than the activity of solar-mass stars in the Pleiades ( S = 0.031 ± 0.003). Solar-type stars in the Hyades rotate faster than the Sun (< P> = 8.6 d ), but slower than similar Pleiades stars.

A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster.

PubMed

Meibom, Søren; Barnes, Sydney A; Platais, Imants; Gilliland, Ronald L; Latham, David W; Mathieu, Robert D

2015-01-29

The ages of the most common stars--low-mass (cool) stars like the Sun, and smaller--are difficult to derive because traditional dating methods use stellar properties that either change little as the stars age or are hard to measure. The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology. To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years, and gyrochronology ages for older stars have been inferred from model predictions. Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.

SEARCH FOR RED DWARF STARS IN GLOBULAR CLUSTER NGC 6397

NASA Technical Reports Server (NTRS)

2002-01-01

Left A NASA Hubble Space Telescope image of a small region (1.4 light-years across) in the globular star cluster NGC 6397. Simulated stars (diamonds) have been added to this view of the same region of the cluster to illustrate what astronomers would have expected to see if faint red dwarf stars were abundant in the Milky Way Galaxy. The field would then contain 500 stars, according to theoretical calculations. Right The unmodified HST image shows far fewer stars than would be expected, according to popular theories of star formation. HST resolves about 200 stars. The stellar density is so low that HST can literally see right through the cluster and resolve far more distant background galaxies. From this observation, scientists have identified the surprising cutoff point below which nature apparently doesn't make many stars smaller that 1/5 the mass of our Sun. These HST findings provide new insights into star formation in our Galaxy. Technical detail:The globular cluster NGC 6397, one of the nearest and densest agglomerations of stars, is located 7,200 light-years away in the southern constellation Ara. This visible-light picture was taken on March 3, 1994 with the Wide Field Planetary Camera 2, as part the HST parallel observing program. Credit: F. Paresce, ST ScI and ESA and NASA

VizieR Online Data Catalog: Shape parameters for 154 Galactic open clusters (Zhai+, 2017)

NASA Astrophysics Data System (ADS)

Zhai, M.; Abt, H.; Zhao, G.; Li, C.

2017-06-01

The data used are from database WEBDA (http://www.univie.ac.at/webda/). We have found 946 open clusters with equatorial coordinates for each cluster member. Since cluster members are easily contaminated by field stars, we have only adopted stars with membership probabilities higher than 70% as cluster members. It is rarely possible to determine a cluster's shape with a small number of members, so we have only considered relatively richer clusters, which host more than 20 of the most probable member stars. After these selections, there are 154 clusters left. (1 data file).

A comprehensive HST BVI catalogue of star clusters in five Hickson compact groups of galaxies

NASA Astrophysics Data System (ADS)

Fedotov, K.; Gallagher, S. C.; Durrell, P. R.; Bastian, N.; Konstantopoulos, I. S.; Charlton, J.; Johnson, K. E.; Chandar, R.

2015-05-01

We present a photometric catalogue of star cluster candidates in Hickson compact groups (HCGs) 7, 31, 42, 59, and 92, based on observations with the Advanced Camera for Surveys and the Wide Field Camera 3 on the Hubble Space Telescope. The catalogue contains precise cluster positions (right ascension and declination), magnitudes, and colours in the BVI filters. The number of detected sources ranges from 2200 to 5600 per group, from which we construct the high-confidence sample by applying a number of criteria designed to reduce foreground and background contaminants. Furthermore, the high-confidence cluster candidates for each of the 16 galaxies in our sample are split into two subpopulations: one that may contain young star clusters and one that is dominated by globular older clusters. The ratio of young star cluster to globular cluster candidates varies from group to group, from equal numbers to the extreme of HCG 31 which has a ratio of 8 to 1, due to a recent starburst induced by interactions in the group. We find that the number of blue clusters with MV < -9 correlates well with the current star formation rate in an individual galaxy, while the number of globular cluster candidates with MV < -7.8 correlates well (though with large scatter) with the stellar mass. Analyses of the high-confidence sample presented in this paper show that star clusters can be successfully used to infer the gross star formation history of the host groups and therefore determine their placement in a proposed evolutionary sequence for compact galaxy groups.

A youthful cluster

NASA Image and Video Library

2015-08-24

Shown here in a new image taken with the Advanced Camera for Surveys (ACS) on board the NASA/ESA Hubble Space Telescope, is the globular cluster NGC 1783. This is one of the biggest globular clusters in the Large Magellanic Cloud, a satellite galaxy of our own galaxy, the Milky Way, in the southern hemisphere constellation of Dorado. First observed by John Herschel in 1835, NGC 1783 is nearly 160 000 light-years from Earth, and has a mass around 170 000 times that of the Sun. Globular clusters are dense collections of stars held together by their own gravity, which orbit around galaxies like satellites. The image clearly shows the symmetrical shape of NGC 1783 and the concentration of stars towards the centre, both typical features of globular clusters. By measuring the colour and brightness of individual stars, astronomers can deduce an overall age for a cluster and a picture of its star formation history. NGC 1783 is thought to be under one and a half billion years old — which is very young for globular clusters, which are typically several billion years old. During that time, it is thought to have undergone at least two periods of star formation, separated by 50 to 100 million years. This ebb and flow of star-forming activity is an indicator of how much gas is available for star formation at any one time. When the most massive stars created in the first burst of formation explode as supernovae they blow away the gas needed to form further stars, but the gas reservoir can later be replenished by less massive stars which last longer and shed their gas less violently. After this gas flows to the dense central regions of the star cluster, a second phase of star formation can take place and once again the short-lived massive stars blow away any leftover gas. This cycle can continue a few times, at which time the remaining gas reservoir is thought to be too small to form any new stars. A version of this image was entered into the Hubble's Hidden Treasures image pr

Supergiants and their shells in young globular clusters

NASA Astrophysics Data System (ADS)

Szécsi, Dorottya; Mackey, Jonathan; Langer, Norbert

2018-04-01

Context. Anomalous surface abundances are observed in a fraction of the low-mass stars of Galactic globular clusters, that may originate from hot-hydrogen-burning products ejected by a previous generation of massive stars. Aims: We aim to present and investigate a scenario in which the second generation of polluted low-mass stars can form in shells around cool supergiant stars within a young globular cluster. Methods: Simulations of low-metallicity massive stars (Mi 150-600 M⊙) show that both core-hydrogen-burning cool supergiants and hot ionizing stellar sources are expected to be present simulaneously in young globular clusters. Under these conditions, photoionization-confined shells form around the supergiants. We have simulated such a shell, investigated its stability and analysed its composition. Results: We find that the shell is gravitationally unstable on a timescale that is shorter than the lifetime of the supergiant, and the Bonnor-Ebert mass of the overdense regions is low enough to allow star formation. Since the low-mass stellar generation formed in this shell is made up of the material lost from the supergiant, its composition necessarily reflects the composition of the supergiant wind. We show that the wind contains hot-hydrogen-burning products, and that the shell-stars therefore have very similar abundance anomalies that are observed in the second generation stars of globular clusters. Considering the mass-budget required for the second generation star-formation, we offer two solutions. Either a top-heavy initial mass function is needed with an index of -1.71 to -2.07. Alternatively, we suggest the shell-stars to have a truncated mass distribution, and solve the mass budget problem by justifiably accounting for only a fraction of the first generation. Conclusions: Star-forming shells around cool supergiants could form the second generation of low-mass stars in Galactic globular clusters. Even without forming a photoionizaton-confined shell, the cool supergiant stars predicted at low-metallicity could contribute to the pollution of the interstellar medium of the cluster from which the second generation was born. Thus, the cool supergiant stars should be regarded as important contributors to the evolution of globular clusters.

The Evolution of Dusty Star formation in Galaxy Clusters to z = 1: Spitzer Infrared Observations of the First Red-Sequence Cluster Survey

NASA Astrophysics Data System (ADS)

Webb, T. M. A.; O'Donnell, D.; Yee, H. K. C.; Gilbank, David; Coppin, Kristen; Ellingson, Erica; Faloon, Ashley; Geach, James E.; Gladders, Mike; Noble, Allison; Muzzin, Adam; Wilson, Gillian; Yan, Renbin

2013-10-01

We present the results of an infrared (IR) study of high-redshift galaxy clusters with the MIPS camera on board the Spitzer Space Telescope. We have assembled a sample of 42 clusters from the Red-Sequence Cluster Survey-1 over the redshift range 0.3 < z < 1.0 and spanning an approximate range in mass of 1014-15 M ⊙. We statistically measure the number of IR-luminous galaxies in clusters above a fixed inferred IR luminosity of 2 × 1011 M ⊙, assuming a star forming galaxy template, per unit cluster mass and find it increases to higher redshift. Fitting a simple power-law we measure evolution of (1 + z)5.1 ± 1.9 over the range 0.3 < z < 1.0. These results are tied to the adoption of a single star forming galaxy template; the presence of active galactic nuclei, and an evolution in their relative contribution to the mid-IR galaxy emission, will alter the overall number counts per cluster and their rate of evolution. Under the star formation assumption we infer the approximate total star formation rate per unit cluster mass (ΣSFR/M cluster). The evolution is similar, with ΣSFR/M cluster ~ (1 + z)5.4 ± 1.9. We show that this can be accounted for by the evolution of the IR-bright field population over the same redshift range; that is, the evolution can be attributed entirely to the change in the in-falling field galaxy population. We show that the ΣSFR/M cluster (binned over all redshift) decreases with increasing cluster mass with a slope (ΣSFR/M_{cluster} \\sim M_{cluster}^{-1.5+/- 0.4}) consistent with the dependence of the stellar-to-total mass per unit cluster mass seen locally. The inferred star formation seen here could produce ~5%-10% of the total stellar mass in massive clusters at z = 0, but we cannot constrain the descendant population, nor how rapidly the star-formation must shut-down once the galaxies have entered the cluster environment. Finally, we show a clear decrease in the number of IR-bright galaxies per unit optical galaxy in the cluster cores, confirming star formation continues to avoid the highest density regions of the universe at z ~ 0.75 (the average redshift of the high-redshift clusters). While several previous studies appear to show enhanced star formation in high-redshift clusters relative to the field we note that these papers have not accounted for the overall increase in galaxy or dark matter density at the location of clusters. Once this is done, clusters at z ~ 0.75 have the same or less star formation per unit mass or galaxy as the field.

An Observational Study of Blended Young Stellar Clusters in the Galactic Plane - Do Massive Stars form First?

NASA Astrophysics Data System (ADS)

Martínez-Galarza, Rafael; Protopapas, Pavlos; Smith, Howard A.; Morales, Esteban

2018-01-01

From an observational point of view, the early life of massive stars is difficult to understand partly because star formation occurs in crowded clusters where individual stars often appear blended together in the beams of infrared telescopes. This renders the characterization of the physical properties of young embedded clusters via spectral energy distribution (SED) fitting a challenging task. Of particular relevance for the testing of star formation models is the question of whether the claimed universality of the IMF (references) is reflected in an equally universal integrated galactic initial mass function (IGIMF) of stars. In other words, is the set of all stellar masses in the galaxy sampled from a single universal IMF, or does the distribution of masses depend on the environment, making the IGIMF different from the canonical IMF? If the latter is true, how different are the two? We present a infrared SED analysis of ~70 Spitzer-selected, low mass ($<100~\\rm{M}_{\\odot}$), galactic blended clusters. For all of the clusters we obtain the most probable individual SED of each member and derive their physical properties, effectively deblending the confused emission from individual YSOs. Our algorithm incorporates a combined probabilistic model of the blended SEDs and the unresolved images in the long-wavelength end. We find that our results are compatible with competitive accretion in the central regions of young clusters, with the most massive stars forming early on in the process and less massive stars forming about 1Myr later. We also find evidence for a relationship between the total stellar mass of the cluster and the mass of the most massive member that favors optimal sampling in the cluster and disfavors random sampling for the canonical IMF, implying that star formation is self-regulated, and that the mass of the most massive star in a cluster depends on the available resources. The method presented here is easily adapted to future observations of clustered regions of star formation with JWST and other high resolution facilities.

Formation of massive black holes through runaway collisions in dense young star clusters.

PubMed

Zwart, Simon F Portegies; Baumgardt, Holger; Hut, Piet; Makino, Junichiro; McMillan, Stephen L W

2004-04-15

A luminous X-ray source is associated with MGG 11--a cluster of young stars approximately 200 pc from the centre of the starburst galaxy M 82 (refs 1, 2). The properties of this source are best explained by invoking a black hole with a mass of at least 350 solar masses (350 M(o)), which is intermediate between stellar-mass and supermassive black holes. A nearby but somewhat more massive cluster (MGG 9) shows no evidence of such an intermediate-mass black hole, raising the issue of just what physical characteristics of the clusters can account for this difference. Here we report numerical simulations of the evolution and motion of stars within the clusters, where stars are allowed to merge with each other. We find that for MGG 11 dynamical friction leads to the massive stars sinking rapidly to the centre of the cluster, where they participate in a runaway collision. This produces a star of 800-3,000 M(o) which ultimately collapses to a black hole of intermediate mass. No such runaway occurs in the cluster MGG 9, because the larger cluster radius leads to a mass segregation timescale a factor of five longer than for MGG 11.

Primordial binary populations in low-density star clusters as seen by Chandra: globular clusters versus old open clusters

NASA Astrophysics Data System (ADS)

van den Berg, Maureen C.

2015-08-01

The binaries in the core of a star cluster are the energy source that prevents the cluster from experiencing core collapse. To model the dynamical evolution of a cluster, it is important to have constraints on the primordial binary content. X-ray observations of old star clusters are very efficient in detecting the close interacting binaries among the cluster members. The X-ray sources in star clusters are a mix of binaries that were dynamically formed and primordial binaries. In massive, dense star clusters, dynamical encounters play an important role in shaping the properties and numbers of the binaries. In contrast, in the low-density clusters the impact of dynamical encounters is presumed to be very small, and the close binaries detected in X-rays represent a primordial population. The lowest density globular clusters have current masses and central densities similar to those of the oldest open clusters in our Milky Way. I will discuss the results of studies with the Chandra X-ray Observatory that have nevertheless revealed a clear dichotomy: far fewer (if any at all) X-ray sources are detected in the central regions of the low-density globular clusters compared to the number of secure cluster members that have been detected in old open clusters (above a limiting X-ray luminosity of typically 4e30 erg/s). The low stellar encounter rates imply that dynamical destruction of binaries can be ignored at present, therefore an explanation must be sought elsewhere. I will discuss several factors that can shed light on the implied differences between the primordial close binary populations in the two types of star clusters.

Radiative Feedback of Forming Star Clusters on Their GMC Environments: Theory and Simulation

NASA Astrophysics Data System (ADS)

Howard, C. S.; Pudritz, R. E.; Harris, W. E.

2013-07-01

Star clusters form from dense clumps within a molecular cloud. Radiation from these newly formed clusters feeds back on their natal molecular cloud through heating and ionization which ultimately stops gas accretion into the cluster. Recent studies suggest that radiative feedback effects from a single cluster may be sufficient to disrupt an entire cloud over a short timescale. Simulating cluster formation on a large scale, however, is computationally demanding due to the high number of stars involved. For this reason, we present a model for representing the radiative output of an entire cluster which involves randomly sampling an initial mass function (IMF) as the cluster accretes mass. We show that this model is able to reproduce the star formation histories of observed clusters. To examine the degree to which radiative feedback shapes the evolution of a molecular cloud, we use the FLASH adaptive-mesh refinement hydrodynamics code to simulate cluster formation in a turbulent cloud. Unlike previous studies, sink particles are used to represent a forming cluster rather than individual stars. Our cluster model is then coupled with a raytracing scheme to treat radiative transfer as the clusters grow in mass. This poster will outline the details of our model and present preliminary results from our 3D hydrodynamical simulations.

Multiple stellar populations in Magellanic Cloud clusters - VI. A survey of multiple sequences and Be stars in young clusters

NASA Astrophysics Data System (ADS)

Milone, A. P.; Marino, A. F.; Di Criscienzo, M.; D'Antona, F.; Bedin, L. R.; Da Costa, G.; Piotto, G.; Tailo, M.; Dotter, A.; Angeloni, R.; Anderson, J.; Jerjen, H.; Li, C.; Dupree, A.; Granata, V.; Lagioia, E. P.; Mackey, A. D.; Nardiello, D.; Vesperini, E.

2018-06-01

The split main sequences (MSs) and extended MS turnoffs (eMSTOs) detected in a few young clusters have demonstrated that these stellar systems host multiple populations differing in a number of properties such as rotation and, possibly, age. We analyse Hubble Space Telescope photometry for 13 clusters with ages between ˜40 and ˜1000 Myr and of different masses. Our goal is to investigate for the first time the occurrence of multiple populations in a large sample of young clusters. We find that all the clusters exhibit the eMSTO phenomenon and that MS stars more massive than ˜1.6 M_{⊙} define a blue and a red MS, with the latter hosting the majority of MS stars. The comparison between the observations and isochrones suggests that the blue MSs are made of slow-rotating stars, while the red MSs host stars with rotational velocities close to the breakup value. About half of the bright MS stars in the youngest clusters are H α emitters. These Be stars populate the red MS and the reddest part of the eMSTO, thus supporting the idea that the red MS is made of fast rotators. We conclude that the split MS and the eMSTO are a common feature of young clusters in both Magellanic Clouds. The phenomena of a split MS and an eMSTO occur for stars that are more massive than a specific threshold, which is independent of the host-cluster mass. As a by-product, we report the serendipitous discovery of a young Small Magellanic Cloud cluster, GALFOR 1.

White Dwarfs in Star Clusters: The Initial-Final Mass Relation for Stars from 0.85 to 8 M$_\\odot$

NASA Astrophysics Data System (ADS)

Cummings, Jeffrey; Kalirai, Jason; Tremblay, P.-E.; Ramírez-Ruiz, Enrico

2018-01-01

The spectroscopic study of white dwarfs provides both their mass, cooling age, and intrinsic photometric properties. For white dwarfs in the field of well-studied star clusters, this intrinsic photometry can be used to determine if they are members of that star cluster. Comparison of a member white dwarf's cooling age to its total cluster's age provides the evolutionary timescale of its progenitor star, and hence the mass. This is the initial-final mass relation (IFMR) for stars, which gives critical information on how a progenitor star evolves and loses mass throughout its lifetime, and how this changes with progenitor mass. Our work, for the first time, presents a uniform analysis of 85 white dwarf cluster members spanning from progenitor masses of 0.85 to 8 M$_\\odot$. Comparison of our work to theoretical IFMRs shows remarkable consistency in their shape but differences remain. We will discuss possible explanations for these differences, including the effects of stellar rotation.

Hubble Space Telescope/Near-Infrared Camera and Multi-Object Spectrometer Observations of the GLIMPSE9 Stellar Cluster

NASA Astrophysics Data System (ADS)

Messineo, Maria; Figer, Donald F.; Davies, Ben; Kudritzki, R. P.; Rich, R. Michael; MacKenty, John; Trombley, Christine

2010-01-01

We present Hubble Space Telescope/Near-Infrared Camera and Multi-Object Spectrometer photometry, and low-resolution K-band spectra of the GLIMPSE9 stellar cluster. The newly obtained color-magnitude diagram shows a cluster sequence with H - KS = ~1 mag, indicating an interstellar extinction A _K_s = 1.6 ± 0.2 mag. The spectra of the three brightest stars show deep CO band heads, which indicate red supergiants with spectral type M1-M2. Two 09-B2 supergiants are also identified, which yield a spectrophotometric distance of 4.2 ± 0.4 kpc. Presuming that the population is coeval, we derive an age between 15 and 27 Myr, and a total cluster mass of 1600 ± 400 M sun, integrated down to 1 M sun. In the vicinity of GLIMPSE9 are several H II regions and supernova remnants, all of which (including GLIMPSE9) are probably associated with a giant molecular cloud (GMC) in the inner galaxy. GLIMPSE9 probably represents one episode of massive star formation in this GMC. We have identified several other candidate stellar clusters of the same complex.

Gas expulsion vs gas retention in young stellar clusters II: effects of cooling and mass segregation

NASA Astrophysics Data System (ADS)

Silich, Sergiy; Tenorio-Tagle, Guillermo

2018-05-01

Gas expulsion or gas retention is a central issue in most of the models for multiple stellar populations and light element anti-correlations in globular clusters. The success of the residual matter expulsion or its retention within young stellar clusters has also a fundamental importance in order to understand how star formation proceeds in present-day and ancient star-forming galaxies and if proto-globular clusters with multiple stellar populations are formed in the present epoch. It is usually suggested that either the residual gas is rapidly ejected from star-forming clouds by stellar winds and supernova explosions, or that the enrichment of the residual gas and the formation of the second stellar generation occur so rapidly, that the negative stellar feedback is not significant. Here we continue our study of the early development of star clusters in the extreme environments and discuss the restrictions that strong radiative cooling and stellar mass segregation provide on the gas expulsion from dense star-forming clouds. A large range of physical initial conditions in star-forming clouds which include the star-forming cloud mass, compactness, gas metallicity, star formation efficiency and effects of massive stars segregation are discussed. It is shown that in sufficiently massive and compact clusters hot shocked winds around individual massive stars may cool before merging with their neighbors. This dramatically reduces the negative stellar feedback, prevents the development of the global star cluster wind and expulsion of the residual and the processed matter into the ambient interstellar medium. The critical lines which separate the gas expulsion and the gas retention regimes are obtained.

ALMA reveals sunburn: CO dissociation around AGB stars in the globular cluster 47 Tucanae

NASA Astrophysics Data System (ADS)

McDonald, I.; Zijlstra, A. A.; Lagadec, E.; Sloan, G. C.; Boyer, M. L.; Matsuura, M.; Smith, R. J.; Smith, C. L.; Yates, J. A.; van Loon, J. Th.; Jones, O. C.; Ramstedt, S.; Avison, A.; Justtanont, K.; Olofsson, H.; Blommaert, J. A. D. L.; Goldman, S. R.; Groenewegen, M. A. T.

2015-11-01

Atacama Large Millimetre Array observations show a non-detection of carbon monoxide around the four most luminous asymptotic giant branch (AGB) stars in the globular cluster 47 Tucanae. Stellar evolution models and star counts show that the mass-loss rates from these stars should be ˜1.2-3.5 × 10-7 M⊙ yr-1. We would naïvely expect such stars to be detectable at this distance (4.5 kpc). By modelling the ultraviolet radiation field from post-AGB stars and white dwarfs in 47 Tuc, we conclude that CO should be dissociated abnormally close to the stars. We estimate that the CO envelopes will be truncated at a few hundred stellar radii from their host stars and that the line intensities are about two orders of magnitude below our current detection limits. The truncation of CO envelopes should be important for AGB stars in dense clusters. Observing the CO (3-2) and higher transitions and targeting stars far from the centres of clusters should result in the detections needed to measure the outflow velocities from these stars.

The Origin of IRS 16: Dynamically Driven In-Spiral of a Dense Star Cluster to the Galactic Center?

NASA Astrophysics Data System (ADS)

Portegies Zwart, Simon F.; McMillan, Stephen L. W.; Gerhard, Ortwin

2003-08-01

We use direct N-body simulations to study the in-spiral and internal evolution of dense star clusters near the Galactic center. These clusters sink toward the center owing to dynamical friction with the stellar background and may go into core collapse before being disrupted by the Galactic tidal field. If a cluster reaches core collapse before disruption, its dense core, which has become rich in massive stars, survives to reach close to the Galactic center. When it eventually dissolves, the cluster deposits a disproportionate number of massive stars in the innermost parsec of the Galactic nucleus. Comparing the spatial distribution and kinematics of the massive stars with observations of IRS 16, a group of young He I stars near the Galactic center, we argue that this association may have formed in this way.

The Initial Mass Function of the Arches Cluster

NASA Astrophysics Data System (ADS)

Hosek, Matthew; Lu, Jessica; Anderson, Jay; Ghez, Andrea; Morris, Mark; Do, Tuan; Clarkson, William; Albers, Saundra; Weisz, Daniel

2018-01-01

The Arches star cluster is only 26 pc (in projection) from Sgr A*, the supermassive black hole at the Galactic Center. This young massive cluster allows us to examine the impact of the extreme Galactic Center environment on the stellar Initial Mass Function (IMF). However, measuring the IMF of the Arches is challenging due to the highly variable extinction along the line of sight, which makes it difficult to separate cluster members from the field stars. We use high-precision proper motion and photometric measurements obtained with the Hubble Space Telescope to calculate cluster membership probabilities for stars down to ~2 M_sun out to the outskirts of the cluster (3 pc). In addition, we measure the effective temperatures of a small sample of cluster members in order to calibrate the mass-luminosity relationship using using Keck OSIRS K-band spectroscopy. We forward model these observations to simultaneously constrain the cluster IMF, age, distance, and extinction. We obtain an IMF that is shallower than what is observed locally, with a higher fraction of high-mass stars to low mass stars (i.e., “top-heavy”). We will compare the IMF of the Arches to similar clusters in the Galactic disk and quantify the effect of the GC environment on the star formation process.

Chemodynamical Clustering Applied to APOGEE Data: Rediscovering Globular Clusters

NASA Astrophysics Data System (ADS)

Chen, Boquan; D’Onghia, Elena; Pardy, Stephen A.; Pasquali, Anna; Bertelli Motta, Clio; Hanlon, Bret; Grebel, Eva K.

2018-06-01

We have developed a novel technique based on a clustering algorithm that searches for kinematically and chemically clustered stars in the APOGEE DR12 Cannon data. As compared to classical chemical tagging, the kinematic information included in our methodology allows us to identify stars that are members of known globular clusters with greater confidence. We apply our algorithm to the entire APOGEE catalog of 150,615 stars whose chemical abundances are derived by the Cannon. Our methodology found anticorrelations between the elements Al and Mg, Na and O, and C and N previously identified in the optical spectra in globular clusters, even though we omit these elements in our algorithm. Our algorithm identifies globular clusters without a priori knowledge of their locations in the sky. Thus, not only does this technique promise to discover new globular clusters, but it also allows us to identify candidate streams of kinematically and chemically clustered stars in the Milky Way.

New atlas of open star clusters

NASA Astrophysics Data System (ADS)

Seleznev, Anton F.; Avvakumova, Ekaterina; Kulesh, Maxim; Filina, Julia; Tsaregorodtseva, Polina; Kvashnina, Alvira

2017-11-01

Due to numerous new discoveries of open star clusters in the last two decades, astronomers need an easy-touse resource to get visual information on the relative position of clusters in the sky. Therefore we propose a new atlas of open star clusters. It is based on a table compiled from the largest modern cluster catalogues. The atlas shows the positions and sizes of 3291 clusters and associations, and consists of two parts. The first contains 108 maps of 12 by 12 degrees with an overlapping of 2 degrees in three strips along the Galactic equator. The second one is an online web application, which shows a square field of an arbitrary size, either in equatorial coordinates or in galactic coordinates by request. The atlas is proposed for the sampling of clusters and cluster stars for further investigation. Another use is the identification of clusters among overdensities in stellar density maps or among stellar groups in images of the sky.

STAR CLUSTERS IN A NUCLEAR STAR FORMING RING: THE DISAPPEARING STRING OF PEARLS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Väisänen, Petri; Barway, Sudhanshu; Randriamanakoto, Zara, E-mail: petri@saao.ac.za

2014-12-20

An analysis of the star cluster population in a low-luminosity early-type galaxy, NGC 2328, is presented. The clusters are found in a tight star forming nuclear spiral/ring pattern and we also identify a bar from structural two-dimensional decomposition. These massive clusters are forming very efficiently in the circumnuclear environment and they are young, possibly all less than 30 Myr of age. The clusters indicate an azimuthal age gradient, consistent with a ''pearls-on-a-string'' formation scenario, suggesting bar-driven gas inflow. The cluster mass function has a robust down turn at low masses at all age bins. Assuming clusters are born with a power-lawmore » distribution, this indicates extremely rapid disruption at timescales of just several million years. If found to be typical, it means that clusters born in dense circumnuclear rings do not survive to become old globular clusters in non-interacting systems.« less

Chandra/ACIS Observations of the 30 Doradus Star-Forming Complex

NASA Astrophysics Data System (ADS)

Townsley, Leisa; Broos, Patrick; Feigelson, Eric; Burrows, David; Chu, You-Hua; Garmire, Gordon; Griffiths, Richard; Maeda, Yoshitomo; Pavlov, George; Tsuboi, Yohko

2002-04-01

30 Doradus is the archetype giant extragalactic H II region, a massive star-forming complex in the Large Magellanic Cloud. We examine high-spatial-resolution X-ray images and spectra of the essential parts of 30 Doradus, obtained with the Advanced CCD Imaging Spectrometer (ACIS) aboard the Chandra X-ray Observatory. The central cluster of young high-mass stars, R136, is resolved at the arcsecond level, allowing spectral analysis of bright constituents; other OB/Wolf-Rayet binaries and multiple systems (e.g. R139, R140) are also detected. Spatially-resolved spectra are presented for N157B, the composite SNR containing a 16-msec pulsar. The spectrally soft superbubble structures seen by ROSAT are dramatically imaged by Chandra; we explore the spectral differences they exhibit. Taken together, the components of 30 Doradus give us an excellent microscopic view of high-energy phenomena seen on larger scales in more distant galaxies as starbursts and galactic winds.

Advances in stellar evolution; Proceedings of the Workshop on Stellar Ecology, Marciana Marina, Italy, June 23-29, 1996

NASA Astrophysics Data System (ADS)

Rood, R. T.; Renzini, A.

1997-01-01

The present volume on stellar evolution discusses fundamentals of stellar evolution and star clusters, variable stars, AGB stars and planetary nebulae, white dwarfs, binary star evolution, and stars in galaxies. Attention is given to the stellar population in the Galactic bulge, a photometric study of NGC 458, and HST observations of high-density globular clusters. Other topics addressed include the Cepheid instability strip in external galaxies, Hyades cluster white dwarfs and the initial-final mass relation, element diffusion in novae, mass function of the stars in the solar neighborhood, synthetic spectral indices for elliptical galaxies, and stars at the Galactic center.

Zodiacal Exoplanets in Time: Searching for Young Stars in K2

NASA Astrophysics Data System (ADS)

Morris, Nathan; Mann, Andrew W.

2017-06-01

Nearby young, open clusters such as the Hyades, Pleiades, and Praesepe provide an important reference point for the properties of stellar systems in general. In each cluster, all stars are of the same known age. As such, observations of planetary systems around these stars can be used to gain insight into the early stages of planetary system formation. K2, the revived Kepler mission, has provided a vast number of light curves for young stars in the and elsewhere in the K2 field. We aim to compute rotational periods from sunspot patterns for all K2 target stars and use gyrochronometric relationships derived from cluster stars to determine their ages. From there, we will search for planets around young stars outside the clusters with the ultimate goal of shedding light on how planets and planetary systems evolve with time.

The Nature and Origin of UCDs in the Coma Cluster

NASA Astrophysics Data System (ADS)

Chiboucas, Kristin; Tully, R. Brent; Madrid, Juan; Phillipps, Steven; Carter, David; Peng, Eric

2018-01-01

UCDs are super massive star clusters found largely in dense regions but have also been found around individual galaxies and in smaller groups. Their origin is still under debate but currently favored scenarios include formation as giant star clusters, either as the brightest globular clusters or through mergers of super star clusters, themselves formed during major galaxy mergers, or as remnant nuclei from tidal stripping of nucleated dwarf ellipticals. Establishing the nature of these enigmatic objects has important implications for our understanding of star formation, star cluster formation, the missing satellite problem, and galaxy evolution. We are attempting to disentangle these competing formation scenarios with a large survey of UCDs in the Coma cluster. Using ACS two-passband imaging from the HST/ACS Coma Cluster Treasury Survey, we are using colors and sizes to identify the UCD cluster members. With a large size limited sample of the UCD population within the core region of the Coma cluster, we are investigating the population size, properties, and spatial distribution, and comparing that with the Coma globular cluster and nuclear star cluster populations to discriminate between the threshing and globular cluster scenarios. In previous work, we had found a possible correlation of UCD colors with host galaxy and a possible excess of UCDs around a non-central giant galaxy with an unusually large globular cluster population, both suggestive of a globular cluster origin. With a larger sample size and additional imaging fields that encompass the regions around these giant galaxies, we have found that the color correlation with host persists and the giant galaxy with unusually large globular cluster population does appear to host a large UCD population as well. We present the current status of the survey.

Hubble Catches Stellar Exodus in Action

NASA Image and Video Library

2015-05-14

Using NASA’s Hubble Space Telescope, astronomers have captured for the first time snapshots of fledging white dwarf stars beginning their slow-paced, 40-million-year migration from the crowded center of an ancient star cluster to the less populated suburbs. White dwarfs are the burned-out relics of stars that rapidly lose mass, cool down and shut off their nuclear furnaces. As these glowing carcasses age and shed weight, their orbits begin to expand outward from the star cluster’s packed downtown. This migration is caused by a gravitational tussle among stars inside the cluster. Globular star clusters sort out stars according to their mass, governed by a gravitational billiard ball game where lower mass stars rob momentum from more massive stars. The result is that heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to the edge. This process is known as "mass segregation." Until these Hubble observations, astronomers had never definitively seen the dynamical conveyor belt in action. Astronomers used Hubble to watch the white-dwarf exodus in the globular star cluster 47 Tucanae, a dense swarm of hundreds of thousands of stars in our Milky Way galaxy. The cluster resides 16,700 light-years away in the southern constellation Tucana. Read more: www.nasa.gov/feature/goddard/hubble-catches-stellar-exodu... Credits: NASA, ESA, and H. Richer and J. Heyl (University of British Columbia, Vancouver, Canada); acknowledgement: J. Mack (STScI) and G. Piotto (University of Padova, Italy) NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

Formation of globular cluster candidates in merging proto-galaxies at high redshift: a view from the FIRE cosmological simulations

DOE PAGES

Kim, Ji-hoon; Ma, Xiangcheng; Grudić, Michael Y.; ...

2017-11-23

Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypotheses of present-day metal-poor globular clusters. Here, we find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ≳ 10 5–6 M ⊙ collectively to high density, at which point it rapidly turns into stars beforemore » stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical time-scale, tff ≲ 3 Myr, shorter than most stellar feedback time-scales. Our simulation then allows us to trace how clusters could become virialized and tightly bound to survive for up to ~420 Myr till the end of the simulation. Finally, because the cluster's tightly bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.« less

Formation of globular cluster candidates in merging proto-galaxies at high redshift: a view from the FIRE cosmological simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Ji-hoon; Ma, Xiangcheng; Grudić, Michael Y.

Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypotheses of present-day metal-poor globular clusters. Here, we find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ≳ 10 5–6 M ⊙ collectively to high density, at which point it rapidly turns into stars beforemore » stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical time-scale, tff ≲ 3 Myr, shorter than most stellar feedback time-scales. Our simulation then allows us to trace how clusters could become virialized and tightly bound to survive for up to ~420 Myr till the end of the simulation. Finally, because the cluster's tightly bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.« less

STAR CLUSTER FORMATION WITH STELLAR FEEDBACK AND LARGE-SCALE INFLOW

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matzner, Christopher D.; Jumper, Peter H., E-mail: matzner@astro.utoronto.ca

2015-12-10

During star cluster formation, ongoing mass accretion is resisted by stellar feedback in the form of protostellar outflows from the low-mass stars and photo-ionization and radiation pressure feedback from the massive stars. We model the evolution of cluster-forming regions during a phase in which both accretion and feedback are present and use these models to investigate how star cluster formation might terminate. Protostellar outflows are the strongest form of feedback in low-mass regions, but these cannot stop cluster formation if matter continues to flow in. In more massive clusters, radiation pressure and photo-ionization rapidly clear the cluster-forming gas when itsmore » column density is too small. We assess the rates of dynamical mass ejection and of evaporation, while accounting for the important effect of dust opacity on photo-ionization. Our models are consistent with the census of protostellar outflows in NGC 1333 and Serpens South and with the dust temperatures observed in regions of massive star formation. Comparing observations of massive cluster-forming regions against our model parameter space, and against our expectations for accretion-driven evolution, we infer that massive-star feedback is a likely cause of gas disruption in regions with velocity dispersions less than a few kilometers per second, but that more massive and more turbulent regions are too strongly bound for stellar feedback to be disruptive.« less

Formation of globular cluster candidates in merging proto-galaxies at high redshift: a view from the FIRE cosmological simulations

NASA Astrophysics Data System (ADS)

Kim, Ji-hoon; Ma, Xiangcheng; Grudić, Michael Y.; Hopkins, Philip F.; Hayward, Christopher C.; Wetzel, Andrew; Faucher-Giguère, Claude-André; Kereš, Dušan; Garrison-Kimmel, Shea; Murray, Norman

2018-03-01

Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypotheses of present-day metal-poor globular clusters. We find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ≳ 105-6 M⊙ collectively to high density, at which point it rapidly turns into stars before stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical time-scale, tff ≲ 3 Myr, shorter than most stellar feedback time-scales. Our simulation then allows us to trace how clusters could become virialized and tightly bound to survive for up to ˜420 Myr till the end of the simulation. Because the cluster's tightly bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.

Collisions in Compact Star Clusters.

NASA Astrophysics Data System (ADS)

Portegies Zwart, S. F.

The high stellar densities in young compact star clusters, such as the star cluster R136 in the 30 Doradus region, may lead to a large number of stellar collisions. Such collisions were recently found to be much more frequent than previous estimates. The number of collisions scales with the number of stars for clusters with the same initial relaxation time. These collisions take place in a few million years. The collision products may finally collapse into massive black holes. The fraction of the total mass in the star cluster which ends up in a single massive object scales with the total mass of the cluster and its relaxation time. This mass fraction is rather constant, within a factor two or so. Wild extrapolation from the relatively small masses of the studied systems to the cores of galactic nuclei may indicate that the massive black holes in these systems have formed in a similar way.

A Search for Variable Stars in Ruprecht 134 (Abstract)

NASA Astrophysics Data System (ADS)

El Hamri, R.; Blake, M.

2018-06-01

(Abstract only) Contact binary stars have been found in many old open clusters. These stars are useful for obtaining the distances to these star clusters and for understanding the stellar populations and evolution of the old clusters. Ruprecht 134 is a relatively neglected, old open cluster with an age of about 1 Gyr. We have obtained observations of Ruprecht 134 using the 1-meter telescope at Cerro Tololo Interamerican Observatory for the purpose of identifying candidate contact binaries. We present the preliminary results of this search and discuss future observations.

Photometry Using Kepler "Superstamps" of Open Clusters NGC 6791 & NGC 6819

NASA Astrophysics Data System (ADS)

Kuehn, Charles A.; Drury, Jason A.; Bellamy, Beau R.; Stello, Dennis; Bedding, Timothy R.; Reed, Mike; Quick, Breanna

2015-09-01

The Kepler space telescope has proven to be a gold mine for the study of variable stars. Usually, Kepler only reads out a handful of pixels around each pre-selected target star, omitting a large number of stars in the Kepler field. Fortunately, for the open clusters NGC 6791 and NGC 6819, Kepler also read out larger "superstamps" which contained complete images of the central region of each cluster. These cluster images can be used to study additional stars in the open clusters that were not originally on Kepler's target list. We discuss our work on using two photometric techniques to analyze these superstamps and present sample results from this project to demonstrate the value of this technique for a wide variety of variable stars.

Near-infrared variability study of the central 2.3 × 2.3 arcmin2 of the Galactic Centre - II. Identification of RR Lyrae stars in the Milky Way nuclear star cluster

NASA Astrophysics Data System (ADS)

Dong, Hui; Schödel, Rainer; Williams, Benjamin F.; Nogueras-Lara, Francisco; Gallego-Cano, Eulalia; Gallego-Calvente, Teresa; Wang, Q. Daniel; Rich, R. Michael; Morris, Mark R.; Do, Tuan; Ghez, Andrea

2017-11-01

Because of strong and spatially highly variable interstellar extinction and extreme source crowding, the faint (K ≥ 15) stellar population in the Milky Way's nuclear star cluster is still poorly studied. RR Lyrae stars provide us with a tool to estimate the mass of the oldest, relative dim stellar population. Recently, we analysed HST/WFC3/IR observations of the central 2.3 × 2.3 arcmin2 of the Milky Way and found 21 variable stars with periods between 0.2 and 1 d. Here, we present a further comprehensive analysis of these stars. The period-luminosity relationship of RR Lyrae is used to derive their extinctions and distances. Using multiple approaches, we classify our sample as 4 RRc stars, 4 RRab stars, 3 RRab candidates and 10 binaries. Especially, the four RRab stars show sawtooth light curves and fall exactly on to the Oosterhoff I division in the Bailey diagram. Compared to the RRab stars reported by Minniti et al., our new RRab stars have higher extinction (AK > 1.8) and should be closer to the Galactic Centre. The extinction and distance of one RRab stars match those for the Milky Way's nuclear star cluster given in previous works. We perform simulations and find that after correcting for incompleteness, there could be not more than 40 RRab stars within the Milky Way's nuclear star cluster and in our field of view. Through comparing with the known globular clusters of the Milky Way, we estimate that if there exists an old, metal-poor (-1.5 < [Fe/H] < -1) stellar population in the Milky Way nuclear star cluster on a scale of 5 × 5 pc, then it contributes at most 4.7 × 105 M⊙, I.e. ˜18 per cent of the stellar mass.

The Gaia-ESO Survey: the present-day radial metallicity distribution of the Galactic disc probed by pre-main-sequence clusters

NASA Astrophysics Data System (ADS)

Spina, L.; Randich, S.; Magrini, L.; Jeffries, R. D.; Friel, E. D.; Sacco, G. G.; Pancino, E.; Bonito, R.; Bravi, L.; Franciosini, E.; Klutsch, A.; Montes, D.; Gilmore, G.; Vallenari, A.; Bensby, T.; Bragaglia, A.; Flaccomio, E.; Koposov, S. E.; Korn, A. J.; Lanzafame, A. C.; Smiljanic, R.; Bayo, A.; Carraro, G.; Casey, A. R.; Costado, M. T.; Damiani, F.; Donati, P.; Frasca, A.; Hourihane, A.; Jofré, P.; Lewis, J.; Lind, K.; Monaco, L.; Morbidelli, L.; Prisinzano, L.; Sousa, S. G.; Worley, C. C.; Zaggia, S.

2017-05-01

Context. The radial metallicity distribution in the Galactic thin disc represents a crucial constraint for modelling disc formation and evolution. Open star clusters allow us to derive both the radial metallicity distribution and its evolution over time. Aims: In this paper we perform the first investigation of the present-day radial metallicity distribution based on [Fe/H] determinations in late type members of pre-main-sequence clusters. Because of their youth, these clusters are therefore essential for tracing the current interstellar medium metallicity. Methods: We used the products of the Gaia-ESO Survey analysis of 12 young regions (age < 100 Myr), covering Galactocentric distances from 6.67 to 8.70 kpc. For the first time, we derived the metal content of star forming regions farther than 500 pc from the Sun. Median metallicities were determined through samples of reliable cluster members. For ten clusters the membership analysis is discussed in the present paper, while for other two clusters (I.e. Chamaeleon I and Gamma Velorum) we adopted the members identified in our previous works. Results: All the pre-main-sequence clusters considered in this paper have close-to-solar or slightly sub-solar metallicities. The radial metallicity distribution traced by these clusters is almost flat, with the innermost star forming regions having [Fe/H] values that are 0.10-0.15 dex lower than the majority of the older clusters located at similar Galactocentric radii. Conclusions: This homogeneous study of the present-day radial metallicity distribution in the Galactic thin disc favours models that predict a flattening of the radial gradient over time. On the other hand, the decrease of the average [Fe/H] at young ages is not easily explained by the models. Our results reveal a complex interplay of several processes (e.g. star formation activity, initial mass function, supernova yields, gas flows) that controlled the recent evolution of the Milky Way. Based on observations made with the ESO/VLT, at Paranal Observatory, under program 188.B-3002 (The Gaia-ESO Public Spectroscopic Survey).Full Table 1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A70

Two massive stars possibly ejected from NGC 3603 via a three-body encounter

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.; Kniazev, A. Y.; Chené, A.-N.; Schnurr, O.

2013-03-01

We report the discovery of a bow-shock-producing star in the vicinity of the young massive star cluster NGC 3603 using archival data of the Spitzer Space Telescope. Follow-up optical spectroscopy of this star with Gemini-South led to its classification as O6 V. The orientation of the bow shock and the distance to the star (based on its spectral type) suggest that the star was expelled from the cluster, while the young age of the cluster (˜2 Myr) implies that the ejection was caused by a dynamical few-body encounter in the cluster's core. The relative position on the sky of the O6 V star and a recently discovered O2 If*/WN6 star (located on the opposite side of NGC 3603) allows us to propose that both objects were ejected from the cluster via the same dynamical event - a three-body encounter between a single (O6 V) star and a massive binary (now the O2 If*/WN6 star). If our proposal is correct, then one can `weigh' the O2 If*/WN6 star using the conservation of the linear momentum. Given a mass of the O6 V star of ≈30 M⊙, we found that at the moment of ejection the mass of the O2 If*/WN6 star was ≈175 M⊙. Moreover, the observed X-ray luminosity of the O2 If*/WN6 star (typical of a single star) suggests that the components of this originally binary system have merged (e.g., because of encounter hardening).

On the determination of age and mass functions of stars in young open star clusters from the analysis of their luminosity functions

NASA Astrophysics Data System (ADS)

Piskunov, A. E.; Belikov, A. N.; Kharchenko, N. V.; Sagar, R.; Subramaniam, A.

2004-04-01

We construct the observed luminosity functions of the remote young open clusters NGC 2383, 2384, 4103, 4755, 7510 and Hogg 15 from CCD observations of them. The observed LFs are corrected for field star contamination determined with the help of a Galactic star count model. In the case of Hogg 15 and NGC 2383 we also consider the additional contamination from neighbouring clusters NGC 4609 and 2384, respectively. These corrections provide a realistic pattern of cluster LF in the vicinity of the main-sequence (MS) turn-on point and at fainter magnitudes reveal the so-called H-feature arising as a result of the transition of the pre-MS phase to the MS, which is dependent on the cluster age. The theoretical LFs are constructed representing a cluster population model with continuous star formation for a short time-scale and a power-law initial mass function (IMF), and these are fitted to the observed LF. As a result, we are able to determine for each cluster a set of parameters describing the cluster population (the age, duration of star formation, IMF slope and percentage of field star contamination). It is found that in spite of the non-monotonic behaviour of observed LFs, cluster IMFs can be described as power-law functions with slopes similar to Salpeter's value. The present main-sequence turn-on cluster ages are several times lower than those derived from the fitting of theoretical isochrones to the turn-off region of the upper main sequences.

Extended Star Formation or a Range of Stellar Rotation Velocities? The Nature of Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters

NASA Astrophysics Data System (ADS)

Goudfrooij, Paul

2016-10-01

Recently, deep color-magnitude diagrams (CMDs) from HST data revealed that several massive intermediate-age star clusters in the Magellanic Clouds exhibit extended main-sequence turn-offs (eMSTOs), and in some cases also dual red clumps. This poses serious questions regarding the mechanisms responsible for the formation of massive star clusters and their well-known light-element abundance variations. The nature of eMSTOs is currently a hotly debated topic of study. Several recent studies indicate that the eMSTOs are caused by an age spread of about 100-500 Myr among cluster stars, while other studies indicate that eMSTOs can be caused by a coeval population in which the relevant stars span a range of rotation velocities. Formal evidence to (dis-)prove either scenario still remains at large, mainly because the available stellar tracks that incorporate the effects of rotation are only available for masses > 1.7 Msun whereas the stars in the known eMSTOs of intermediate-age clusters are less massive. To circumvent this issue, we identified a massive star cluster in the Large Magellanic Cloud (LMC) that has the right dynamical properties to host an eMSTO along with an age at which the effects of age spreads to CMD morphology are substantially different from those of spreads of rotation rates: the 600 Myr old cluster NGC 1831. We propose to obtain deep WFC3/UVIS imaging with filters F336W and F814W to analyze the morphologies of the MSTO and upper MS regions of NGC 1831 at high precision and compare with model predictions. This will have a lasting impact on our understanding of the eMSTO phenomenon and of star cluster formation in general.

Search for Hot and Bright Stars for H_3^+ Spectroscopy Near the Galactic Center

NASA Astrophysics Data System (ADS)

Oka, Takeshi; Geballe, T. R.

2009-06-01

It is becoming increasingly clear that H_3^+ is abnormally abundant near the Galactic center and that it is a powerful probe for studying the gas in that region. To date we have observed a dozen sightlines toward bright and hot stars close to the Galactic plane (within 3 pc) and located in the region from the center to 30 pc east of the center. They are mostly stars belonging to the super-massive Quintuplet Cluster and the Central Cluster, but also include few lying between the two clusters. All sightlines showed H_3^+ with column densities on the order of 4 × 10^{15} cm^{-2} demonstrating the ubiquity of H_3^+, its high volume filling factor, and high ionization rate of H_{2} in the region. We plan to expand the region in which we have probed for H_3^+ by two orders of magnitude in solid angle by covering the whole of the Central Molecular Zone (CMZ), the region with a radius of ˜ 200 pc from the center. For this purpose, the first requirement is to find bright and hot stars suitable for the H_3^+ spectroscopy in this more extended region, in which few if any such stars are known outside of the clusters. We are using the recent GLIMPSE Point Source Catalogue provided by the Spitzer Space Telescope together with 2MASS photometry to identify such stars. Out of the over one million stars in GLIMPSE that are in the sightline to the CMZ, we have selected those few thousand stars with L < 7.5 mag. We then use results of J, K, L photometry to eliminate likely late-type stars, whose complex photospheric spectra would make it difficult to isolate the weak interstellar lines of H_3^+. For the few hundred stars thus chosen, we are obtaining medium resolution (R ˜ 2000) spectroscopy from 1.6 to 2.4 μm. The presence or absence of CO overtone bands (2-0, 3-1, 4-2, ...) near 2.3 microns allow us clearly discriminate the hot stars from late-type stars. So far we have observed 84 candidate hot stars and found a dozen that are usable for H_3^+ spectroscopy. Some of them are probably foreground stars. High resolution spectroscopy of low excitation CO lines and of H_3^+ are required to establish the positions of these stars along the line of sight and the environments in which any H_3^+ is located. The completion of this project will take several years. Quite apart from the application to the H_3^+ spectroscopy, finding bona fide young stars near the Galactic center is itself an interesting discovery. M. Goto, Usuda, Nagata, Geballe, McCall, Indriolo, Suto, Henning, Morong, and Oka, ApJ, 688, 306 (2008)

Field O stars: formed in situ or as runaways?

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.; Weidner, C.; Kroupa, P.; Pflamm-Altenburg, J.

2012-08-01

A significant fraction of massive stars in the Milky Way and other galaxies are located far from star clusters and star-forming regions. It is known that some of these stars are runaways, i.e. possess high space velocities (determined through the proper motion and/or radial velocity measurements), and therefore most likely were formed in embedded clusters and then ejected into the field because of dynamical few-body interactions or binary-supernova explosions. However, there exists a group of field O stars whose runaway status is difficult to prove via direct proper motion measurements (e.g. in the Magellanic Clouds) or whose (measured) low space velocities and/or young ages appear to be incompatible with their large separation from known star clusters. The existence of this group led some authors to believe that field O stars can form in situ. Since the question of whether or not O stars can form in isolation is of crucial importance for star formation theory, it is important to thoroughly test candidates of such stars in order to improve the theory. In this paper, we examine the runaway status of the best candidates for isolated formation of massive stars in the Milky Way and the Magellanic Clouds by searching for bow shocks around them, by using the new reduction of the Hipparcos data, and by searching for stellar systems from which they could originate within their lifetimes. We show that most of the known O stars thought to have formed in isolation are instead very likely runaways. We show also that the field must contain a population of O stars whose low space velocities and/or young ages are in apparent contradiction to the large separation of these stars from their parent clusters and/or the ages of these clusters. These stars (the descendants of runaway massive binaries) cannot be traced back to their parent clusters and therefore can be mistakenly considered as having formed in situ. We argue also that some field O stars could be detected in optical wavelengths only because they are runaways, while their cousins residing in the deeply embedded parent clusters might still remain totally obscured. The main conclusion of our study is that there is no significant evidence whatsoever in support of the in situ proposal on the origin of massive stars.

HIERARCHICAL STRUCTURE FORMATION AND MODES OF STAR FORMATION IN HICKSON COMPACT GROUP 31

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gallagher, S. C.; Durrell, P. R.; Elmegreen, D. M.

2010-02-15

The handful of low-mass, late-type galaxies that comprise Hickson Compact Group 31 (HCG 31) is in the midst of complex, ongoing gravitational interactions, evocative of the process of hierarchical structure formation at higher redshifts. With sensitive, multicolor Hubble Space Telescope imaging, we characterize the large population of < 10 Myr old star clusters (SCs) that suffuse the system. From the colors and luminosities of the young SCs, we find that the galaxies in HCG 31 follow the same universal scaling relations as actively star-forming galaxies in the local universe despite the unusual compact group environment. Furthermore, the specific frequency ofmore » the globular cluster system is consistent with the low end of galaxies of comparable masses locally. This, combined with the large mass of neutral hydrogen and tight constraints on the amount of intragroup light, indicate that the group is undergoing its first epoch of interaction-induced star formation. In both the main galaxies and the tidal-dwarf candidate, F, stellar complexes, which are sensitive to the magnitude of disk turbulence, have both sizes and masses more characteristic of z = 1-2 galaxies. After subtracting the light from compact sources, we find no evidence for an underlying old stellar population in F-it appears to be a truly new structure. The low-velocity dispersion of the system components, available reservoir of H I, and current star formation rate of {approx}10 M {sub sun} yr{sup -1} indicate that HCG 31 is likely to both exhaust its cold gas supply and merge within {approx}1 Gyr. We conclude that the end product will be an isolated, X-ray-faint, low-mass elliptical.« less

A preliminary comparison of photometric (MWSC) and trigonometric (TGAS) distances of open cluster stars

NASA Astrophysics Data System (ADS)

Kovaleva, Dana; Piskunov, Anatoly; Kharchenko, Nina; Scholz, Ralf-Dieter

2017-12-01

The goal of this researchwas to compare the open cluster photometric distance scale of the global survey of star clusters in the MilkyWay (MWSC) with the distances derived fromtrigonometric parallaxes fromthe Gaia DR1/TGAS catalogue and to investigate towhich degree and extent both scales agree.We compared the parallax-based and photometrybased distances of 5743 cluster stars selected as members of 1118 clusters based on their kinematic and photometric MWSC membership probabilities. We found good overall agreement between trigonometric and photometric distances of open cluster stars. The residuals between them were small and unbiased up to log(d, [pc]) ≈ 2.8. If we considered only the most populated clusters and used cluster distances obtained from the mean trigonometric parallax of their MWSC members, the good agreement of the distance scales continued up to log(d, [pc]) ≈ 3.3.

Rotation periods of open-cluster stars, 2

NASA Technical Reports Server (NTRS)

Prosser, Charles F.; Shetrone, Matthew D.; Marilli, Ettore; Catalano, Santo; Williams, Scott D.; Backman, Dana E.; Laaksonen, Bentley D.; Adige, Vikram; Marschall, Laurence A.; Stauffer, John R.

1993-01-01

We present the results from a photometric monitoring program of 21 stars observed during 1992 in the Pleiades and Alpha Persei open clusters. Period determinations for 16 stars are given, 13 of which are the first periods reported for these stars. Brightness variations for an additional five cluster stars are also given. One K dwarf member of the alpha Per cluster is observed to have a period of rotation of only 4.39 hr. perhaps the shortest period currently known among BY Draconis variables. The individual photometric measurements have been deposited with the NSSDC. Combining current X-ray flux determinations with known photometric periods, we illustrate the X-ray activity/rotation relation among Pleiades K dwarfs based on available data.

The Structure of the Young Star Cluster NGC 6231. I. Stellar Population

NASA Astrophysics Data System (ADS)

Kuhn, Michael A.; Medina, Nicolás; Getman, Konstantin V.; Feigelson, Eric D.; Gromadzki, Mariusz; Borissova, Jordanka; Kurtev, Radostin

2017-09-01

NGC 6231 is a young cluster (age ˜2-7 Myr) dominating the Sco OB1 association (distance ˜1.59 kpc) with ˜100 O and B stars and a large pre-main-sequence stellar population. We combine a reanalysis of archival Chandra X-ray data with multiepoch near-infrared (NIR) photometry from the VISTA Variables in the Vía Lactéa (VVV) survey and published optical catalogs to obtain a catalog of 2148 probable cluster members. This catalog is 70% larger than previous censuses of probable cluster members in NGC 6231. It includes many low-mass stars detected in the NIR but not in the optical and some B stars without previously noted X-ray counterparts. In addition, we identify 295 NIR variables, about half of which are expected to be pre-main-sequence stars. With the more complete sample, we estimate a total population in the Chandra field of 5700-7500 cluster members down to 0.08 {M}⊙ (assuming a universal initial mass function) with a completeness limit at 0.5 {M}⊙ . A decrease in stellar X-ray luminosities is noted relative to other younger clusters. However, within the cluster, there is little variation in the distribution of X-ray luminosities for ages less than 5 Myr. The X-ray spectral hardness for B stars may be useful for distinguishing between early-B stars with X-rays generated in stellar winds and B-star systems with X-rays from a pre-main-sequence companion (>35% of B stars). A small fraction of catalog members have unusually high X-ray median energies or reddened NIR colors, which might be explained by absorption from thick or edge-on disks or being background field stars.

The Effects of Single and Close Binary Evolution on the Stellar Mass Function

NASA Astrophysics Data System (ADS)

Schneider, R. N. F.; Izzard, G. R.; de Mink, S.; Langer, N., Stolte, A., de Koter, A.; Gvaramadze, V. V.; Hussmann, B.; Liermann, A.; Sana, H.

2013-06-01

Massive stars are almost exclusively born in star clusters, where stars in a cluster are expected to be born quasi-simultaneously and with the same chemical composition. The distribution of their birth masses favors lower over higher stellar masses, such that the most massive stars are rare, and the existence of an stellar upper mass limit is still debated. The majority of massive stars are born as members of close binary systems and most of them will exchange mass with a close companion during their lifetime. We explore the influence of single and binary star evolution on the high mass end of the stellar mass function using a rapid binary evolution code. We apply our results to two massive Galactic star clusters and show how the shape of their mass functions can be used to determine cluster ages and comment on the stellar upper mass limit in view of our new findings.

Not all stars form in clusters - measuring the kinematics of OB associations with Gaia

NASA Astrophysics Data System (ADS)

Ward, Jacob L.; Kruijssen, J. M. Diederik

2018-04-01

It is often stated that star clusters are the fundamental units of star formation and that most (if not all) stars form in dense stellar clusters. In this monolithic formation scenario, low-density OB associations are formed from the expansion of gravitationally bound clusters following gas expulsion due to stellar feedback. N-body simulations of this process show that OB associations formed this way retain signs of expansion and elevated radial anisotropy over tens of Myr. However, recent theoretical and observational studies suggest that star formation is a hierarchical process, following the fractal nature of natal molecular clouds and allowing the formation of large-scale associations in situ. We distinguish between these two scenarios by characterizing the kinematics of OB associations using the Tycho-Gaia Astrometric Solution catalogue. To this end, we quantify four key kinematic diagnostics: the number ratio of stars with positive radial velocities to those with negative radial velocities, the median radial velocity, the median radial velocity normalized by the tangential velocity, and the radial anisotropy parameter. Each quantity presents a useful diagnostic of whether the association was more compact in the past. We compare these diagnostics to models representing random motion and the expanding products of monolithic cluster formation. None of these diagnostics show evidence of expansion, either from a single cluster or multiple clusters, and the observed kinematics are better represented by a random velocity distribution. This result favours the hierarchical star formation model in which a minority of stars forms in bound clusters and large-scale, hierarchically structured associations are formed in situ.

New science from the phase space of old stellar systems

NASA Astrophysics Data System (ADS)

Varri, Anna Lisa; Breen, Philip G.; Heggie, Douglas C.; Tiongco, Maria; Vesperini, Enrico

2017-06-01

Our traditional interpretative picture of the internal dynamics of globular clusters has been recently revolutionized by a series of discoveries about their chemical, structural, and kinematic properties. The empirical evidence that their velocity space is much more complex than usually expected encourages us to use them as refreshingly novel phase space laboratories for some long-forgotten aspects of collisional gravitational dynamics. Such a realization, coupled with the discovery that the stars in clusters were not all born at once in a single population, makes them new, challenging chemodynamical puzzles.Thanks to the proper motions of thousands of stars that will be available from the Gaia mission, we are about to enter a new ''golden age'' for the study of the dynamics of this class of stellar systems, as the full phase space of several Galactic globular clusters will be soon unlocked for the first time. In this context, I will present the highlights of a more realistic dynamical paradigm for these intriguing stellar systems, with emphasis on the role of angular momentum, velocity anisotropy and external tidal field. Such a fundamental understanding of the emerging phase space complexity of globulars will allow us to address many open questions about their rich dynamical evolution, their elusive stellar populations and putative black holes, and their role within the history of our Galaxy.

AN ASTEROSEISMIC MEMBERSHIP STUDY OF THE RED GIANTS IN THREE OPEN CLUSTERS OBSERVED BY KEPLER: NGC 6791, NGC 6819, AND NGC 6811

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stello, Dennis; Huber, Daniel; Bedding, Timothy R.

Studying star clusters offers significant advances in stellar astrophysics due to the combined power of having many stars with essentially the same distance, age, and initial composition. This makes clusters excellent test benches for verification of stellar evolution theory. To fully exploit this potential, it is vital that the star sample is uncontaminated by stars that are not members of the cluster. Techniques for determining cluster membership therefore play a key role in the investigation of clusters. We present results on three clusters in the Kepler field of view based on a newly established technique that uses asteroseismology to identifymore » fore- or background stars in the field, which demonstrates advantages over classical methods such as kinematic and photometry measurements. Four previously identified seismic non-members in NGC 6819 are confirmed in this study, and three additional non-members are found-two in NGC 6819 and one in NGC 6791. We further highlight which stars are, or might be, affected by blending, which needs to be taken into account when analyzing these Kepler data.« less

Radial Velocities of RR Lyrae Stars in and around NGC 6441

NASA Astrophysics Data System (ADS)

Kunder, Andrea; Mills, Arthur; Edgecomb, Joseph; Thomas, Mathew; Schilter, Levi; Boyle, Craig; Parker, Stephen; Bellevue, Gordon; Rich, R. Michael; Koch, Andreas; Johnson, Christian I.; Nataf, David M.

2018-04-01

Detailed elemental abundance patterns of metal-poor ([Fe/H] ∼ ‑1 dex) stars in the Galactic bulge indicate that a number of them are consistent with globular cluster (GC) stars and may be former members of dissolved GCs. This would indicate that a few per cent of the Galactic bulge was built up from destruction and/or evaporation of GCs. Here, an attempt is made to identify such presumptive stripped stars originating from the massive, inner Galaxy GC NGC 6441 using its rich RR Lyrae variable star (RRL) population. We present radial velocities of 40 RRLs centered on the GC NGC 6441. All 13 of the RRLs observed within the cluster tidal radius have velocities consistent with cluster membership, with an average radial velocity of 24 ± 5 km s‑1 and a star-to-star scatter of 11 km s‑1. This includes two new RRLs that were previously not associated with the cluster. Eight RRLs with radial velocities consistent with cluster membership but up to three time the distance from the tidal radius are also reported. These potential extra-tidal RRLs also have exceptionally long periods, which is a curious characteristic of the NGC 6441 RRL population that hosts RRLs with periods longer than seen anywhere else in the Milky Way. As expected of stripped cluster stars, most are inline with the cluster’s orbit. Therefore, either the tidal radius of NGC 6441 is underestimated and/or we are seeing dissolving cluster stars stemming from NGC 6441 that are building up the old spheroidal bulge.

Hubble Checks out a Home for Old Stars

NASA Image and Video Library

2017-12-08

This image, taken with the Wide Field Planetary Camera 2 on board the NASA/ESA Hubble Space Telescope, shows the globular cluster Terzan 1. Lying around 20,000 light-years from us in the constellation of Scorpius (The Scorpion), it is one of about 150 globular clusters belonging to our galaxy, the Milky Way. Typical globular clusters are collections of around a hundred thousand stars, held together by their mutual gravitational attraction in a spherical shape a few hundred light-years across. It is thought that every galaxy has a population of globular clusters. Some, like the Milky Way, have a few hundred, while giant elliptical galaxies can have several thousand. They contain some of the oldest stars in a galaxy, hence the reddish colors of the stars in this image — the bright blue ones are foreground stars, not part of the cluster. The ages of the stars in the globular cluster tell us that they were formed during the early stages of galaxy formation! Studying them can also help us to understand how galaxies formed. Terzan 1, like many globular clusters, is a source of X-rays. It is likely that these X-rays come from binary star systems that contain a dense neutron star and a normal star. The neutron star drags material from the companion star, causing a burst of X-ray emission. The system then enters a quiescent phase in which the neutron star cools, giving off X-ray emission with different characteristics, before enough material from the companion builds up to trigger another outburst. Image credit: NASA & ESA, Acknowledgement: Judy Schmidt NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

Chemical Abundances of Giants in Globular Clusters

NASA Astrophysics Data System (ADS)

Gratton, Raffaele G.; Bragaglia, Angela; Carretta, Eugenio; D'Orazi, Valentina; Lucatello, Sara

A large fraction of stars form in clusters. According to a widespread paradigma, stellar clusters are prototypes of single stellar populations. According to this concept, they formed on a very short time scale, and all their stars share the same chemical composition. Recently it has been understood that massive stellar clusters (the globular clusters) rather host various stellar populations, characterized by different chemical composition: these stellar populations have also slightly different ages, stars of the second generations being formed from the ejecta of part of those of an earlier one. Furthermore, it is becoming clear that the efficiency of the process is quite low: many more stars formed within this process than currently present in the clusters. This implies that a significant, perhaps even dominant fraction of the ancient population of galaxies formed within the episodes that lead to formation the globular clusters.

Color gradients in cooling flow cluster central galaxies and the ionization of cluster emission line systems

NASA Technical Reports Server (NTRS)

Romanishin, W.

1988-01-01

Preliminary results are given for a program to measure color gradients in the central galaxies in clusters with a variety of cooling flow rates. The objectives are to search for extended blue continuum regions indicative of star formation, to study the spatial distribution of star formation, and to make a quantitative measure of the amount of light from young stars, which can lead to a measure of the star formation rate (for an assumed initial mass function). Four clusters with large masses and large cluster H-alpha emission fluxes are found to have an excess of blue light concentrated to the centers of the cluster central galaxy. Assumption of a disk IMF leads to the conclusion that the starlight might play a major role in ionizing the emission line gas in these clusters.

Horizontal branch stars, and galactic and magellanic cloud globular clusters

NASA Technical Reports Server (NTRS)

Deboer, K. S.

1981-01-01

Seven blue horizontal branch stars in the field were observed and a few HB stars were isolated in globular clusters. Energy distributions are compared to assess possible differences and also used in comparison with model atmospheres. Observed energy distributions of HB stars in NGC 6397 are used to estimate the total number of HB stars which produced the integrated fluxes as observed by ANS. Preliminary results are given for colors of globular clusters observed in the Magellanic Clouds and for their extent, based on the Washburn IUE extraction.

Probing the formation history of the nuclear star cluster at the Galactic Centre with millisecond pulsars

NASA Astrophysics Data System (ADS)

Abbate, F.; Mastrobuono-Battisti, A.; Colpi, M.; Possenti, A.; Sippel, A. C.; Dotti, M.

2018-01-01

The origin of the nuclear star cluster in the centre of our Galaxy is still unknown. One possibility is that it formed after the disruption of stellar clusters that spiralled into the Galactic Centre due to dynamical friction. We trace the formation of the nuclear star cluster around the central black hole, using state-of-the-art N-body simulations, and follow the dynamics of the neutron stars born in the clusters. We then estimate the number of millisecond pulsars (MSPs) that are released in the nuclear star cluster during its formation. The assembly and tidal dismemberment of globular clusters lead to a population of MSPs distributed over a radius of about 20 pc, with a peak near 3 pc. No clustering is found on the subparsec scale. We simulate the detectability of this population with future radio telescopes like the MeerKAT radio telescope and SKA1, and find that about an order of 10 MSPs can be observed over this large volume, with a paucity of MSPs within the central parsec. This helps discriminating this scenario from the in situ formation model for the nuclear star cluster that would predict an overabundance of MSPs closer to the black hole. We then discuss the potential contribution of our MSP population to the gamma-ray excess at the Galactic Centre.

History of Chandra X-Ray Observatory

NASA Image and Video Library

2002-12-18

At a distance of 6,000 light years from Earth, the star cluster RCW 38 is a relatively close star-forming region. This area is about 5 light years across, and contains thousands of hot, very young stars formed less than a million years ago, 190 of which exposed x-rays to Chandra. Enveloping the star cluster, the diffused cloud of x-rays shows an excess of high energy x-rays, which indicates that the x-rays come from trillion-volt electrons moving in a magnetic field. Such particles are typically produced by exploding stars, or in the strong magnetic fields around neutron stars or black holes, none of which are evident in RCW 38. One possible origin for the particles, could be an undetected supernova that occurred in the cluster, possibly thousands of years ago, producing a shock wave that is interacting with the young stars. Regardless of the origin of these energetic electrons, their presence could change the chemistry of the disks that will eventually form planets around the stars in the cluster.

Lithium Abundances in the Young Open Cluster IC 2602

NASA Technical Reports Server (NTRS)

Randich, S.; Aharpour, N.; Pallavicini, R.; Prosser, C. F.; Stauffer, J. R.

1997-01-01

We have obtained high-resolution spectra for 28 candidate late-type stars in the 30 Myr old cluster IC 2602. NLTE Li abundances have been derived from measured equivalent widths. The log n(Li) - T(sub eff) and log n(Li) - mass distributions for our sample stars have been compared with those of the Pleiades and alpha Persei. Our data show that F stars in the three clusters have the same lithium content, which corresponds to the initial content for Pop. I stars. G and early-K IC 2602 stars are, on average, somewhat more Li-rich than their counterparts in the two slightly older clusters. Finally, the latest-type IC 2602 stars are heavily Li depleted, with their Li content being as low as the lowest measured among the Pleiades. As in the Pleiades and alpha Per, a star-to-star scatter in lithium is observed among 30 Myr old late-K/early-K dwarfs in IC 2602, indicating that this spread develops in the pre-main sequence phases.

PANCHROMATIC HUBBLE ANDROMEDA TREASURY. XVI. STAR CLUSTER FORMATION EFFICIENCY AND THE CLUSTERED FRACTION OF YOUNG STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, L. Clifton; Sandstrom, Karin; Seth, Anil C.

We use the Panchromatic Hubble Andromeda Treasury survey data set to perform spatially resolved measurements of star cluster formation efficiency (Γ), the fraction of stellar mass formed in long-lived star clusters. We use robust star formation history and cluster parameter constraints, obtained through color–magnitude diagram analysis of resolved stellar populations, to study Andromeda’s cluster and field populations over the last ∼300 Myr. We measure Γ of 4%–8% for young, 10–100 Myr-old populations in M31. We find that cluster formation efficiency varies systematically across the M31 disk, consistent with variations in mid-plane pressure. These Γ measurements expand the range of well-studiedmore » galactic environments, providing precise constraints in an H i-dominated, low-intensity star formation environment. Spatially resolved results from M31 are broadly consistent with previous trends observed on galaxy-integrated scales, where Γ increases with increasing star formation rate surface density (Σ{sub SFR}). However, we can explain observed scatter in the relation and attain better agreement between observations and theoretical models if we account for environmental variations in gas depletion time ( τ {sub dep}) when modeling Γ, accounting for the qualitative shift in star formation behavior when transitioning from a H{sub 2}-dominated to a H i-dominated interstellar medium. We also demonstrate that Γ measurements in high Σ{sub SFR} starburst systems are well-explained by τ {sub dep}-dependent fiducial Γ models.« less

Young and Exotic Stellar Zoo

NASA Astrophysics Data System (ADS)

2005-03-01

Summary Super star clusters are groups of hundreds of thousands of very young stars packed into an unbelievably small volume. They represent the most extreme environments in which stars and planets can form. Until now, super star clusters were only known to exist very far away, mostly in pairs or groups of interacting galaxies. Now, however, a team of European astronomers [1] have used ESO's telescopes to uncover such a monster object within our own Galaxy, the Milky Way, almost, but not quite, in our own backyard! The newly found massive structure is hidden behind a large cloud of dust and gas and this is why it took so long to unveil its true nature. It is known as "Westerlund 1" and is a thousand times closer than any other super star cluster known so far. It is close enough that astronomers may now probe its structure in some detail. Westerlund 1 contains hundreds of very massive stars, some shining with a brilliance of almost one million suns and some two-thousand times larger than the Sun (as large as the orbit of Saturn)! Indeed, if the Sun were located at the heart of this remarkable cluster, our sky would be full of hundreds of stars as bright as the full Moon. Westerlund 1 is a most unique natural laboratory for the study of extreme stellar physics, helping astronomers to find out how the most massive stars in our Galaxy live and die. From their observations, the astronomers conclude that this extreme cluster most probably contains no less than 100,000 times the mass of the Sun, and all of its stars are located within a region less than 6 light-years across. Westerlund 1 thus appears to be the most massive compact young cluster yet identified in the Milky Way Galaxy. PR Photo 09a/05: The Super Star Cluster Westerlund 1 (2.2m MPG/ESO + WFI) PR Photo 09b/05: Properties of Young Massive Clusters Super Star Clusters Stars are generally born in small groups, mostly in so-called "open clusters" that typically contain a few hundred stars. From a wide range of observations, astronomers infer that the Sun itself was born in one such cluster, some 4,500 million years ago. In some active ("starburst") galaxies, scientists have observed violent episodes of star formation (see, for example, ESO Press Photo 31/04), leading to the development of super star clusters, each containing several million stars. Such events were obviously common during the Milky Way's childhood, more than 12,000 million years ago: the many galactic globular clusters - which are nearly as old as our Galaxy (e.g. ESO PR 20/04) - are indeed thought to be the remnants of early super star clusters. All super star clusters so far observed in starburst galaxies are very distant. It is not possible to distinguish their individual stars, even with the most advanced technology. This dramatically complicates their study and astronomers have therefore long been eager to find such clusters in our neighbourhood in order to probe their structure in much more detail. Now, a team of European astronomers [1] has finally succeeded in doing so, using several of ESO's telescopes at the La Silla observatory (Chile). Westerlund 1 ESO PR Photo 09a/05 ESO PR Photo 09a/05 The Super Star Cluster Westerlund 1 (2.2m MPG/ESO + WFI) [Preview - JPEG: 400 x 472 pix - 58k] [Normal - JPEG: 800 x 943 pix - 986k] [Full Res - JPEG: 1261 x 1486 pix - 2.4M] Caption: ESO PR Photo 09a/05 is a composite image of the super star cluster "Westerlund 1" from 2.2-m MPG/ESO Wide-Field Imager (WFI) observations. The image covers a 5 x 5 arcmin sky region and is based on observations made in the V-band (550 nm, 2 min exposure time, associated to the blue channel), R-band (650nm, 1 min, green channel) and I-band (784nm, 18 sec, red channel). Only the central CCD of WFI was used, as the entire cluster fits comfortably inside it. The foreground stars appear blue, while the hot massive members of the cluster look orange, and the cool massive ones come out red. The open cluster Westerlund 1 is located in the Southern constellation Ara (the Altar). It was discovered in 1961 from Australia by Swedish astronomer Bengt Westerlund, who later moved from there to become ESO Director in Chile (1970 - 74). This cluster is behind a huge interstellar cloud of gas and dust, which blocks most of its visible light. The dimming factor is more than 100,000 - and this is why it has taken so long to uncover the true nature of this particular cluster. In 2001, the team of astronomers identified more than a dozen extremely hot and peculiar massive stars in the cluster, so-called "Wolf-Rayet" stars. They have since studied Westerlund 1 extensively with various ESO telescopes. They used images from the Wide Field Imager (WFI) attached to the 2.2-m ESO/MPG as well as from the SUperb Seeing Imager 2 (SuSI2) camera on the ESO 3.5-m New Technology Telescope (NTT). From these observations, they were able to identify about 200 cluster member stars. To establish the true nature of these stars, the astronomers then performed spectroscopic observations of about one quarter of them. For this, they used the Boller & Chivens spectrograph on the ESO 1.52-m telescope and the ESO Multi-Mode Instrument (EMMI) on the NTT. An Exotic Zoo These observations have revealed a large population of very bright and massive, quite extreme stars. Some would fill the solar system space within the orbit of Saturn (about 2,000 times larger than the Sun!), others are as bright as a million Suns. Westerlund 1 is obviously a fantastic stellar zoo, with a most exotic population and a true astronomical bonanza. All stars identified are evolved and very massive, spanning the full range of stellar oddities from Wolf-Rayet stars, OB supergiants, Yellow Hypergiants (nearly as bright as a million Suns) and Luminous Blue Variables (similar to the exceptional Eta Carinae object - see ESO PR 31/03). All stars so far analysed in Westerlund 1 weigh at least 30-40 times more than the Sun. Because such stars have a rather short life - astronomically speaking - Westerlund 1 must be very young. The astronomers determine an age somewhere between 3.5 and 5 million years. So, Westerlund 1 is clearly a "newborn" cluster in our Galaxy! The Most Massive Cluster ESO PR Photo 09b/05 ESO PR Photo 09b/05 Properties of Young Massive Clusters [Preview - JPEG: 400 x 511 pix - 20k] [Normal - JPEG: 800 x 1021 pix - 122k] Caption: ESO PR Photo 09b/05 shows the properties of young massive clusters in our Galaxy and in the Large Magellanic Clouds, as well as of Super Star Clusters in star-forming galaxies. The diagram shows the mass and radius of these clusters and also the position of Westerlund 1 (indicated Wd 1). Westerlund 1 is incredibly rich in monster stars - just as one example, it contains as many Yellow Hypergiants as were hitherto known in the entire Milky Way! "If the Sun were located at the heart of Westerlund 1, the sky would be full of stars, many of them brighter than the full Moon", comments Ignacio Negueruela of the Universidad de Alicante in Spain and member of the team. The large quantity of very massive stars implies that Westerlund 1 must contain a huge number of stars. "In our Galaxy, explains Simon Clark of the University College London (UK) and one of the authors of this study, "there are more than 100 solar-like stars for every star weighing 10 times as much as the Sun. The fact that we see hundreds of massive stars in Westerlund 1 means that it probably contains close to half a million stars, but most of these are not bright enough to peer through the obscuring cloud of gas and dust". This is ten times more than any other known young clusterin the Milky Way. Westerlund 1 is presumably much more massive than the dense clusters of heavy stars present in the central region of our Galaxy, like the Arches and Quintuplet clusters. Further deep infrared observations will be required to confirm this. This super star cluster now provides astronomers with a unique perspective towards one of the most extreme environments in the Universe. Westerlund 1 will certainly provide new opportunities in the long-standing quest for more and finer details about how stars, and especially massive ones, do form. ... and the Most Dense The large number of stars in Westerlund 1 was not the only surprise awaiting Clark and his colleagues. From their observations, the team members also found that all these stars are packed into an amazingly small volume of space, indeed less than 6 light-years across. In fact, this is more or less comparable to the 4 light-year distance to the star nearest to the Sun, Proxima Centauri! It is incredible: the concentration in Westerlund 1 is so high that the mean separation between stars is quite similar to the extent of the Solar System. "With so many stars in such a small volume, some of them may collide", envisages Simon Clark. "This could lead to the formation of an intermediate-mass black hole more massive than 100 solar masses. It may well be that such a monster has already formed at the core of Westerlund 1." The huge population of massive stars in Westerlund 1 suggests that it will have a very significant impact on its surroundings. The cluster contains so many massive stars that in a time span of less than 40 million years, it will be the site of more than 1,500 supernovae. A gigantic firework that may drive a fountain of galactic material! Because Westerlund 1 is at a distance of only about 10,000 light-years, high-resolution cameras such as NAOS/CONICA on ESO's Very Large Telescope can resolve its individual stars. Such observations are now starting to reveal smaller stars in Westerlund 1, including some that are less massive than the Sun. Astronomers will thus soon be able to study this exotic galactic zoo in great depth. More information The research presented in this ESO Press Release will soon appear in the leading research journal Astronomy and Astrophysics ("On the massive stellar population of the Super Star Cluster Westerlund 1" by J.S. Clark and colleagues). The PDF file is available at the A&A web site. A second paper ("Further Wolf-Rayet stars in the starburst cluster Westerlund 1", by Ignacio Negueruela and Simon Clark) will also soon be published in Astronomy and Astrophysics. It is available as astro-ph/0503303. A Spanish press release issued by Universidad de Alicante is available on the web site of Ignacio Negueruela.

The Hyades cluster-supercluster connection - Evidence for a local concentration of dark matter

NASA Technical Reports Server (NTRS)

Casertano, Stefano; Iben, Icko, Jr.; Shiels, Aaron

1993-01-01

Stars that evaporate from the Hyades cluster will remain within a few hundred parsecs of the cluster only if they are dynamically bound to a much more massive entity containing the cluster. A local mass enhancement of at least (5-10) x 10 exp 5 solar masses, with a radius of about 100 pc, can trap stars with an origin related to that of the Hyades cluster and explains the excess of stars with velocities near the Hyades velocity that constitutes the Hyades supercluster. Part of this mass enhancement can be in visible stars, but a substantial fraction is likely to be in the form of dark matter.

Stellar Vampires Unmasked

NASA Astrophysics Data System (ADS)

2006-10-01

Astronomers have found possible proofs of stellar vampirism in the globular cluster 47 Tucanae. Using ESO's Very Large Telescope, they found that some hot, bright, and apparently young stars in the cluster present less carbon and oxygen than the majority of their sisters. This indicates that these few stars likely formed by taking their material from another star. "This is the first detection of a chemical signature clearly pointing to a specific scenario to form so-called 'Blue straggler stars' in a globular cluster", said Francesco Ferraro, from the Astronomy Department of Bologna University (Italy) and lead-author of the paper presenting the results. Blue stragglers are unexpectedly young-looking stars found in stellar aggregates, such as globular clusters, which are known to be made up of old stars. These enigmatic objects are thought to be created in either direct stellar collisions or through the evolution and coalescence of a binary star system in which one star 'sucks' material off the other, rejuvenating itself. As such, they provide interesting constraints on both binary stellar evolution and star cluster dynamics. To date, the unambiguous signatures of either stellar traffic accidents or stellar vampirism have not been observed, and the formation mechanisms of Blue stragglers are still a mystery. The astronomers used ESO's Very Large Telescope to measure the abundance of chemical elements at the surface of 43 Blue straggler stars in the globular cluster 47 Tucanae [1]. They discovered that six of these Blue straggler stars contain less carbon and oxygen than the majority of these peculiar objects. Such an anomaly indicates that the material at the surface of the blue stragglers comes from the deep interiors of a parent star [2]. Such deep material can reach the surface of the blue straggler only during the mass transfer process occurring between two stars in a binary system. Numerical simulations indeed show that the coalescence of stars should not result in anomalous abundances. ESO PR Photo 37/06 ESO PR Photo 37/06 Abundances in Blue Straggler Stars In the core of a globular cluster, stars are packed extremely close to each other: more than 4000 stars are found in the innermost light-year-sized cube of 47 Tucanae. Thus, stellar collisions are thought to be very frequent and the collision channel for the formation of blue stragglers should be extremely efficient. The chemical signature detected by these observations demonstrates that also the binary mass-transfer scenario is fully active even in a high-density cluster like 47 Tuc. "Our discovery is therefore a fundamental step toward the solution of the long-standing mystery of blue straggler formation in globular clusters," said Ferraro. Measurements of so many faint stars are only possible since the advent of 8-m class telescopes equipped with multiplexing capability spectrographs. In this case, the astronomers used the FLAMES/Giraffe instrument that allows the simultaneous observation of up to 130 targets at a time, making it ideally suited for surveying individual stars in closely populated fields.

Spectroscopy of Hot Horizontal Branch Stars in Globular Clusters

NASA Astrophysics Data System (ADS)

Moni-Bidin, C. M.

2006-06-01

We will present our latest results on spectroscopy of hot horizontal branch stars in globular clusters. This class of stars still presents many puzzling features, and many aspects of their formation and evolution are still unclear. Extreme Horizontal Branch (EHB) stars, also known as Subdwarf B (sdB) stars, are post-He flash stars with a He-burning core and high effective temperature (T_{eff} ≥ 20000 K). They originate from stars of low initial mass that during their evolution have lost great part of their external envelope. Many channel for the formation of these stars have been studied in literature. The scenarios involving dynamical interactions inside close binary systems, deeply investigated by Han et al. (2003, MNRAS, 341, 669), have been recently preferred, since between field sdB stars many close binary systems have been detected. (Morales-Rueda et al. 2003, MNRAS, 338, 752). Maxted et al. (2001, MNRAS, 326, 1391) estimated that 69+/-9% of field sdB stars are close binary systems. Latest results indicates that also this scenario presents some problems (Lisker et al. 2005, A&A, 430, 223), and Napiwotzki et al. (2004) found a lower fraction of binaries among their sample (42%). Moni Bidin et al. (2005, A&A, submitted) recently showed that in globular cluster NGC6752 the binary fraction among EHB stars is sensibly lower than what observed among field sdBs, estimating an upper limit of 20%. This difference between field and cluster sdBs is quite surprising. We are performing further investigation of these stars extending our search for close binary systems to other two clusters with a rich population of EHB stars. This will allow us to tell if the results on NGC6752 indicate a pecular cluster or the lack of binaries is a common trend of EHB stars in globular clusters. Moreover, with a larger sample we will be able to better estimate the binary fraction, or an upper limit for it. With our contribution we are going to show our results on this investigation that at the moment is still a work in progress.

Morphologies and ages of star cluster pairs and multiplets in the Small Magellanic Cloud

NASA Astrophysics Data System (ADS)

de Oliveira, M. R.; Dutra, C. M.; Bica, E.; Dottori, H.

2000-10-01

An isophotal atlas of 75 star cluster pairs and multiplets in the Small Magellanic Cloud is presented, comprising 176 objects. They are concentrated in the SMC main body. The isophotal contours were made from Digitized Sky Survey* images and showed relevant structural features possibly related to interactions in about 25% of the sample. Previous N-body simulations indicate that such shapes could be due to tidal tails, bridges or common envelopes. The diameter ratio between the members of a pair is preferentially in the range 1 - 2, with a peak at 1. The projected separation is in the range ~ 3 - 22 pc with a pronounced peak at ~ 13 pc. For 91 objects it was possible to derive ages from Colour-Magnitude Diagrams using the OGLE-II photometric survey. The cluster multiplets in general occur in OB stellar associations and/or HII region complexes. This indicates a common origin and suggests that multiplets coalesce into pairs or single clusters in a short time scale. Pairs in the SMC appear to be mostly coeval and consequently captures are a rare phenomenon. We find evidence that star cluster pairs and multiplets may have had an important role in the dynamical history of clusters presently seen as large single objects. The images in this study are based on photographic data obtained using the UK Schmidt Telescope, which was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council, until 1988 June, and thereafter by the Anglo-Australian Observatory. Original plate material is copyright by the Royal Observatory Edinburgh and the Anglo-Australian Observatory. The plates were processed into the present compressed digital form with their permission. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166.

The massive stellar population of W49: A spectroscopic survey

NASA Astrophysics Data System (ADS)

Wu, Shi-Wei; Bik, Arjan; Bestenlehner, Joachim M.; Henning, Thomas; Pasquali, Anna; Brandner, Wolfgang; Stolte, Andrea

2016-05-01

Context. Massive stars form on different scales that range from large, dispersed OB associations to compact, dense starburst clusters. The complex structure of regions of massive star formation and the involved short timescales provide a challenge for our understanding of their birth and early evolution. As one of the most massive and luminous star-forming region in our Galaxy, W49 is the ideal place to study the formation of the most massive stars. Aims: By classifying the massive young stars that are deeply embedded in the molecular cloud of W49, we aim to investigate and trace the star formation history of this region. Methods: We analyse near-infrared K-band spectroscopic observations of W49 from LBT/LUCI combined with JHK images obtained with NTT/SOFI and LBT/LUCI. Based on JHK-band photometry and K-band spectroscopy, the massive stars are placed in a Hertzsprung Russell diagram. By comparison with evolutionary models, their age and hence the star formation history of W49 can be investigated. Results: Fourteen O-type stars, as well as two young stellar objects (YSOs), are identified by our spectroscopic survey. Eleven O stars are main sequence stars with subtypes ranging from O3 to O9.5 and masses ranging from ~20 M⊙ to ~120 M⊙. Three of the O stars show strong wind features and are considered to be Of-type supergiants with masses beyond 100 M⊙. The two YSOs show CO emission, which is indicative of the presence of circumstellar disks in the central region of the massive cluster. The age of the cluster is estimated as ~1.5 Myr, with star formation continuing in different parts of the region. The ionising photons from the central massive stars have not yet cleared the molecular cocoon surrounding the cluster. W49 is comparable to extragalactic star-forming regions, and it provides us with a unique chance to study a starburst in detail. Based on data acquired using the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in Germany, Italy and the United States. LBT Corporation partners are: LBT Beteiligungsgesellschaft, Germany, representing the Max Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; Istituto Nazionale di Astrofisica, Italy; The University of Arizona on behalf of the Arizona university system; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia.Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 67.C-0514 and 073.D-0837.The reduced spectra (FITS files) are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/589/A16

HST-WFPC2 Observations of the Star Clusters in the Giant H II Regions of M33

NASA Astrophysics Data System (ADS)

Lee, Myung Gyoon; Park, Hong Soo; Kim, Sang Chul; Waller, William H.; Parker, Joel Wm.; Malumuth, Eliot M.; Hodge, Paul W.

We present a photometric study of the stars in ionizing star clusters embedded in several giant H II regions of M33 (CC93, IC 142, NGC 595, MA2, NGC 604 and NGC 588). Our photometry is based on the HST-WFPC2 images of these clusters. Color-magnitude diagrams and color-color diagrams of these clusters are obtained and are used for estimating the reddenings and ages of the clusters. The luminosity functions (LFs) and initial mass functions (IMFs) of the massive stars in these clusters are also derived. The slopes of the IMFs range from Γ = -0.5 to -2.1. Interestingly, it is found that the IMFs get steeper with increasing galactocentric distance and with decreasing [O/H] abundance.

Peculiarities in velocity dispersion and surface density profiles of star clusters

NASA Astrophysics Data System (ADS)

Küpper, Andreas H. W.; Kroupa, Pavel; Baumgardt, Holger; Heggie, Douglas C.

2010-10-01

Based on our recent work on tidal tails of star clusters we investigate star clusters of a few 104Msolar by means of velocity dispersion profiles and surface density profiles. We use a comprehensive set of N-body computations of star clusters on various orbits within a realistic tidal field to study the evolution of these profiles with time, and ongoing cluster dissolution. From the velocity dispersion profiles we find that the population of potential escapers, i.e. energetically unbound stars inside the Jacobi radius, dominates clusters at radii above about 50 per cent of the Jacobi radius. Beyond 70 per cent of the Jacobi radius nearly all stars are energetically unbound. The velocity dispersion therefore significantly deviates from the predictions of simple equilibrium models in this regime. We furthermore argue that for this reason this part of a cluster cannot be used to detect a dark matter halo or deviations from the Newtonian gravity. By fitting templates to about 104 computed surface density profiles we estimate the accuracy which can be achieved in reconstructing the Jacobi radius of a cluster in this way. We find that the template of King works well for extended clusters on nearly circular orbits, but shows significant flaws in the case of eccentric cluster orbits. This we fix by extending this template with three more free parameters. Our template can reconstruct the tidal radius over all fitted ranges with an accuracy of about 10 per cent, and is especially useful in the case of cluster data with a wide radial coverage and for clusters showing significant extra-tidal stellar populations. No other template that we have tried can yield comparable results over this range of cluster conditions. All templates fail to reconstruct tidal parameters of concentrated clusters, however. Moreover, we find that the bulk of a cluster adjusts to the mean tidal field which it experiences and not to the tidal field at perigalacticon as has often been assumed in other investigations, i.e. a fitted tidal radius is a cluster's time average mean tidal radius and not its perigalactic one. Furthermore, we study the tidal debris in the vicinity of the clusters and find it to be well represented by a power law with a slope of -4 to -5. This steep slope we ascribe to the epicyclic motion of escaped stars in the tidal tails. Star clusters close to apogalacticon show a significantly shallower slope of up to -1, however. We suggest that clusters at apogalacticon can be identified by measuring this slope.

Multiple stellar populations in Magellanic Cloud clusters - III. The first evidence of an extended main sequence turn-off in a young cluster: NGC 1856

NASA Astrophysics Data System (ADS)

Milone, A. P.; Bedin, L. R.; Piotto, G.; Marino, A. F.; Cassisi, S.; Bellini, A.; Jerjen, H.; Pietrinferni, A.; Aparicio, A.; Rich, R. M.

2015-07-01

Recent studies have shown that the extended main-sequence turn-off (eMSTO) is a common feature of intermediate-age star clusters in the Magellanic Clouds (MCs). The most simple explanation is that these stellar systems harbour multiple generations of stars with an age difference of a few hundred million years. However, while an eMSTO has been detected in a large number of clusters with ages between ˜1-2 Gyr, several studies of young clusters in both MCs and in nearby galaxies do not find any evidence for a prolonged star formation history, i. e. for multiple stellar generations. These results have suggested alternative interpretation of the eMSTOs observed in intermediate-age star clusters. The eMSTO could be due to stellar rotation mimicking an age spread or to interacting binaries. In these scenarios, intermediate-age MC clusters would be simple stellar populations, in close analogy with younger clusters. Here, we provide the first evidence for an eMSTO in a young stellar cluster. We exploit multiband Hubble Space Telescope photometry to study the ˜300-Myr old star cluster NGC 1856 in the Large Magellanic Cloud and detected a broadened MSTO that is consistent with a prolonged star formation which had a duration of about 150 Myr. Below the turn-off, the main sequence (MS) of NGC 1856 is split into a red and blue component, hosting 33 ± 5 and 67 ± 5 per cent of the total number of MS stars, respectively. We discuss these findings in the context of multiple-stellar-generation, stellar-rotation, and interacting-binary hypotheses.

A Binary System in the Hyades Cluster Hosting a Neptune-Sized Planet

NASA Astrophysics Data System (ADS)

Feinstein, Adina; Ciardi, David; Crossfield, Ian; Schlieder, Joshua; Petigura, Erik; David, Trevor J.; Bristow, Makennah; Patel, Rahul; Arnold, Lauren; Benneke, Björn; Christiansen, Jessie; Dressing, Courtney; Fulton, Benjamin; Howard, Andrew; Isaacson, Howard; Sinukoff, Evan; Thackeray, Beverly

2018-01-01

We report the discovery of a Neptune-size planet (Rp = 3.0Rearth) in the Hyades Cluster. The host star is in a binary system, comprising a K5V star and M7/8V star with a projected separation of 40 AU. The planet orbits the primary star with an orbital period of 17.3 days and a transit duration of 3 hours. The host star is bright (V = 11.2, J = 9.1) and so may be a good target for precise radial velocity measurements. The planet is the first Neptune-sized planet to be found orbiting in a binary system within an open cluster. The Hyades is the nearest star cluster to the Sun, has an age of 625-750 Myr, and forms one of the fundamental rungs in the distance ladder; understanding the planet population in such a well-studied cluster can help us understand and set contraints on the formation and evolution of planetary systems.

Dynamical Mass Segregation Versus Disruption of Binary Stars in Dense Stellar Systems

NASA Astrophysics Data System (ADS)

de Grijs, Richard; Li, C.; Deng, L.

2013-01-01

Upon their formation, dynamically cool (collapsing) star clusters will, within only a few million years, achieve stellar mass segregation for stars down to a few solar masses due to gravitational two-body encounters. Since binary systems are, on average, more massive than single stars, one would expect them to also rapidly mass segregate dynamically. Contrary to these expectations and based on high-resolution Hubble Space Telescope observations, we show that the compact, 15-30 Myr-old Large Magellanic Cloud cluster NGC 1818 is characterized by an increasing fraction of F-star binary systems (with combined masses of 1.3-1.6 solar masses) with increasing distance from the cluster center. This offers unprecedented support of the theoretically predicted but thus far unobserved dynamical disruption processes of the significant population of "soft" binary systems (with relatively low binding energies compared to the kinetic energy of their stellar members) in star clusters, which we could unravel by virtue of the cluster's unique combination of youth and high stellar density.

Robo-AO Discovery and Basic Characterization of Wide Multiple Star Systems in the Pleiades, Praesepe, and NGC 2264 Clusters

NASA Astrophysics Data System (ADS)

Hillenbrand, Lynne A.; Zhang, Celia; Riddle, Reed L.; Baranec, Christoph; Ziegler, Carl; Law, Nicholas M.; Stauffer, John

2018-02-01

We identify and roughly characterize 66 candidate binary star systems in the Pleiades, Praesepe, and NGC 2264 star clusters, based on robotic adaptive optics imaging data obtained using Robo-AO at the Palomar 60″ telescope. Only ∼10% of our imaged pairs were previously known. We detect companions at red optical wavelengths, with physical separations ranging from a few tens to a few thousands of au. A three-sigma contrast curve generated for each final image provides upper limits to the brightness ratios for any undetected putative companions. The observations are sensitive to companions with a maximum contrast of ∼6m at larger separations. At smaller separations, the mean (best) raw contrast at 2″ is 3.ͫ8 (6m), at 1″ is 3.ͫ0 (4.ͫ5), and at 0.″5 is 1.ͫ9 (3m). Point-spread function subtraction can recover nearly the full contrast in the closer separations. For detected candidate binary pairs, we report separations, position angles, and relative magnitudes. Theoretical isochrones appropriate to the Pleiades and Praesepe clusters are then used to determine the corresponding binary mass ratios, which range from 0.2 to 0.9 in q={m}2/{m}1. For our sample of roughly solar-mass (FGK type) stars in NGC 2264 and sub-solar-mass (K and early M-type) primaries in the Pleiades and Praesepe, the overall binary frequency is measured at ∼15.5% ± 2%. However, this value should be considered a lower limit to the true binary fraction within the specified separation and mass ratio ranges in these clusters, given that complex and uncertain corrections for sensitivity and completeness have not been applied.

MOCCA code for star cluster simulation: comparison with optical observations using COCOA

NASA Astrophysics Data System (ADS)

Askar, Abbas; Giersz, Mirek; Pych, Wojciech; Olech, Arkadiusz; Hypki, Arkadiusz

2016-02-01

We introduce and present preliminary results from COCOA (Cluster simulatiOn Comparison with ObservAtions) code for a star cluster after 12 Gyr of evolution simulated using the MOCCA code. The COCOA code is being developed to quickly compare results of numerical simulations of star clusters with observational data. We use COCOA to obtain parameters of the projected cluster model. For comparison, a FITS file of the projected cluster was provided to observers so that they could use their observational methods and techniques to obtain cluster parameters. The results show that the similarity of cluster parameters obtained through numerical simulations and observations depends significantly on the quality of observational data and photometric accuracy.

Proper motions of five OB stars with candidate dusty bow shocks in the Carina Nebula

NASA Astrophysics Data System (ADS)

Kiminki, Megan M.; Smith, Nathan; Reiter, Megan; Bally, John

2017-06-01

We constrain the proper motions of five OB stars associated with candidate stellar wind bow shocks in the Carina Nebula using Hubble Space Telescope ACS imaging over 9-10 yr baselines. These proper motions allow us to directly compare each star's motion to the orientation of its candidate bow shock. Although these stars are saturated in our imaging, we assess their motion by the shifts required to minimize residuals in their airy rings. The results limit the direction of each star's motion to sectors less than 90° wide. None of the five stars are moving away from the Carina Nebula's central clusters as runaway stars would be, confirming that a candidate bow shock is not necessarily indicative of a runaway star. Two of the five stars are moving tangentially relative to the orientation of their candidate bow shocks, both of which point at the OB cluster Trumpler 14. In these cases, the large-scale flow of the interstellar medium, powered by feedback from the cluster, appears to dominate over the motion of the star in producing the observed candidate bow shock. The remaining three stars all have some component of motion towards the central clusters, meaning that we cannot distinguish whether their candidate bow shocks are indicators of stellar motion, of the flow of ambient gas or of density gradients in their surroundings. In addition, these stars' lack of outward motion hints that the distributed massive-star population in Carina's South Pillars region formed in place, rather than migrating out from the association's central clusters.

Applying Machine Learning to Star Cluster Classification

NASA Astrophysics Data System (ADS)

Fedorenko, Kristina; Grasha, Kathryn; Calzetti, Daniela; Mahadevan, Sridhar

2016-01-01

Catalogs describing populations of star clusters are essential in investigating a range of important issues, from star formation to galaxy evolution. Star cluster catalogs are typically created in a two-step process: in the first step, a catalog of sources is automatically produced; in the second step, each of the extracted sources is visually inspected by 3-to-5 human classifiers and assigned a category. Classification by humans is labor-intensive and time consuming, thus it creates a bottleneck, and substantially slows down progress in star cluster research.We seek to automate the process of labeling star clusters (the second step) through applying supervised machine learning techniques. This will provide a fast, objective, and reproducible classification. Our data is HST (WFC3 and ACS) images of galaxies in the distance range of 3.5-12 Mpc, with a few thousand star clusters already classified by humans as a part of the LEGUS (Legacy ExtraGalactic UV Survey) project. The classification is based on 4 labels (Class 1 - symmetric, compact cluster; Class 2 - concentrated object with some degree of asymmetry; Class 3 - multiple peak system, diffuse; and Class 4 - spurious detection). We start by looking at basic machine learning methods such as decision trees. We then proceed to evaluate performance of more advanced techniques, focusing on convolutional neural networks and other Deep Learning methods. We analyze the results, and suggest several directions for further improvement.

Sizing the star cluster population of the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Piatti, Andrés E.

2018-04-01

The number of star clusters that populate the Large Magellanic Cloud (LMC) at deprojected distances <4 deg has been recently found to be nearly double the known size of the system. Because of the unprecedented consequences of this outcome in our knowledge of the LMC cluster formation and dissolution histories, we closely revisited such a compilation of objects and found that only ˜35 per cent of the previously known catalogued clusters have been included. The remaining entries are likely related to stellar overdensities of the LMC composite star field, because there is a remarkable enhancement of objects with assigned ages older than log(t yr-1) ˜ 9.4, which contrasts with the existence of the LMC cluster age gap; the assumption of a cluster formation rate similar to that of the LMC star field does not help to conciliate so large amount of clusters either; and nearly 50 per cent of them come from cluster search procedures known to produce more than 90 per cent of false detections. The lack of further analyses to confirm the physical reality as genuine star clusters of the identified overdensities also glooms those results. We support that the actual size of the LMC main body cluster population is close to that previously known.

Truncation of the Binary Distribution Function in Globular Cluster Formation

NASA Astrophysics Data System (ADS)

Vesperini, E.; Chernoff, David F.

1996-02-01

We investigate a population of primordial binaries during the initial stage of evolution of a star cluster. For our calculations we assume that equal-mass stars form rapidly in a tidally truncated gas cloud, that ˜10% of the stars are in binaries, and that the resulting star cluster undergoes an epoch of violent relaxation. We study the collisional interaction of the binaries and single stars, in particular, the ionization of the binaries and the energy exchange between binaries and single stars. We find that for large N systems (N > 1000), even the most violent beginning leaves the binary distribution function largely intact. Hence, the binding energy originally tied up in the cloud's protostellar pairs is preserved during the relaxation process, and the binaries are available to interact at later times within the virialized cluster.

The Clusters AgeS Experiment (CASE). Variable stars in the field of the globular cluster NGC 362

NASA Astrophysics Data System (ADS)

Rozyczka, M.; Thompson, I. B.; Narloch, W.; Pych, W.; Schwarzenberg-Czerny, A.

2016-09-01

The field of the globular cluster NGC 362 was monitored between 1997 and 2015 in a search for variable stars. BV light curves were obtained for 151 periodic or likely periodic variable stars, over a hundred of which are new detections. Twelve newly detected variable stars are proper-motion members of the cluster: two SX Phe and two RR Lyr pulsators, one contact binary, three detached or semi-detached eclipsing binaries, and four spotted variable stars. The most interesting objects among these are the binary blue straggler V20 with an asymmetric light curve, and the 8.1 d semidetached binary V24 located on the red giant branch of NGC 362, which is a Chandra X-ray source. We also provide substantial new data for 24 previously known variable stars.

DISCOVERY OF 14 NEW SLOWLY PULSATING B STARS IN THE OPEN CLUSTER NGC 7654

DOE Office of Scientific and Technical Information (OSTI.GOV)

Luo, Y. P.; Han, Z. W.; Zhang, X. B.

2012-02-10

We carried out time-series BV CCD photometric observations of the open cluster NGC 7654 (Messier 52) to search for variable stars. Eighteen slowly pulsating B (SPB) stars have been detected, among which 14 candidates are newly discovered, three known ones are confirmed, and a previously found {delta} Scuti star is also identified as an SPB candidate. Twelve SPBs are probable cluster members based on membership analysis. This makes NGC 7654 the richest galactic open cluster in terms of SPB star content. It is also a new discovery that NGC 7654 hosts three {gamma} Dor star candidates. We found that allmore » these stars (18 SPB and 3 {gamma} Dor stars) have periods longer than their corresponding fundamental radial mode. With such a big sample of g-mode pulsators in a single cluster, it is clear that multi-mode pulsation is more common in the upper part of the main sequence than in the lower part. All the stars span a narrow strip on the period-luminosity plane, which also includes the {gamma} Dor stars at the low-luminosity extension. This result implies that there may be a single period-luminosity relation applicable to all g-mode main-sequence pulsators. As a by-product, three EA-type eclipsing binaries and an EW-type eclipsing binary are also discovered.« less

Photometric search for variable stars in the young open cluster Berkeley 59

NASA Astrophysics Data System (ADS)

Lata, Sneh; Pandey, A. K.; Maheswar, G.; Mondal, Soumen; Kumar, Brijesh

2011-12-01

We present the time series photometry of stars located in the extremely young open cluster Berkeley 59. Using the 1.04-m telescope at Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, we have identified 42 variables in a field of ˜13 × 13 arcmin2 around the cluster. The probable members of the cluster have been identified using a (V, V-I) colour-magnitude diagram and a (J-H, H-K) colour-colour diagram. 31 variables have been found to be pre-main-sequence stars associated with the cluster. The ages and masses of the pre-main-sequence stars have been derived from the colour-magnitude diagram by fitting theoretical models to the observed data points. The ages of the majority of the probable pre-main-sequence variable candidates range from 1 to 5 Myr. The masses of these pre-main-sequence variable stars have been found to be in the range of ˜0.3 to ˜3.5 M⊙, and these could be T Tauri stars. The present statistics reveal that about 90 per cent T Tauri stars have period <15 d. The classical T Tauri stars are found to have a larger amplitude than the weak-line T Tauri stars. There is an indication that the amplitude decreases with an increase in mass, which could be due to the dispersal of the discs of relatively massive stars.

Be Stars in M31

NASA Astrophysics Data System (ADS)

Peters, Matthew L.; Wisniewski, John; Choi, Yumi; Williams, Ben; Lomax, Jamie; Bjorkman, Karen; Durbin, Meredith; Johnson, Lent Cliff; Lewis, Alexia; Lutz, Julie; Sigut, Aaron; Wallach, Aislynn; Dalcanton, Julianne

2018-01-01

We identify Be candidate stars in M31 using two-epoch F625W + F658N photometry from HST/ACS+WFC3 combined with the Panchromatic Hubble Andromeda Treasury (PHAT) Catalog. Using the PHAT catalog allows us to extract stellar parameters such as surface temperature and gravity, thereby allowing us to identify the main sequence B type stars in the field of view. Be candidate stars are identified by comparing their HST narrow-band Hα excess magnitudes with that predicted by Kurucz spectra. We find 314 Be candidate stars out of 5699 B + Be candidate stars (5.51%) in our first epoch and 301 Be candidate stars out of 5769 B + Be candidate stars (5.22%) in our second epoch. Our Be fraction, while lower than that of the SMC, LMC, and MW, is possibly consistent with the fact the M31 has a higher metallicity than the other galaxies because Be fraction varies inversely with metallicity. We note that earlier spectral types have the largest Be fraction, and that the Be fraction strictly declines as the spectral type increases to later types. We then match our Be candidate stars with clusters, establishing that 39 of 314 are cluster stars in epoch one and 36 of 301 stars are cluster stars in epoch two. We assign ages, using the cluster age to characterize cluster Be candidate stars and star formation histories to characterize field Be candidate stars. Finally, we determine which Be candidate stars exhibited disk loss or disk growth between epochs, finding that, of the Be stars that did not show source confusion or low SNR in one of the epochs, 65 / 265 (24.5%) showed disk loss or renewal, while 200 / 265 (75.5%) showed only small changes in Hα excess. Our research provides context for the parameters of candidate Be stars in M31, which will be useful in further determining the nature of Be stars. This paper was supported by a grant from STScI via GO-13857.

A census of variability in globular cluster M 68 (NGC 4590)

NASA Astrophysics Data System (ADS)

Kains, N.; Arellano Ferro, A.; Figuera Jaimes, R.; Bramich, D. M.; Skottfelt, J.; Jørgensen, U. G.; Tsapras, Y.; Street, R. A.; Browne, P.; Dominik, M.; Horne, K.; Hundertmark, M.; Ipatov, S.; Snodgrass, C.; Steele, I. A.; Lcogt/Robonet Consortium; Alsubai, K. A.; Bozza, V.; Calchi Novati, S.; Ciceri, S.; D'Ago, G.; Galianni, P.; Gu, S.-H.; Harpsøe, K.; Hinse, T. C.; Juncher, D.; Korhonen, H.; Mancini, L.; Popovas, A.; Rabus, M.; Rahvar, S.; Southworth, J.; Surdej, J.; Vilela, C.; Wang, X.-B.; Wertz, O.; Mindstep Consortium

2015-06-01

Aims: We analyse 20 nights of CCD observations in the V and I bands of the globular cluster M 68 (NGC 4590) and use them to detect variable objects. We also obtained electron-multiplying CCD (EMCCD) observations for this cluster in order to explore its core with unprecedented spatial resolution from the ground. Methods: We reduced our data using difference image analysis to achieve the best possible photometry in the crowded field of the cluster. In doing so, we show that when dealing with identical networked telescopes, a reference image from any telescope may be used to reduce data from any other telescope, which facilitates the analysis significantly. We then used our light curves to estimate the properties of the RR Lyrae (RRL) stars in M 68 through Fourier decomposition and empirical relations. The variable star properties then allowed us to derive the cluster's metallicity and distance. Results: M 68 had 45 previously confirmed variables, including 42 RRL and 2 SX Phoenicis (SX Phe) stars. In this paper we determine new periods and search for new variables, especially in the core of the cluster where our method performs particularly well. We detect 4 additional SX Phe stars and confirm the variability of another star, bringing the total number of confirmed variable stars in this cluster to 50. We also used archival data stretching back to 1951 to derive period changes for some of the single-mode RRL stars, and analyse the significant number of double-mode RRL stars in M 68. Furthermore, we find evidence for double-mode pulsation in one of the SX Phe stars in this cluster. Using the different classes of variables, we derived values for the metallicity of the cluster of [Fe/H] = -2.07 ± 0.06 on the ZW scale, or -2.20 ± 0.10 on the UVES scale, and found true distance moduli μ0 = 15.00 ± 0.11 mag (using RR0 stars), 15.00 ± 0.05 mag (using RR1 stars), 14.97 ± 0.11 mag (using SX Phe stars), and 15.00 ± 0.07 mag (using the MV -[Fe/H] relation for RRL stars), corresponding to physical distances of 10.00 ± 0.49, 9.99 ± 0.21, 9.84 ± 0.50, and 10.00 ± 0.30 kpc, respectively. Thanks to the first use of difference image analysis on time-series observations of M 68, we are now confident that we have a complete census of the RRL stars in this cluster. The full Table 2 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/578/A128

Optical-near-IR analysis of globular clusters in the IKN dwarf spheroidal: a complex star formation history

NASA Astrophysics Data System (ADS)

Tudorica, A.; Georgiev, I. Y.; Chies-Santos, A. L.

2015-09-01

Context. Age, metallicity, and spatial distribution of globular clusters (GCs) provide a powerful tool for reconstructing major star-formation episodes in galaxies. IKN is a faint dwarf spheroidal (dSph) in the M 81 group of galaxies. It contains five old GCs, which makes it the galaxy with the highest known specific frequency (SN = 126). Aims: We estimate the photometric age, metallicity, and spatial distribution of the poorly studied IKN GCs. We search SDSS for GC candidates beyond the HST/ACS field of view, which covers half of IKN. Methods: To break the age-metallicity degeneracy in the colour, we used WHT/LIRIS KS-band photometry and derived photometric ages and metallicities by comparison with SSP models in the V,I,Ks colour space. Results: IKN GCs' VIKs colours are consistent with old ages (≥8 Gyr) and a metallicity distribution with a higher mean than is typical for such a dSph ([Fe/H] ≃ -1.4-0.2+0.6 dex). Their photometric mass range (0.5 < ℳGC< 4 × 105 M⊙) implies an unusually high mass ratio between GCs and field stars, of 10.6%. Mixture model analysis of the RGB field stars' metallicity suggests that 72% of the stars may have formed together with the GCs. Using the most massive GC-SFR relation, we calculated a star formation rate (SFR) of ~10 M⊙/yr during its formation epoch. We note that the more massive GCs are closer to the galaxy photometric centre. IKN GCs also appear spatially aligned along a line close to the major axis of the IKN and nearly orthogonal to the plane of spatial distribution of galaxies in the M 81 group. We identify one new IKN GC candidate based on colour and the PSF analysis of the SDSS data. Conclusions: The evidence of i) broad and high metallicity distribution of the field IKN RGB stars and its GCs, ii) high fraction, and iii) spatial alignment of IKN GCs supports a scenario for tidally triggered, complex IKN's star formation history in the context of interactions with galaxies in the M 81 group.

Ages of young star clusters, massive blue stragglers, and the upper mass limit of stars: Analyzing age-dependent stellar mass functions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schneider, F. R. N.; Izzard, R. G.; Langer, N.

2014-01-10

Massive stars rapidly change their masses through strong stellar winds and mass transfer in binary systems. The latter aspect is important for populations of massive stars as more than 70% of all O stars are expected to interact with a binary companion during their lifetime. We show that such mass changes leave characteristic signatures in stellar mass functions of young star clusters that can be used to infer their ages and to identify products of binary evolution. We model the observed present-day mass functions of the young Galactic Arches and Quintuplet star clusters using our rapid binary evolution code. Wemore » find that the shaping of the mass function by stellar wind mass loss allows us to determine the cluster ages as 3.5 ± 0.7 Myr and 4.8 ± 1.1 Myr, respectively. Exploiting the effects of binary mass exchange on the cluster mass function, we find that the most massive stars in both clusters are rejuvenated products of binary mass transfer, i.e., the massive counterpart of classical blue straggler stars. This resolves the problem of an apparent age spread among the most luminous stars exceeding the expected duration of star formation in these clusters. We perform Monte Carlo simulations to probe stochastic sampling, which support the idea of the most massive stars being rejuvenated binary products. We find that the most massive star is expected to be a binary product after 1.0 ± 0.7 Myr in Arches and after 1.7 ± 1.0 Myr in Quintuplet. Today, the most massive 9 ± 3 stars in Arches and 8 ± 3 in Quintuplet are expected to be such objects. Our findings have strong implications for the stellar upper mass limit and solve the discrepancy between the claimed 150 M {sub ☉} limit and observations of four stars with initial masses of 165-320 M {sub ☉} in R136 and of supernova 2007bi, which is thought to be a pair-instability supernova from an initial 250 M {sub ☉} star. Using the stellar population of R136, we revise the upper mass limit to values in the range 200-500 M {sub ☉}.« less

Ages of Young Star Clusters, Massive Blue Stragglers, and the Upper Mass Limit of Stars: Analyzing Age-dependent Stellar Mass Functions

NASA Astrophysics Data System (ADS)

Schneider, F. R. N.; Izzard, R. G.; de Mink, S. E.; Langer, N.; Stolte, A.; de Koter, A.; Gvaramadze, V. V.; Hußmann, B.; Liermann, A.; Sana, H.

2014-01-01

Massive stars rapidly change their masses through strong stellar winds and mass transfer in binary systems. The latter aspect is important for populations of massive stars as more than 70% of all O stars are expected to interact with a binary companion during their lifetime. We show that such mass changes leave characteristic signatures in stellar mass functions of young star clusters that can be used to infer their ages and to identify products of binary evolution. We model the observed present-day mass functions of the young Galactic Arches and Quintuplet star clusters using our rapid binary evolution code. We find that the shaping of the mass function by stellar wind mass loss allows us to determine the cluster ages as 3.5 ± 0.7 Myr and 4.8 ± 1.1 Myr, respectively. Exploiting the effects of binary mass exchange on the cluster mass function, we find that the most massive stars in both clusters are rejuvenated products of binary mass transfer, i.e., the massive counterpart of classical blue straggler stars. This resolves the problem of an apparent age spread among the most luminous stars exceeding the expected duration of star formation in these clusters. We perform Monte Carlo simulations to probe stochastic sampling, which support the idea of the most massive stars being rejuvenated binary products. We find that the most massive star is expected to be a binary product after 1.0 ± 0.7 Myr in Arches and after 1.7 ± 1.0 Myr in Quintuplet. Today, the most massive 9 ± 3 stars in Arches and 8 ± 3 in Quintuplet are expected to be such objects. Our findings have strong implications for the stellar upper mass limit and solve the discrepancy between the claimed 150 M ⊙ limit and observations of four stars with initial masses of 165-320 M ⊙ in R136 and of supernova 2007bi, which is thought to be a pair-instability supernova from an initial 250 M ⊙ star. Using the stellar population of R136, we revise the upper mass limit to values in the range 200-500 M ⊙.

Hubble Peers into the Most Crowded Place in the Milky Way

NASA Image and Video Library

2015-05-29

This NASA/ESA Hubble Space Telescope image presents the Arches Cluster, the densest known star cluster in the Milky Way. It is located about 25,000 light-years from Earth in the constellation of Sagittarius (The Archer), close to the heart of our galaxy, the Milky Way. It is, like its neighbor the Quintuplet Cluster, a fairly young astronomical object at between two and four million years old. The Arches cluster is so dense that in a region with a radius equal to the distance between the sun and its nearest star there would be over 100,000 stars! At least 150 stars within the cluster are among the brightest ever discovered in the Milky Way. These stars are so bright and massive that they will burn their fuel within a short time (on a cosmological scale that means just a few million years). Then they will die in spectacular supernova explosions. Due to the short lifetime of the stars in the cluster the gas between the stars contains an unusually high amount of heavier elements, which were produced by earlier generations of stars. Despite its brightness the Arches Cluster cannot be seen with the naked eye. The visible light from the cluster is completely obscured by gigantic clouds of dust in this region. To make the cluster visible astronomers have to use detectors which can collect light from the X-ray, infrared, and radio bands, as these wavelengths can pass through the dust clouds. This observation shows the Arches Cluster in the infrared and demonstrates the leap in Hubble’s performance since its 1999 image of same object. Credit: NASA/ESA NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

The GALAH survey: chemical tagging of star clusters and new members in the Pleiades

NASA Astrophysics Data System (ADS)

Kos, Janez; Bland-Hawthorn, Joss; Freeman, Ken; Buder, Sven; Traven, Gregor; De Silva, Gayandhi M.; Sharma, Sanjib; Asplund, Martin; Duong, Ly; Lin, Jane; Lind, Karin; Martell, Sarah; Simpson, Jeffrey D.; Stello, Dennis; Zucker, Daniel B.; Zwitter, Tomaž; Anguiano, Borja; Da Costa, Gary; D'Orazi, Valentina; Horner, Jonathan; Kafle, Prajwal R.; Lewis, Geraint; Munari, Ulisse; Nataf, David M.; Ness, Melissa; Reid, Warren; Schlesinger, Katie; Ting, Yuan-Sen; Wyse, Rosemary

2018-02-01

The technique of chemical tagging uses the elemental abundances of stellar atmospheres to 'reconstruct' chemically homogeneous star clusters that have long since dispersed. The GALAH spectroscopic survey - which aims to observe one million stars using the Anglo-Australian Telescope - allows us to measure up to 30 elements or dimensions in the stellar chemical abundance space, many of which are not independent. How to find clustering reliably in a noisy high-dimensional space is a difficult problem that remains largely unsolved. Here, we explore t-distributed stochastic neighbour embedding (t-SNE) - which identifies an optimal mapping of a high-dimensional space into fewer dimensions - whilst conserving the original clustering information. Typically, the projection is made to a 2D space to aid recognition of clusters by eye. We show that this method is a reliable tool for chemical tagging because it can: (i) resolve clustering in chemical space alone, (ii) recover known open and globular clusters with high efficiency and low contamination, and (iii) relate field stars to known clusters. t-SNE also provides a useful visualization of a high-dimensional space. We demonstrate the method on a data set of 13 abundances measured in the spectra of 187 000 stars by the GALAH survey. We recover seven of the nine observed clusters (six globular and three open clusters) in chemical space with minimal contamination from field stars and low numbers of outliers. With chemical tagging, we also identify two Pleiades supercluster members (which we confirm kinematically), one as far as 6° - one tidal radius away from the cluster centre.

Constraints on Helium Enhancement in the Globular Cluster M3 (NGC 5272): The Horizontal Branch Test

NASA Technical Reports Server (NTRS)

Catelan, M.; Grundahl, F.; Sweigart, A. V.; Valcarce, A. A. R.; Cortes, C.

2007-01-01

It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is a common feature among globular star clusters. In this scenario, such a helium enhancement would be particularly apparent in the enhanced luminosity of thc blue horizontal branch (HB) stars compared to the red HB stars. In this Letter, wc test this scenario in the case of the Galactic globular cluster M3 (NGC 5272), using high-precision Stromgren photometry and spectroscopic gravities for blue HB stars. We find that any helium enhancement among the cluster's blue HB stars must be significantly less than I%, thus ruling out the much higher helium enhancements that have been proposed in the literature.

Photometrically-derived properties of massive-star clusters obtained with different massive-star evolution tracks and deterministic models

NASA Astrophysics Data System (ADS)

Wofford, Aida; Charlot, Stéphane; Eldridge, John

2015-08-01

We compute libraries of stellar + nebular spectra of populations of coeval stars with ages of <100 Myr and metallicities of Z=0.001 to 0.040, using different sets of massive-star evolution tracks, i.e., new Padova tracks for single non-rotating stars, the Geneva tracks for single non-rotating and rotating stars, and the Auckland tracks for single non-rotating and binary stars. For the stellar component, we use population synthesis codes galaxev, starburst99, and BPASS, depending on the set of tracks. For the nebular component we use photoionization code cloudy. From these spectra, we obtain magnitudes in filters F275W, F336W, F438W, F547M, F555W, F657N, and F814W of the Hubble Space Telescope (HST) Wide Field Camera Three. We use i) our computed magnitudes, ii) new multi-band photometry of massive-star clusters in nearby (<11 Mpc) galaxies spanning the metallicity range 12+log(O/H)=7.2-9.2, observed as part of HST programs 13364 (PI Calzetti) and 13773 (PI Chandar), and iii) Bayesian inference to a) establish how well the different models are able to constrain the metallicities, extinctions, ages, and masses of the star clusters, b) quantify differences in the cluster properties obtained with the different models, and c) assess how properties of lower-mass clusters are affected by the stochastic sampling of the IMF. In our models, the stellar evolution tracks, stellar atmospheres, and nebulae have similar chemical compositions. Different metallicities are available with different sets of tracks and we compare results from models of similar metallicities. Our results have implications for studies of the formation and evolution of star clusters, the cluster age and mass functions, and the star formation histories of galaxies.

A black hole in a globular cluster.

PubMed

Maccarone, Thomas J; Kundu, Arunav; Zepf, Stephen E; Rhode, Katherine L

2007-01-11

Globular star clusters contain thousands to millions of old stars packed within a region only tens of light years across. Their high stellar densities make it very probable that their member stars will interact or collide. There has accordingly been considerable debate about whether black holes should exist in these star clusters. Some theoretical work suggests that dynamical processes in the densest inner regions of globular clusters may lead to the formation of black holes of approximately 1,000 solar masses. Other numerical simulations instead predict that stellar interactions will eject most or all of the black holes that form in globular clusters. Here we report the X-ray signature of an accreting black hole in a globular cluster associated with the giant elliptical galaxy NGC 4472 (in the Virgo cluster). This object has an X-ray luminosity of about 4 x 10(39) erg s(-1), which rules out any object other than a black hole in such an old stellar population. The X-ray luminosity varies by a factor of seven in a few hours, which excludes the possibility that the object is several neutron stars superposed.

OGLE Collection of Star Clusters. New Objects in the Magellanic Bridge and the Outskirts of the Small Magellanic Cloud

NASA Astrophysics Data System (ADS)

Sitek, M.; Szymański, M. K.; Udalski, A.; Skowron, D. M.; Kostrzewa-Rutkowska, Z.; Skowron, J.; Karczmarek, P.; Cieślar, M.; Wyrzykowski, Ł.; Kozłowski, S.; Pietrukowicz, P.; Soszyński, I.; Mróz, P.; Pawlak, M.; Poleski, R.; Ulaczyk, K.

2017-12-01

The Magellanic System (MS) encompasses the nearest neighbors of the Milky Way, the Large (LMC) and Small (SMC) Magellanic Clouds, and the Magellanic Bridge (MBR). This system contains a diverse sample of star clusters. Their parameters, such as the spatial distribution, chemical composition and age distribution yield important information about the formation scenario of the whole Magellanic System. Using deep photometric maps compiled in the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV) we present the most complete catalog of star clusters in the Magellanic System ever constructed from homogeneous, long time-scale photometric data. In this second paper of the series, we show the collection of star clusters found in the area of about 360 square degrees in the MBR and in the outer regions of the SMC. Our sample contains 198 visually identified star cluster candidates, 75 of which were not listed in any of the previously published catalogs. The new discoveries are mainly young small open clusters or clusters similar to associations.

Integrated light chemical tagging analyses of seven M31 outer halo globular clusters from the Pan-Andromeda Archaeological Survey

NASA Astrophysics Data System (ADS)

Sakari, Charli M.; Venn, Kim A.; Mackey, Dougal; Shetrone, Matthew D.; Dotter, Aaron; Ferguson, Annette M. N.; Huxor, Avon

2015-04-01

Detailed chemical abundances are presented for seven M31 outer halo globular clusters (with projected distances from M31 greater than 30 kpc), as derived from high-resolution integrated light spectra taken with the Hobby-Eberly Telescope. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS) - this paper presents the first determinations of integrated Fe, Na, Mg, Ca, Ti, Ni, Ba, and Eu abundances for these clusters. Four of the target clusters (PA06, PA53, PA54, and PA56) are metal poor ([Fe/H] < -1.5), α-enhanced (though they are possibly less α-enhanced than Milky Way stars at the 1σ level), and show signs of star-to-star Na and Mg variations. The other three globular clusters (H10, H23, and PA17) are more metal rich, with metallicities ranging from [Fe/H] = -1.4 to -0.9. While H23 is chemically similar to Milky Way field stars, Milky Way globular clusters, and other M31 clusters, H10 and PA17, have moderately low [Ca/Fe], compared to Milky Way field stars and clusters. Additionally, PA17's high [Mg/Ca] and [Ba/Eu] ratios are distinct from Milky Way stars, and are in better agreement with the stars and clusters in the Large Magellanic Cloud. None of the clusters studied here can be conclusively linked to any of the identified streams from PAndAS; however, based on their locations, kinematics, metallicities, and detailed abundances, the most metal-rich PAndAS clusters H23 and PA17 may be associated with the progenitor of the Giant Stellar Stream, H10 may be associated with the SW cloud, and PA53 and PA56 may be associated with the eastern cloud.

A catalogue of masses, structural parameters and velocity dispersion profiles of 112 Milky Way globular clusters

NASA Astrophysics Data System (ADS)

Baumgardt, H.; Hilker, M.

2018-05-01

We have determined masses, stellar mass functions and structural parameters of 112 Milky Way globular clusters by fitting a large set of N-body simulations to their velocity dispersion and surface density profiles. The velocity dispersion profiles were calculated based on a combination of more than 15,000 high-precision radial velocities which we derived from archival ESO/VLT and Keck spectra together with ˜20, 000 published radial velocities from the literature. Our fits also include the stellar mass functions of the globular clusters, which are available for 47 clusters in our sample, allowing us to self-consistently take the effects of mass segregation and ongoing cluster dissolution into account. We confirm the strong correlation between the global mass functions of globular clusters and their relaxation times recently found by Sollima & Baumgardt (2017). We also find a correlation of the escape velocity from the centre of a globular cluster and the fraction of first generation stars (FG) in the cluster recently derived for 57 globular clusters by Milone et al. (2017), but no correlation between the FG star fraction and the global mass function of a globular cluster. This could indicate that the ability of a globular cluster to keep the wind ejecta from the polluting star(s) is the crucial parameter determining the presence and fraction of second generation stars and not its later dynamical mass loss.

Filaments, ridges and a mini-starburst - HOBYS' view of high mass star formation with Herschel

NASA Astrophysics Data System (ADS)

Hill, T.; Motte, F.; Didelon, P.

2012-03-01

With its unprecedented spatial resolution and high sensitivity, Herschel is revolutionising our understanding of high mass star formation and the interstellar medium (ISM). In particular, Herschel is unveiling the filamentary structure and molecular cloud constituents of the ISM where star formation takes place. The Herschel Imaging Survey of OB Young Stellar objects (HOBYS; Motte, Zavagno, Bontemps, see http://www.herschel.fr/cea/hobys/en/index.php) key program targets burgeoning young stellar objects with the aim of characterising them and the environments in which they form. HOBYS has already proven fruitful with many clear examples of high-mass star formation in nearby molecular cloud complexes (e.g. Motte et al., 2010). Through multi-wavelength Herschel observations I will introduce select regions of the HOBYS program, including Vela C, M16 and W48 to start. These data are rich with filamentary structures and a wealth of sources which span a large mass range including, low, intermediate and high-mass objects in the pre-collapse or protostellar phase of formation, many of which will proceed to form stars. The natal filaments themselves come in many shapes and sizes, they can form thick ridge-like structures, be dispersed in low column density regions or cluster in higher density regions. In Vela C, high-mass star formation proceeds preferentially in high column density supercritical filaments, called ridges, which may result from the constructive convergence of flows (Hill et al., 2011). I will present other examples of ridges identified in HOBYS regions. In addition, I will present the latest results on the Eagle Nebula (M16). This region was made iconic by Hubble, but only Herschel can trace the cold, dense early prestellar phases of star formation, and their natal interstellar filaments, in this infamous star-forming complex. The cavity ionised by the nearby OB cluster in M16 serves to heat the Pillars of Creation and the surrounding interstellar filaments. We draw hypotheses regarding the long, cold resilient (enduring) filament in the eastern portion of M16, offset from the ionised cavity. In W48, the IRDC G035.39-00.33 is likely undergoing a mini star-burst of star formation (Nuygen-Luong et al., 2011).

A Slice of Orion

NASA Technical Reports Server (NTRS)

2006-01-01

[figure removed for brevity, see original site] Extended Orion Nebula Cloud

This image composite shows a part of the Orion constellation surveyed by NASA's Spitzer Space Telescope. The shape of the main image was designed by astronomers to roughly follow the shape of Orion cloud A, an enormous star-making factory containing about 1,800 young stars. This giant cloud includes the famous Orion nebula (bright circular area in 'blade' part of hockey stick-shaped box at the bottom), which is visible to the naked eye on a clear, dark night as a fuzzy star in the hunter constellation's sword.

The region that makes up the shaft part of the hockey stick box stretches 70 light-years beyond the Orion nebula. This particular area does not contain massive young stars like those of the Orion nebula, but is filled with 800 stars about the same mass as the sun. These sun-like stars don't live in big 'cities,' or clusters, of stars like the one in the Orion nebula; instead, they can be found in small clusters (right inset), or in relative isolation (middle insert).

In the right inset, developing stars are illuminating the dusty cloud, creating small wisps that appear greenish. The stars also power speedy jets of gas (also green), which glow as the jets ram into the cloudy material.

Since infrared light can penetrate through dust, we see not only stars within the cloud, but thousands of stars many light-years behind it, which just happen to be in the picture like unwanted bystanders. Astronomers carefully separate the young stars in the Orion cloud complex from the bystanders by looking for their telltale infrared glow.

The infrared image shows light captured by Spitzer's infrared array camera. Light with wavelengths of 8 and 5.8 microns (red and orange) comes mainly from dust that has been heated by starlight. Light of 4.5 microns (green) shows hot gas and dust; and light of 3.6 microns (blue) is from starlight.

Star clusters in evolving galaxies

NASA Astrophysics Data System (ADS)

Renaud, Florent

2018-04-01

Their ubiquity and extreme densities make star clusters probes of prime importance of galaxy evolution. Old globular clusters keep imprints of the physical conditions of their assembly in the early Universe, and younger stellar objects, observationally resolved, tell us about the mechanisms at stake in their formation. Yet, we still do not understand the diversity involved: why is star cluster formation limited to 105M⊙ objects in the Milky Way, while some dwarf galaxies like NGC 1705 are able to produce clusters 10 times more massive? Why do dwarfs generally host a higher specific frequency of clusters than larger galaxies? How to connect the present-day, often resolved, stellar systems to the formation of globular clusters at high redshift? And how do these links depend on the galactic and cosmological environments of these clusters? In this review, I present recent advances on star cluster formation and evolution, in galactic and cosmological context. The emphasis is put on the theory, formation scenarios and the effects of the environment on the evolution of the global properties of clusters. A few open questions are identified.

Ruprecht 3: An old star cluster remnant?

NASA Astrophysics Data System (ADS)

Pavani, D. B.; Bica, E.; Ahumada, A. V.; Clariá, J. J.

2003-02-01

2MASS J and H photometry and integrated spectroscopy are employed to study the nature of the poorly populated compact concentration of stars Ruprecht 3, which was previously catalogued as an open cluster. The integrated spectrum remarkably resembles that of a moderately metal-rich globular cluster. The distribution of the object stars in the colour-magnitude diagram is compatible with that of a 1.5 +/- 0.5 Gyr open cluster or older, depending on whether the bluer stars are interpreted as turnoff stars or blue stragglers, respectively. We derive for the object a distance from the Sun dsun = 0.72 +0.04-0.03 kpc and a colour excess E(B-V) = 0.04. Although a globular cluster remnant cannot be ruled out, the integrated spectrum resemblance to that of a globular cluster probably reflects a stochastic effect owing to the few brighter stars. The structural and photometric properties of Ruprecht 3 are compatible with what would be expected for an intermediate-age open cluster remnant. Based on observations made at Complejo Astronómico El Leoncito, which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba and San Juan, Argentina.

Gone with the Wind: Watching Galaxy Transformation in Abell 2125

NASA Astrophysics Data System (ADS)

Keel, W.; Owen, F.; Ledlow, M.; Wang, D.

2003-12-01

Dense environments clearly foster the transformation of galaxies, but it has proven difficult to untangle the roles of various processes in cluster environments. We have found a uniquely strong case for ongoing stripping of gas from the galaxy C153 in Abell 2125. The cluster, at z=0.25, includes merging subsystems with a relative line-of-sight velocity near 2000 km/s. C153, identified using the VLA as a strong radio source powered by star formation, is the brightest cluster member with activity of this kind, and part of the less populous blueshifted grouping. Several lines of evidence indicate that it is being swept by a stripping event. (1) A tail of ionized gas is seen in [O II] emission, which extends at least 70 kpc toward the cluster core, coinciding with a soft X-ray feature seen in the Chandra observations reported by Wang et al. (2) HST WFPC2 images reveal disturbed and clumpy morphology, including luminous star-forming complexes and chaotic dust features. (3) The spectral energy distribution and Gemini GMOS absorption-line spectrum indicate a massive burst of star formation ≈ 108 years ago superimposed on an older and much fainter population. (4) The stellar and gas kinematics are decoupled, with multiple gas velocity systems including counter-rotating components. The large velocity difference between the galaxy and (most of the) intracluster medium may contribute to the signatures being more prominent than hitherto seen. The starburst age is consistent with estimates of the time since the closest encounter of the major subsystems during the cluster-level merger. We continue to explore whether a starburst outflow or tidal damage has added to the role of stripping by the ICM, and how star formation has proceeded in the gas after leaving the galaxy disk. This work was supported by NASA through HST grant GO-07279.01-96A, and by the NSF through facilities at NRAO, Kitt Peak, and Gemini-North.

The Grism Lens-Amplified Survey from Space (GLASS). VII. The Diversity of the Distribution of Star Formation in Cluster and Field Galaxies at 0.3 less than or equal to z less than or equal to 0.7

NASA Astrophysics Data System (ADS)

Vulcani, Benedetta; Treu, Tommaso; Schmidt, Kasper B.; Morishita, Takahiro; Dressler, Alan; Poggianti, Bianca M.; Abramson, Louis; Bradač, Marusa; Brammer, Gabriel B.; Hoag, Austin; Malkan, Matthew; Pentericci, Laura; Trenti, Michele

2016-12-01

Exploiting the slitless spectroscopy taken as part of the Grism Lens-Amplified Survey from Space (GLASS), we present an extended analysis of the spatial distribution of star formation in 76 galaxies in 10 clusters at 0.3\\lt z\\lt 0.7. We use 85 foreground and background galaxies in the same redshift range as a field sample. The samples are well matched in stellar mass (108-1011 {M}⊙ ) and star formation rate (0.5-50 {M}⊙ {{yr}}-1). We visually classify galaxies in terms of broad band morphology, Hα morphology, and likely physical process acting on the galaxy. Most Hα emitters have a spiral morphology (41% ± 8% in clusters, 51% ± 8% in the field), followed by mergers/interactions (28% ± 8%, 31% ± 7%, respectively) and early-type galaxies (remarkably as high as 29% ± 8% in clusters and 15% ± 6% in the field). A diversity of Hα morphologies is detected, suggesting a diversity of physical processes. In clusters, 30% ± 8% of the galaxies present a regular morphology, mostly consistent with star formation diffused uniformly across the stellar population (mostly in the disk component, when present). The second most common morphology (28% ± 8%) is asymmetric/jellyfish, consistent with ram-pressure stripping or other non-gravitational processes in 18% ± 8% of the cases. Ram-pressure stripping appears significantly less prominent in the field (2% ± 2%), where the most common morphology/mechanism appears to be consistent with minor gas-rich mergers or clump accretion. This work demonstrates that while environment-specific mechanisms affect galaxy evolution at this redshift, they are diverse and their effects are subtle. A full understanding of this complexity requires larger samples and detailed and spatially resolved physical models.

Simulations of Fractal Star Cluster Formation. I. New Insights for Measuring Mass Segregation of Star Clusters with Substructure

NASA Astrophysics Data System (ADS)

Yu, Jincheng; Puzia, Thomas H.; Lin, Congping; Zhang, Yiwei

2017-05-01

We compare the existent methods, including the minimum spanning tree based method and the local stellar density based method, in measuring mass segregation of star clusters. We find that the minimum spanning tree method reflects more the compactness, which represents the global spatial distribution of massive stars, while the local stellar density method reflects more the crowdedness, which provides the local gravitational potential information. It is suggested to measure the local and the global mass segregation simultaneously. We also develop a hybrid method that takes both aspects into account. This hybrid method balances the local and the global mass segregation in the sense that the predominant one is either caused by dynamical evolution or purely accidental, especially when such information is unknown a priori. In addition, we test our prescriptions with numerical models and show the impact of binaries in estimating the mass segregation value. As an application, we use these methods on the Orion Nebula Cluster (ONC) observations and the Taurus cluster. We find that the ONC is significantly mass segregated down to the 20th most massive stars. In contrast, the massive stars of the Taurus cluster are sparsely distributed in many different subclusters, showing a low degree of compactness. The massive stars of Taurus are also found to be distributed in the high-density region of the subclusters, showing significant mass segregation at subcluster scales. Meanwhile, we also apply these methods to discuss the possible mechanisms of the dynamical evolution of the simulated substructured star clusters.

Simulations of Fractal Star Cluster Formation. I. New Insights for Measuring Mass Segregation of Star Clusters with Substructure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yu, Jincheng; Puzia, Thomas H.; Lin, Congping

2017-05-10

We compare the existent methods, including the minimum spanning tree based method and the local stellar density based method, in measuring mass segregation of star clusters. We find that the minimum spanning tree method reflects more the compactness, which represents the global spatial distribution of massive stars, while the local stellar density method reflects more the crowdedness, which provides the local gravitational potential information. It is suggested to measure the local and the global mass segregation simultaneously. We also develop a hybrid method that takes both aspects into account. This hybrid method balances the local and the global mass segregationmore » in the sense that the predominant one is either caused by dynamical evolution or purely accidental, especially when such information is unknown a priori. In addition, we test our prescriptions with numerical models and show the impact of binaries in estimating the mass segregation value. As an application, we use these methods on the Orion Nebula Cluster (ONC) observations and the Taurus cluster. We find that the ONC is significantly mass segregated down to the 20th most massive stars. In contrast, the massive stars of the Taurus cluster are sparsely distributed in many different subclusters, showing a low degree of compactness. The massive stars of Taurus are also found to be distributed in the high-density region of the subclusters, showing significant mass segregation at subcluster scales. Meanwhile, we also apply these methods to discuss the possible mechanisms of the dynamical evolution of the simulated substructured star clusters.« less

Massive open star clusters using the VVV survey. I. Presentation of the data and description of the approach

NASA Astrophysics Data System (ADS)

Chené, A.-N.; Borissova, J.; Clarke, J. R. A.; Bonatto, C.; Majaess, D. J.; Moni Bidin, C.; Sale, S. E.; Mauro, F.; Kurtev, R.; Baume, G.; Feinstein, C.; Ivanov, V. D.; Geisler, D.; Catelan, M.; Minniti, D.; Lucas, P.; de Grijs, R.; Kumar, M. S. N.

2012-09-01

Context. The ESO Public Survey "VISTA Variables in the Vía Láctea" (VVV) provides deep multi-epoch infrared observations for unprecedented 562 sq. degrees of the Galactic bulge, and adjacent regions of the disk. Aims: The VVV observations will foster the construction of a sample of Galactic star clusters with reliable and homogeneously derived physical parameters (e.g., age, distance, and mass, etc.). In this first paper in a series, the methodology employed to establish cluster parameters for the envisioned database are elaborated upon by analysing four known young open clusters: Danks 1, Danks 2, RCW 79, and DBS 132. The analysis offers a first glimpse of the information that can be gleaned from the VVV observations for clusters in the final database. Methods: Wide-field, deep JHKs VVV observations, combined with new infrared spectroscopy, are employed to constrain fundamental parameters for a subset of clusters. Results: Results are inferred from VVV near-infrared photometry and numerous low resolution spectra (typically more than 10 per cluster). The high quality of the spectra and the deep wide-field VVV photometry enables us to precisely and independently determine the characteristics of the clusters studied, which we compare to previous determinations. An anomalous reddening law in the direction of the Danks clusters is found, specifically E(J - H)/E(H - Ks) = 2.20 ± 0.06, which exceeds published values for the inner Galaxy. The G305 star forming complex, which includes the Danks clusters, lies beyond the Sagittarius-Carina spiral arm and occupies the Centaurus arm. Finally, the first deep infrared colour-magnitude diagram of RCW 79 is presented, which reveals a sizeable pre-main sequence population. A list of candidate variable stars in G305 region is reported. Conclusions: This study demonstrates the strength of the dataset and methodology employed, and constitutes the first step of a broader study which shall include reliable parameters for a sizeable number of poorly characterised and/or newly discovered clusters. Based on observations made with NTT telescope at the La Silla Observatory, ESO, under programme ID 087.D-0490A, and with the Clay telescope at the Las Campanas Observatory under programme CN2011A-086. Also based on data from the VVV survey observed under program ID 172.B-2002.Tables 1, 5 and 6 are available in electronic form at http://www.aanda.org

CCD photometry of NGC 6101 - Another globular cluster with blue straggler stars

NASA Technical Reports Server (NTRS)

Sarajedini, Ata; Da Costa, G. S.

1991-01-01

Results are presented on CCD photometric observations of a large sample of stars in the southern globular cluster NGC 6101, and the procedures used to derive the color-magnitude (C-M) diagram of the cluster are described. No indication was found of any difference in age, at the less than 2 Gyr level, between NGC 6101 cluster and other clusters of similar abundance, such as M92. The C-M diagram revealed a significant blue straggler population. It was found that, in NGC 6101, these stars are more centrally concentrated than the cluster subgiants of similar magnitude, indicating that the blue stragglers have larger masses. Results on the magnitude and luminosity function of the sample are consistent with the bianry mass transfer or merger hypotheses for the origin of blue straggler stars.

The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems

NASA Astrophysics Data System (ADS)

Silvestri, Nicole M.; Hawley, Suzanne L.; Oswalt, Terry D.

2005-05-01

We investigate the relationship between age and chromospheric activity for 139 M dwarf stars in wide binary systems with white dwarf companions. The age of each system is determined from the cooling age of its white dwarf component. The current limit for activity-age relations found for M dwarfs in open clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for the exploration of this relationship at ages older than 4 Gyr. The general trend of stars remaining active for a longer time at a later spectral type is confirmed. However, our larger sample and greater age range reveal additional complexity in assigning age based on activity alone. We find that M dwarfs in wide binaries older than 4 Gyr depart from the loglinear relation for clusters and are found to have activity at magnitudes, colors, and masses that are brighter, bluer, and more massive than predicted by the cluster relation. In addition to our activity-age results, we present the measured radial velocities and complete space motions for 161 white dwarf stars in wide binaries. Based on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium; the Cerro Tololo Inter-American Observatory 4.0 m telescope, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation (NSF) as part of the National Optical Astronomy Observatory (NOAO), which also operates Kitt Peak National Observatory in Tucson, Arizona; and the SARA Observatory 0.9 m telescope at Kitt Peak, which is owned and operated by the Southeastern Association for Research in Astronomy (http://www.saraobservatory.org).

A 30 kpc Chain of "Beads on a String" Star Formation between Two Merging Early Type Galaxies in the Core of a Strong-lensing Galaxy Cluster

NASA Astrophysics Data System (ADS)

Tremblay, Grant R.; Gladders, Michael D.; Baum, Stefi A.; O'Dea, Christopher P.; Bayliss, Matthew B.; Cooke, Kevin C.; Dahle, Håkon; Davis, Timothy A.; Florian, Michael; Rigby, Jane R.; Sharon, Keren; Soto, Emmaris; Wuyts, Eva

2014-08-01

New Hubble Space Telescope ultraviolet and optical imaging of the strong-lensing galaxy cluster SDSS J1531+3414 (z = 0.335) reveals two centrally dominant elliptical galaxies participating in an ongoing major merger. The interaction is at least somewhat rich in cool gas, as the merger is associated with a complex network of 19 massive superclusters of young stars (or small tidal dwarf galaxies) separated by ~1 kpc in projection from one another, combining to an estimated total star formation rate of ~5 M ⊙ yr-1. The resolved young stellar superclusters are threaded by narrow Hα, [O II], and blue excess filaments arranged in a network spanning ~27 kpc across the two merging galaxies. This morphology is strongly reminiscent of the well-known "beads on a string" mode of star formation observed on kiloparsec scales in the arms of spiral galaxies, resonance rings, and in tidal tails between interacting galaxies. Nevertheless, the arrangement of this star formation relative to the nuclei of the two galaxies is difficult to interpret in a dynamical sense, as no known "beads on a string" systems associated with kiloparsec-scale tidal interactions exhibit such lopsided morphology relative to the merger participants. In this Letter, we present the images and follow-up spectroscopy and discuss possible physical interpretations for the unique arrangement of the young stellar clusters. While we suggest that this morphology is likely to be dynamically short-lived, a more quantitative understanding awaits necessary multiwavelength follow-up, including optical integral field spectroscopy, ALMA submillimeter interferometry, and Chandra X-ray imaging.

The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud: Probing the LMC Disk

NASA Astrophysics Data System (ADS)

Alves, D. R.; Alcock, C.; Allsman, R. A.; Axelrod, T. S.; Basu, A.; Becker, A. C.; Bennett, D. P.; Cook, K. H.; Drake, A. J.; Freeman, K. C.; Geha, M.; Griest, K.; King, L. J.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Peterson, B. A.; Popowski, P.; Pratt, M. R.; Quinn, P. J.; Rodgers, A. W.; Stubbs, C. W.; Sutherland, W.; Tomaney, A.; Vandehei, T.; Welch, D. L.; MACHO Collaboration

1998-12-01

We present a 9 million star color-magnitude diagram (9M CMD) of the Large Magellanic Cloud (LMC) bar. The 9M CMD reveals a complex superposition of different age and metallicity stellar populations. Young LMC stellar populations are prominent in the 9M CMD. Of these, the red and blue supergiants are potentially useful probes of the late stages of evolution in intermediate mass stars. Old LMC stellar populations are also evident in the 9M CMD. These are used to reconstruct the evolution of the LMC during cosmologically interesting epochs. We first build a plausible model for the old LMC populations consistent with features observed in the 9M CMD. We choose the 1.5 Gyr old cluster NGC 411 and the ancient globular cluster M3, with metal abundances of [Fe/H] = -0.7 and -1.5 dex respectively, as good representations of the giant branch and horizontal branch (HB) stars. The evolved asymptotic giant branch appears bimodal, which supports a model of two discrete older populations in the LMC field. We conclude the old populations in the LMC bar are likely a mix similar to NGC 411 and M3. Next, we infer the old and low metallicity LMC field population has a red HB morphology, which implies this population formed ~ 2 Gyr after the truly ancient LMC clusters formed. We find the surface density profile of this old LMC field population (traced by RRab variable stars) is exponential, favoring a disk-like rather than spheroidal distribution. We conclude the LMC disk formed ~ 10 Gyr ago, at the same time the Milky Way disk formed.

A 30 kpc CHAIN OF ''BEADS ON A STRING'' STAR FORMATION BETWEEN TWO MERGING EARLY TYPE GALAXIES IN THE CORE OF A STRONG-LENSING GALAXY CLUSTER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tremblay, Grant R.; Davis, Timothy A.; Gladders, Michael D.

2014-08-01

New Hubble Space Telescope ultraviolet and optical imaging of the strong-lensing galaxy cluster SDSS J1531+3414 (z = 0.335) reveals two centrally dominant elliptical galaxies participating in an ongoing major merger. The interaction is at least somewhat rich in cool gas, as the merger is associated with a complex network of 19 massive superclusters of young stars (or small tidal dwarf galaxies) separated by ∼1 kpc in projection from one another, combining to an estimated total star formation rate of ∼5 M {sub ☉} yr{sup –1}. The resolved young stellar superclusters are threaded by narrow Hα, [O II], and blue excess filaments arrangedmore » in a network spanning ∼27 kpc across the two merging galaxies. This morphology is strongly reminiscent of the well-known ''beads on a string'' mode of star formation observed on kiloparsec scales in the arms of spiral galaxies, resonance rings, and in tidal tails between interacting galaxies. Nevertheless, the arrangement of this star formation relative to the nuclei of the two galaxies is difficult to interpret in a dynamical sense, as no known ''beads on a string'' systems associated with kiloparsec-scale tidal interactions exhibit such lopsided morphology relative to the merger participants. In this Letter, we present the images and follow-up spectroscopy and discuss possible physical interpretations for the unique arrangement of the young stellar clusters. While we suggest that this morphology is likely to be dynamically short-lived, a more quantitative understanding awaits necessary multiwavelength follow-up, including optical integral field spectroscopy, ALMA submillimeter interferometry, and Chandra X-ray imaging.« less

The formation and evolution of M33 as revealed by its star clusters

NASA Astrophysics Data System (ADS)

San Roman, Izaskun

2012-03-01

Numerical simulations based on the Lambda-Cold Dark Matter (Λ-CDM) model predict a scenario consistent with observational evidence in terms of the build-up of Milky Way-like halos. Under this scenario, large disk galaxies derive from the merger and accretion of many smaller subsystems. However, it is less clear how low-mass spiral galaxies fit into this picture. The best way to answer this question is to study the nearest example of a dwarf spiral galaxy, M33. We will use star clusters to understand the structure, kinematics and stellar populations of this galaxy. Star clusters provide a unique and powerful tool for studying the star formation histories of galaxies. In particular, the ages and metallicities of star clusters bear the imprint of the galaxy formation process. We have made use of the star clusters to uncover the formation and evolution of M33. In this dissertation, we have carried out a comprehensive study of the M33 star cluster system, including deep photometry as well as high signal-to-noise spectroscopy. In order to mitigate the significant incompleteness presents in previous catalogs, we have conducted ground-based and space-based photometric surveys of M33 star clusters. Using archival images, we have analyzed 12 fields using the Advanced Camera for Surveys Wide Field Channel onboard the Hubble Space Telescope (ACS/HST) along the major axis of the galaxy. We present integrated photometry and color-magnitude diagrams for 161 star clusters in M33, of which 115 were previously uncataloged. This survey extends the depth of the existing M33 cluster catalogs by ˜ 1 mag. We have expanded our search through a photometric survey in a 1° x 1° area centered on M33 using the MegaCam camera on the 3.6m Canada-France-Hawaii Telescope (CFHT). In this work we discuss the photometric properties of the sample, including color-color diagrams of 599 new candidate stellar clusters, and 204 confirmed clusters. Comparisons with models of simple stellar populations suggest a large range of ages some as old as ˜ 10 Gyr. In addition, we find in the color-color diagrams a significant population of very young clusters (< 10 Myr) possessing nebular emission. Analysis of the radial density distribution suggests that the cluster system of M33 has suffered from significant depletion, possibly due to interactions with M31. To further understand the properties of M33 star clusters, we have carried out a morphological study 161 star clusters in M33 using ACS/HST images. We have obtained, for the first time, ellipticities, position angles, and surface brightness profiles of a statistically significant number of clusters. Ellipticities show that, on average, M33 clusters are more flattened than those of the Milky Way and M31, and more similar to clusters in the Small Magellanic Cloud. The ellipticities do not show any correlation with age or mass, suggesting that rotation is not the main cause of elongation in the M33 clusters. The position angles of the clusters show a bimodality with a strong peak perpendicular to the position angle of the galaxy. These results support the notion that tidal forces are the reason for the cluster flattening. We have fit analytical models to the surface brightness profiles, and derived structural parameters. The overall analysis shows several differences between the structural properties of the M33 cluster system and cluster systems in nearby galaxies. Finally, we have performed a spectroscopic study of star clusters in the above mentioned catalog. We present high-precision velocity measures of 45 star clusters, based on observations from the 10.4m Gran Telescopio Canarias (GTC) using OSIRIS and 4.2m William Herschel Telescope (WHT) using WYFFOS. All the clusters have been previously confirmed using HST imaging, and ages and integrated photometry are known. The velocity of the clusters with respect to local disk motion increases with age for young and intermediate clusters. The mean dispersion velocity for the intermediate age clusters in our sample is significantly larger than in previous studies. Analysis of these velocities along the major axis of the galaxy show no net rotation of the intermediate age subsample. The small number of old clusters in our sample does not allow for any conclusive evidence in that age division.

Strong evidences for a nonextensive behavior of the rotation period in open clusters

NASA Astrophysics Data System (ADS)

de Freitas, D. B.; Nepomuceno, M. M. F.; Soares, B. B.; Silva, J. R. P.

2014-11-01

Time-dependent nonextensivity in a stellar astrophysical scenario combines nonextensive entropic indices qK derived from the modified Kawaler's parametrization, and q, obtained from rotational velocity distribution. These q's are related through a heuristic single relation given by q≈ q0(1-Δ t/qK) , where t is the cluster age. In a nonextensive scenario, these indices are quantities that measure the degree of nonextensivity present in the system. Recent studies reveal that the index q is correlated to the formation rate of high-energy tails present in the distribution of rotation velocity. On the other hand, the index qK is determined by the stellar rotation-age relationship. This depends on the magnetic-field configuration through the expression qK=1+4aN/3 , where a and N denote the saturation level of the star magnetic field and its topology, respectively. In the present study, we show that the connection q-qK is also consistent with 548 rotation period data for single main-sequence stars in 11 open clusters aged less than 1 Gyr. The value of qK ˜ 2.5 from our unsaturated model shows that the mean magnetic-field topology of these stars is slightly more complex than a purely radial field. Our results also suggest that stellar rotational braking behavior affects the degree of anti-correlation between q and cluster age t. Finally, we suggest that stellar magnetic braking can be scaled by the entropic index q.

FEEDBACK FROM MASSIVE STARS AND GAS EXPULSION FROM PROTO-GLOBULAR CLUSTERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Calura, F.; Romano, D.; D’Ercole, A.

2015-11-20

Globular clusters (GCs) are considerably more complex structures than previously thought, harboring at least two stellar generations that present clearly distinct chemical abundances. Scenarios explaining the abundance patterns in GCs mostly assume that originally the clusters had to be much more massive than today, and that the second generation of stars originates from the gas shed by stars of the first generation (FG). The lack of metallicity spread in most GCs further requires that the supernova-enriched gas ejected by the FG is completely lost within ∼30 Myr, a hypothesis never tested by means of three-dimensional hydrodynamic simulations. In this paper,more » we use 3D hydrodynamic simulations including stellar feedback from winds and supernovae, radiative cooling and self-gravity to study whether a realistic distribution of OB associations in a massive proto-GC of initial mass M{sub tot} ∼ 10{sup 7} M{sub ⊙} is sufficient to expel its entire gas content. Our numerical experiment shows that the coherence of different associations plays a fundamental role: as the bubbles interact, distort, and merge, they carve narrow tunnels that reach deeper and deeper toward the innermost cluster regions, and through which the gas is able to escape. Our results indicate that after 3 Myr, the feedback from stellar winds is responsible for the removal of ∼40% of the pristine gas, and that after 14 Myr, 99% of the initial gas mass has been removed.« less

Highlights of Astronomy, Vol. 16

NASA Astrophysics Data System (ADS)

Montmerle, Thierry

2015-04-01

Part I. Invited Discourses: 1. The Herschel view of star formation; 2. Past, present and future of Chinese astronomy; 3. The zoo of galaxies; 4. Supernovae, the accelerating cosmos, and dark energy; Part II. Joint Discussion: 5. Very massive stars in the local universe; 6. 3-D views of the cycling Sun in stellar context; 7. Ultraviolet emission in early-type galaxies; 8. From meteors and meteorites to their parent bodies: current status and future developments; 9. The connection between radio properties and high-energy emission in AGNs; 10. Space-time reference systems for future research; Part III. Special Sessions: 11. Origin and complexity of massive star clusters; 12. Cosmic evolution of groups and clusters of galaxies; 13. Galaxy evolution through secular processes; 14. New era for studying interstellar and intergalactic magnetic fields; 15. The IR view of massive stars: the main sequence and beyond; 16. Science with large solar telescopes; 17. The impact hazard: current activities and future plans; 18. Calibration of star-formation rate measurements across the electromagnetic spectrum; 19. Future large scale facilities; 20. Dynamics of the star-planet relations strategic plan and the Global Office of Astronomy for Development; 21. Strategic plan and the Global Office of Astronomy for Development; 22. Modern views of the interstellar medium; 23. High-precision tests of stellar physics from high-precision photometry; 24. Communicating astronomy with the public for scientists; 25. Data intensive astronomy; 26. Unexplained spectral phenomena in the interstellar medium; 27. Light pollution: protecting astronomical sites and increasing global awareness through education.

Simulating star clusters with the AMUSE software framework. I. Dependence of cluster lifetimes on model assumptions and cluster dissolution modes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Whitehead, Alfred J.; McMillan, Stephen L. W.; Vesperini, Enrico

2013-12-01

We perform a series of simulations of evolving star clusters using the Astrophysical Multipurpose Software Environment (AMUSE), a new community-based multi-physics simulation package, and compare our results to existing work. These simulations model a star cluster beginning with a King model distribution and a selection of power-law initial mass functions and contain a tidal cutoff. They are evolved using collisional stellar dynamics and include mass loss due to stellar evolution. After studying and understanding that the differences between AMUSE results and results from previous studies are understood, we explored the variation in cluster lifetimes due to the random realization noisemore » introduced by transforming a King model to specific initial conditions. This random realization noise can affect the lifetime of a simulated star cluster by up to 30%. Two modes of star cluster dissolution were identified: a mass evolution curve that contains a runaway cluster dissolution with a sudden loss of mass, and a dissolution mode that does not contain this feature. We refer to these dissolution modes as 'dynamical' and 'relaxation' dominated, respectively. For Salpeter-like initial mass functions, we determined the boundary between these two modes in terms of the dynamical and relaxation timescales.« less

Hubble Space Telescope imaging of the central star forming region in NGC 1140 (exp 1)

NASA Technical Reports Server (NTRS)

Hunter, Deidre A.; O'Connell, Robert W.; Gallagher, John S. Iii

1994-01-01

We present broadband images taken with the Hubble Space Telescope's Planetary Camera of the central supergiant H II region in the amorphous galaxy NGC 1140. These images allow observations to a resolution of about 13 pc at the galaxy, and they reveal that its central 1/2 kpc contains 6-7 blue, luminous, compact super star clusters, many of which would be comparable in luminosity to globular clusters at the same age. A blue arc-shaped structure near the center may be a grouping of less luminous, R136/NGC 2070-sized clusters or a sheet of OB stars. Additional somewhat less luminous and redder clusters are also found farther out from the center. If these clusters are older, they too could have had luminosities comparable to those of the central six clusters at a comparable age. Thus, we find that NGC 1140 is remarkable in the number of extreme clusters that it has formed recently in a relatively small area of the galaxy. Since NGC 1140 exhibits global characteristics that are consistent with a recent merger, these clusters are likely to be a product of that event. This galaxy adds to the number of cases where rapid star formation has evidently produced super star clusters.

Globular Cluster Systems in Interacting Galaxies

NASA Astrophysics Data System (ADS)

Zepf, S.; Murdin, P.

2000-11-01

GLOBULAR CLUSTERS are dynamically bound and dense collections of large numbers of coeval stars. Typical globular clusters have roughly one million stars within a radius of a few parsecs. They are also usually close to spherical, hence the name globular. By virtue of their rich, isolated population of stars they provide an important laboratory for studies of STELLAR EVOLUTION. Moreover, because of...

The B and Be Star Population of NGC 3766

NASA Astrophysics Data System (ADS)

McSwain, M. V.

2006-12-01

I present results from a spectroscopic monitoring program of B and Be stars in the open cluster NGC 3766. From a 4-year time baseline of photometric and spectroscopic data, I have identified 9 Be stars in the cluster that have undergone disk outbursts or whose disks have disappeared. Using Kurucz ATLAS9 model spectra to measure temperatures, gravities, rotational velocities, and abundances among the cluster members, I present preliminary results of the stellar and cluster properties that may affect the long term variability of Be stars. M.V.M. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-0401460.

A Multi-wavelength Study of Star Formation Activity in the S235 Complex

NASA Astrophysics Data System (ADS)

Dewangan, L. K.; Ojha, D. K.; Luna, A.; Anandarao, B. G.; Ninan, J. P.; Mallick, K. K.; Mayya, Y. D.

2016-03-01

We have carried out an extensive multi-wavelength study to investigate the star formation process in the S235 complex. The S235 complex has a spherelike shell appearance at wavelengths longer than 2 μm and harbors an O9.5V type star approximately at its center. A near-infrared extinction map of the complex traces eight subregions (having AV > 8 mag), and five of them appear to be distributed in an almost regularly spaced manner along the spherelike shell surrounding the ionized emission. This picture is also supported by the integrated 12CO and 13CO intensity maps and by Bolocam 1.1 mm continuum emission. The position-velocity analysis of CO reveals an almost semi-ringlike structure, suggesting an expanding H II region. We find that the Bolocam clump masses increase as we move away from the location of the ionizing star. This correlation is seen only for those clumps that are distributed near the edges of the shell. Photometric analysis reveals 435 young stellar objects (YSOs), 59% of which are found in clusters. Six subregions (including five located near the edges of the shell) are very well correlated with the dust clumps, CO gas, and YSOs. The average values of Mach numbers derived using NH3 data for three (East 1, East 2, and Central E) out of these six subregions are 2.9, 2.3, and 2.9, indicating these subregions are supersonic. The molecular outflows are detected in these three subregions, further confirming the ongoing star formation activity. Together, all these results are interpreted as observational evidence of positive feedback of a massive star.

The nature, origin and evolution of embedded star clusters

NASA Technical Reports Server (NTRS)

Lada, Charles J.; Lada, Elizabeth A.

1991-01-01

The recent development of imaging infrared array cameras has enabled the first systematic studies of embedded protoclusters in the galaxy. Initial investigations suggest that rich embedded clusters are quite numerous and that a significant fraction of all stars formed in the galaxy may begin their lives in such stellar systems. These clusters contain extremely young stellar objects and are important laboratories for star formation research. However, observational and theoretical considerations suggest that most embedded clusters do not survive emergence from molecular clouds as bound clusters. Understanding the origin, nature, and evolution of embedded clusters requires understanding the intimate physical relation between embedded clusters and the dense molecular cloud cores from which they form.

X-ray studies of coeval star samples. II - The Pleiades cluster as observed with the Einstein Observatory

NASA Technical Reports Server (NTRS)

Micela, G.; Sciortino, S.; Vaiana, G. S.; Harnden, F. R., Jr.; Rosner, R.

1990-01-01

Coronal X-ray emission of the Pleiades stars is investigated, and maximum likelihood, integral X-ray luminosity functions are computed for Pleiades members in selected color-index ranges. A detailed search is conducted for long-term variability in the X-ray emission of those stars observed more than once. An overall comparison of the survey results with those of previous surveys confirms the ubiquity of X-ray emission in the Pleiades cluster stars and its higher rate of emission with respect to older stars. It is found that the X-ray emission from dA and early dF stars cannot be proven to be dissimilar to that of Hyades and field stars of the same spectral type. The Pleiades cluster members show a real rise of the X-ray luminosity from dA stars to early dF stars. X-ray emission for the young, solarlike Pleiades stars is about two orders of magnitude more intense than for the nearby solarlike stars.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Benjamin; Tan, Jonathan C.; Christie, Duncan

We study giant molecular cloud (GMC) collisions and their ability to trigger star cluster formation. We further develop our three-dimensional magnetized, turbulent, colliding GMC simulations by implementing star formation subgrid models. Two such models are explored: (1) “Density-Regulated,” i.e., fixed efficiency per free-fall time above a set density threshold and (2) “Magnetically Regulated,” i.e., fixed efficiency per free-fall time in regions that are magnetically supercritical. Variations of parameters associated with these models are also explored. In the non-colliding simulations, the overall level of star formation is sensitive to model parameter choices that relate to effective density thresholds. In the GMCmore » collision simulations, the final star formation rates and efficiencies are relatively independent of these parameters. Between the non-colliding and colliding cases, we compare the morphologies of the resulting star clusters, properties of star-forming gas, time evolution of the star formation rate (SFR), spatial clustering of the stars, and resulting kinematics of the stars in comparison to the natal gas. We find that typical collisions, by creating larger amounts of dense gas, trigger earlier and enhanced star formation, resulting in 10 times higher SFRs and efficiencies. The star clusters formed from GMC collisions show greater spatial substructure and more disturbed kinematics.« less

Ashes from the Elder Brethren

NASA Astrophysics Data System (ADS)

2001-03-01

UVES Observes Stellar Abundance Anomalies in Globular Clusters Summary Globular clusters are very massive assemblies of stars. More than 100 are known in the Milky Way galaxy and most of them harbour several million stars. They are very dense - at their centers, the typical distance between individual stars is comparable to the size of the Solar System, or 100 to 1000 times closer than the corresponding distances between stars in the solar neighborhood. Globular clusters are among the oldest objects known , with estimated ages of 11 to 15 billion years [1]. All stars in a globular cluster were formed at nearly the same moment, and from the same parent cloud of gas and dust. The original chemical composition of all stars is therefore the same. But now, an international group of astronomers [2], working with the UVES Spectrograph at the ESO Very Large Telescope (VLT) , have obtained some unexpected results during a detailed analysis of dwarf stars in some globular clusters . Such stars have about the same mass as our Sun and like it, they evolve very slowly. Thus they still ought to have about the same abundances of most chemical elements. Nevertheless, the astronomers found large abundance variations from star to star, especially for the common elements Oxygen, Sodium, Magnesium and Aluminium . This phenomenon has never been seen in such stars before . It appears that those stars must somehow have received "burnt" stellar material from more massive stars that died many billion years ago. In their final phase - as "planetary nebulae" - they eject stellar material that has been enriched with certain chemical elements which were produced by the nuclear processes in their interiors during their active life. Such an acquisition of material from other stars has been proposed but has never before been seen in globular clusters . This new discovery obviously sets stars in globular cluster apart from those in less dense environments, like the solar neighbourhood. PR Photo 06a/01 : The globular cluster NGC 6752 . PR Photo 06b/01 : Spectra of dwarf stars in NGC 6752 Globular clusters ESO PR Photo 06a/01 ESO PR Photo 06a/01 [Preview - JPEG: 400 x 467 pix - 136k] [Normal - JPEG: 800 x 934 pix - 424k] [Hires - JPEG: 3000 x 3503 pix - 3.0M] Caption : PR Photo 06a/01 is an image of the globular cluster NGC 6752 ; stars for which spectra were obtained in the present programme are marked by small circles (only visible in the high-resolution version of this photo). NGC 6752 is a typical globular cluster, containing many hundreds of thousands of stars, of which some tens of thousands are visible in this photo. It is located at a distance of approximately 13,000 light-years and is one of the oldest known objects in the Universe. The bright, round object to the lower right of the cluster is the overexposed image of the 7th magnitude star HD 177999 . Technical information about this photo is available below. Globular clusters are very massive and extremely dense agglomerates of stars: typical distances between stars at their centres are comparable to the size of the Solar System. They were formed very early in the Universe and have very low metal content, down to about 1/200 of the Solar abundance. They are among the oldest objects for which relatively accurate ages can be determined for individual stars by means of their observed colours (for information about the "radioactive" method, see ESO Press Release 02/01. The study of globular clusters therefore plays a basic role in our understanding of the evolution of the Universe and of our own Galaxy. The globular clusters are quite distant and most are located in the Milky Way halo, far above or below the main plane of this galaxy. The nearest globular cluster is Messier 4 (NGC 6121) , about 7,000 light-years away. The globular cluster NGC 6752 , shown in PR Photo 06a/01 , is a typical representative of this class of celestial objects. Its distance is estimated at 13,000 light-years Spectral analysis supports distance and age determinations The vast majority of stars in globular clusters are "dwarfs" like our own Sun. They burn Hydrogen into Helium in their central regions, and like the Sun they spend billions of years in this particular evolutionary phase. When their light is dispersed with a spectrograph , thousands of narrow spectral lines are revealed that are caused by chemical elements like Iron, Sodium, Oxygen, Magnesium and Lithium, present in the outer atmospheres of these stars. "Spectral analysis" is one of the basic tools of astronomy, during which the accurate chemical composition of a star is determined by means of a detailed study of the lines seen in its spectrum. In this context, very detailed observations of dwarf stars in globular clusters are of great importance. They allow to compare directly the properties of stars in distant clusters with those of much closer - and hence more easily observable - similar stars in the solar neighbourhood. Such a comparison contributes to reducing current uncertainties in the determination of distances and ages of the globular clusters. Studies like these will ultimately yield a better determination of the age of our own Galaxy and the Universe, as well as the universal distance scale. Variations in chemical abundances ESO PR Photo 06b/01 ESO PR Photo 06b/01 [Preview - JPEG: 400 x 457 pix - 96k] [Normal - JPEG: 800 x 914 pix - 264k] Caption : PR Photo 06b/01 displays a series of spectra of dwarf stars in the globular cluster NGC 6752 , obtained with the UVES high-dispersion spectrograph at the 8.2-m VLT KUEYEN telescope. Sodium (Na) and Oxygen (O) lines are marked, and the spectra are arranged according to the strength of the Sodium lines, with the strongest at the top. It is obvious that stars with stronger Sodium lines (and therefore with a higher Sodium abundance) have weaker Oxygen lines (and are therefore poorer in Oxygen). Even with UVES, the most powerful high-resolution astronomical spectrograph in the world, exposures of up to 4.5 hours were required to record good spectra of these faint objects (V-mag = 17.2). Detailed observations of dwarf stars in globular clusters are rather difficult because they are quite faint objects; The brightest are at least 10,000 times fainter than the dimmest stars observable with the unaided eye. Nevertheless, the closest globular clusters are seen in the southern sky and with the high efficiency of the UVES spectrograph mounted at the KUEYEN 8.2-m telescope at Paranal (Chile), it has now become possible for the first time to obtain excellent spectra for a significant number of dwarf stars in globular clusters, cf. PR Photo 06b/01 . The UVES spectra cover a wide wavelength interval (350 - 900 nm) and display a very large number of spectral lines that originate from many different elements. The first results obtained from the excellent data for this observational programme immediately brought a great surprise to Raffaele Gratton and his co-investigators. The Italian astronomer reports that "our detailed analysis revealed that, while heavy elements like Iron display an impressively similar abundance in all of the observed dwarf stars, other elements, such as Oxygen, Sodium, Magnesium and Aluminium show large abundance variations from star to star". Moreover, "these variations are apparently not completely random, as there is evidence that certain elements change in a similar pattern from star to star". Evidence for accretion? This result is indeed unexpected, since the dwarf stars in globular clusters originated from the same interstellar material. Which effect may therefore produce the observed variations ? And why are such variations not observed in dwarf stars in the solar neighborhood ? The scientists think they have the answer. It has been known since the early 1970's that large star-to-star variations in the abundances of light elements like Carbon, Nitrogen, Oxygen, Sodium, Magnesium and Aluminium may occur in giant stars . Contrary to dwarf stars that still burn Hydrogen at their centres into Helium, giant stars have exhausted their Hydrogen supplies and have become much more luminous. Most investigators attributed the observed variations to the fact that in giant stars a certain amount of "mixing" occurs between the upper atmospheric layers (that emit the light we see) and the deeper (warmer) layers, in which some nuclear burning is going on, transforming Carbon into Nitrogen, etc. However, it is a well established fact of stellar evolution theory that such mixing and, consequently, the presence of abundance anomalies in the upper atmosphere can only occur in bright, evolved giant stars. It does not happen in dwarf stars, because the central temperature of those objects is not high enough to burn Oxygen or Magnesium, and to produce Sodium and Aluminium. It seems therefore not possible that the abundance anomalies are produced in those stars where they are observed. They should have been produced elsewhere, and transported in some way to the surface layers of the stars where we observe them [3]. ESO astronomer Luca Pasquini from the team explains that "we therefore believe that these observations provide evidence that a certain fraction of stars in some globular cluster has received "burnt" material from more massive stars." He adds that "the stars of that elder generation ended their active lifetimes a long time ago by ejecting their material into surrounding space during a "planetary nebula" phase and have now become very dim "white dwarf stars" [4]. The acquisition of material from other stars is a phenomenon that is apparently unique to globular clusters (except that it has also been observed in a few close binary stars). It clearly distinguishes stars in globular cluster from those found in less dense environments, like the solar neighborhood. More information The research paper ("The O-Na and Mg-Al Anticorrelations in Turn-Off and early Subgiants in Globular Clusters") on which this Press Release is based is now in press in the European journal Astronomy & Astrophysics. It is also available on the web as astro-ph/0012457. Notes [1]: 1 billion = 1,000 million. [2]: The team members in the ESO Large Program 165-L0263 devoted to the analysis of globular cluster dwarf stars, described in this Press Release, are: Raffaele Gratton (PI), Eugenio Carretta , Riccardo Claudi , Silvano Desidera , Sara Lucatello (Osservatorio Astronomico di Padova, Italy), Gisella Clementini , Angela Bragaglia (Osservatorio Astronomico di Bologna, Italy), Paolo Molaro , Piercarlo Bonifacio , Miriam Centurion (Osservatorio Astronomico di Trieste, Italy), Francesca D' Antona (Osservatorio Astronomico di Roma, Italy), Vittorio Castellani (Universita' di Pisa, Italy), Alessandro Chieffi (CNR-IAS, Italy), Oscar Straniero (Osservatorio di Teramo, Italy), Luca Pasquini , Patrick Francois (ESO), Francois Spite , Monique Spite (Observatoire de Meudon, France), Chris Sneden (University of Texas at Austin, USA), Frank Grundahl (University of Aarhus, Denmark). [3]: While it is apparent that some mass is transferred from the Planetary Nebulae to the stars, the details of this process are not clear. It may have happened before the stars here observed were formed, or later. In the latter case, the accretion may have occurred only during a particular evolutionary phase, some 100 million years after the cluster formed, i.e. about 11 to 15 billion years ago, and in very dense environments. Moreover, the accretion rate will depend on the relative velocities: only stars that move slowly with respect to the interstellar medium has a good chance of accreting matter. This may also be (part of) an explanation of the observed, large differences from star to star. [4]: A photo of a large planetary nebula is available as PR Photo 38a/98 and information about VLT observations of white dwarf stars in globular clusters are described in PR 20/99. Technical information about the photo PR Photo 06a/01 The image has been obtained through a v-band filter with the DFOSC multi-mode instrument the Danish 1.5-m Telescope at the ESO La Silla Observatory (Chile). The diameter of the field-of-view is 9 arcmin; the exposure time was 10 min, and the seeing was 1.3 arcsec. A few CCD columns suffer from imaging defects.

STAR FORMATION ACTIVITY IN A YOUNG GALAXY CLUSTER AT Z = 0.866

DOE Office of Scientific and Technical Information (OSTI.GOV)

Laganá, T. F.; Martins, L. P.; Ulmer, M. P.

2016-07-10

The galaxy cluster RX J1257+4738 at z = 0.866 is one of the highest redshift clusters with a richness of multi-wavelength data, and is thus a good target to study the star formation–density relation at early epochs. Using a sample of spectroscopically confirmed cluster members, we derive the star-formation rates (SFRs) of our galaxies using two methods: (1) the relation between SFR and total infrared luminosity extrapolated from the observed Spitzer Multiband Imaging Photometer for Spitzer 24 μ m imaging data; and (2) spectral energy distribution fitting using the MAGPHYS code, including eight different bands. We show that, for thismore » cluster, the SFR–density relation is very weak and seems to be dominated by the two central galaxies and the SFR presents a mild dependence on stellar mass, with more massive galaxies having higher SFR. However, the specific SFR (SSFR) decreases with stellar mass, meaning that more massive galaxies are forming fewer stars per unit of mass, and thus suggesting that the increase in star-forming members is driven by cluster assembly and infall. If the environment is somehow driving the star formation, one would expect a relation between the SSFR and the cluster centric distance, but that is not the case. A possible scenario to explain this lack of correlation is the contamination by infalling galaxies in the inner part of the cluster, which may be on their initial pass through the cluster center. As these galaxies have higher SFRs for their stellar mass, they enhance the mean SSFR in the center of the cluster.« less

Star Formation in NGC 6531-Evidence From the age Spread and Initial Mass Function

NASA Astrophysics Data System (ADS)

Forbes, Douglas

1996-09-01

The results of a photometric UBV study of the young open cluster NGC 6531 are presented. The cluster is found to have a mean reddening E(B-V)=0.28±0.04 (s.d.) and distance modulus (V0-Mv)=10.70±0.13 (s.e.), and 105±11 likely cluster members have been identified within the cluster coronal radius of 9 arcmin. A comparison of the high-luminosity end of the cluster color-magnitude diagram to the evolutionary models by Maeder & Meynet [A&AS, 76, 411(1988)] suggests a nuclear age of (8±2) Myr. The very clear gap in the distribution of stars with 0≤(B-V)0≤0.20, corresponding to the "burn-off" of 3He in stars contracting to the main sequence [Ulrich, ApJ, 168, 57 (1971)], implies a contraction age of (8±3) Myr. There would seem to be no evidence of a spread in the ages of cluster stars, as has been observed in several other young open clusters [Herbst & Miller, AJ, 87, 1478 (1982)]. The initial mass function (IMF) constructed from the cluster luminosity function and the mass-luminosity relation given by Scab (1986) shows good agreement with the field star IMF, and with the IMFS of a number of clusters of similar age and richness. The relative deficiency of low-mass stars seen by Herbst and Miller in NGC 3293 (a cluster of quite similar age and reddening) is not evident in NGC 6531.

NuSTAR Search for Hard X-ray Emission from the Star Formation Regions in Sh2-104

NASA Astrophysics Data System (ADS)

Gotthelf, Eric V.

2016-04-01

We present NuSTAR hard X-ray observations of Sh2-104, a compact Hii region containing several young massive stellar clusters (YMSCs). We have detected distinct hard X-ray sources coincident with localized VERITAS TeV emission recently resolved from the giant gamma-ray complex MGRO J2019+37 in the Cygnus region. Faint, diffuse X-ray emission coincident with the eastern YMSC in Sh2-104 is likely the result of colliding winds of component stars. Just outside the radio shell of Sh2-104 lies 3XMM J201744.7+365045 and nearby nebula NuSTAR J201744.3+364812, whose properties are most consistent with extragalactic objects. The combined XMM-Newton and NuSTAR spectrum of 3XMM J201744.7+365045 is well-fit to an absorbed power-law model with NH = (3.1+/-1.0)E22 1/cm^2 and photon index Gamma = 2.1+/-0.1. Based on possible long-term flux variation and lack of detected pulsations (<43% modulation), this object is likely a background AGN rather than a Galactic pulsar. The spectrum of the NuSTAR nebula shows evidence of an emission line at E = 5.6 keV suggesting an optically obscured galaxy cluster at z = 0.19+/-0.02 (d = 800 Mpc) and Lx = 1.2E44 erg/s. Follow-up Chandra observations of Sh2-104 will help identify the nature of the X-ray sources and their relation to MGRO J2019+37.

HUBBLE SEES A VAST 'CITY' OF STARS

NASA Technical Reports Server (NTRS)

2002-01-01

In these pictures, a 'city' of a million stars glitters like a New York City skyline. The images capture the globular cluster 47 Tucanae, located 15,000 light-years from Earth in the southern constellation Tucana. Using NASA's Hubble Space Telescope, astronomers went hunting in this large city for planetary companions: bloated gaseous planets that snuggle close to their parent stars, completing an orbit in a quick three to five days. To their surprise, they found none. This finding suggests that the cluster's environment is too hostile for breeding planets or that it lacks the necessary elements for making them. The picture at left, taken by a terrestrial telescope, shows most of the cluster, a tightly packed group of middle-aged stars held together by mutual gravitational attraction. The box near the center represents the Hubble telescope's view. The image at right shows the Hubble telescope's close-up look at a swarm of 35,000 stars near the cluster's central region. The stars are tightly packed together: They're much closer together than our Sun and its closest stars. The picture, taken by the Wide Field and Planetary Camera 2, depicts the stars' natural colors and tells scientists about their composition and age. For example, the red stars denote bright red giants nearing the end of their lives; the more common yellow stars are similar to our middle-aged Sun. Most of the stars in the cluster are believed to have formed about 10 billion years ago. The bright, blue stars -- thought to be remnants of stellar collisions and mergers -- provide a few rejuvenated, energetic stars in an otherwise old system. The Hubble picture was taken in July 1999. Credits for Hubble image: NASA and Ron Gilliland (Space Telescope Science Institute) Credits for ground-based image: David Malin, c Anglo-Australian Observatory

Shaping a high-mass star-forming cluster through stellar feedback. The case of the NGC 7538 IRS 1-3 complex

NASA Astrophysics Data System (ADS)

Frau, P.; Girart, J. M.; Zhang, Q.; Rao, R.

2014-07-01

Context. NGC 7538 IRS 1-3 is a high-mass star-forming cluster with several detected dust cores, infrared sources, (ultra)compact H II regions, molecular outflows, and masers. In such a complex environment, interactions and feedback among the embedded objects are expected to play a major role in the evolution of the region. Aims: We study the dust, kinematic, and polarimetric properties of the NGC 7538 IRS 1-3 region to investigate the role of the different forces in the formation and evolution of high-mass star-forming clusters. Methods: We performed SMA high angular resolution observations at 880 μm with the compact configuration. We developed the RATPACKS code to generate synthetic velocity cubes from models of choice to be compared to the observational data. To quantify the stability against gravitational collapse we developed the "mass balance" analysis that accounts for all the energetics on core scales. Results: We detect 14 dust cores from 3.5 M⊙ to 37 M⊙ arranged in two larger scale structures: a central bar and a filamentary spiral arm. The spiral arm presents large-scale velocity gradients in H13CO+ 4-3 and C17O 3-2, and magnetic field segments aligned well to the dust main axis. The velocity gradient is reproduced well by a spiral arm expanding at 9 km s-1 with respect to the central core MM1, which is known to power a large precessing outflow. The energy of the outflow is comparable to the spiral-arm kinetic energy, which dominates gravitational and magnetic energies. In addition, the dynamical ages of the outflow and spiral arm are comparable. On core scales, those embedded in the central bar seem to be unstable against gravitational collapse and prone to forming high-mass stars, while those in the spiral arm have lower masses that seem to be supported by non-thermal motions and magnetic fields. Conclusions: The NGC 7538 IRS 1-3 cluster seems to be dominated by protostellar feedback. The dusty spiral arm appears to be formed in a snowplow fashion owing to the outflow from the MM1 core. We speculate that the external pressure from the redshifted lobe of the outflow could trigger star formation in the spiral arm cores. This scenario would form a small cluster with a few central high-mass stars, surrounded by a number of low-mass stars formed through protostellar feedback. Based on observations carried out with the SMA telescope. The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and the Academia Sinica (http://sma1.sma.hawaii.edu/).Final reduced SMA data cube is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/567/A116

Yellow evolved stars in open clusters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sowell, J.R.

1987-05-01

This paper describes a program in which Galactic cluster post-AGB candidates were first identified and then analyzed for cluster membership via radial velocities, monitored for possible photometric variations, examined for evidence of mass loss, and classified as completely as possible in terms of their basic stellar parameters. The intrinsically brightest supergiants are found in the youngest clusters. With increasing cluster age, the absolute luminosities attained by the supergiants decline. It appears that the evolutionary tracks of luminosity class II stars are more similar to those of class I than of class III. Only two superluminous giant star candidates are foundmore » in open clusters. 154 references.« less

New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve

NASA Technical Reports Server (NTRS)

Brown, T. M.; Lanz, T.; Sweigart, A. V.; Cracraft, Misty; Hubeny, Ivan; Landsman, W. B.

2011-01-01

Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters w Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear sub luminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).

Dense CO in Mrk 71-A: Superwind Suppressed in a Young Super Star Cluster

NASA Astrophysics Data System (ADS)

Oey, M. S.; Herrera, C. N.; Silich, Sergiy; Reiter, Megan; James, Bethan L.; Jaskot, A. E.; Micheva, Genoveva

2017-11-01

We report the detection of CO(J=2-1) coincident with the super star cluster (SSC) Mrk 71-A in the nearby Green Pea analog galaxy, NGC 2366. Our observations with the Northern Extended Millimeter Array reveal a compact, ˜7 pc, molecular cloud whose mass ({10}5 {M}⊙ ) is similar to that of the SSC, consistent with a high star formation efficiency, on the order of 0.5. There are two spatially distinct components separated by 11 {km} {{{s}}}-1. If expanding, these could be due to momentum-driven stellar wind feedback. Alternatively, we may be seeing remnants of the infalling, colliding clouds responsible for triggering the SSC formation. The kinematics are also consistent with a virialized system. These extreme, high-density, star-forming conditions inhibit energy-driven feedback; the co-spatial existence of a massive, molecular cloud with the SSC supports this scenario, and we quantitatively confirm that any wind-driven feedback in Mrk 71-A is momentum-driven, rather than energy-driven. Since Mrk 71-A is a candidate Lyman continuum emitter, this implies that energy-driven superwinds may not be a necessary condition for the escape of ionizing radiation. In addition, the detection of nebular continuum emission yields an accurate astrometric position for the Mrk 71-A. We also detect four other massive molecular clouds in this giant star-forming complex.

The Hyades main sequence

NASA Astrophysics Data System (ADS)

Eggen, O. J.

1982-11-01

Intermediate band, H-beta and RI observations of 72 Hyades cluster stars to V = 11 mag are reported and discussed. A modulus of 3.2 mag is derived on the basis of a comparison with field stars of large parallax. Also presented are observations of 98 main-sequence stars of the Hyades group that were previously found to be group members from kinematical considerations. Parallaxes of the group stars, computed on the assumption that they are members of an extended Hyades cluster, yield mean values of (U, V, W) = (+40.5, -18.4, -4.9) km/s, with dispersions of (2.3, 2.3, 6.0) km/s, compared with (+41.7, -18.4, -2.0) and (2.6, 1.3, 1.9) km/s for the brightest cluster members. It is noted that all the stars discussed can be considered as members of a supercluster in which only a slight relaxation control of the W velocities is present for stars far from the nucleus. Evidence is found, including that of the Praesepe cluster at Z = +80 pc, for some interchange between the U, V, and W velocities in stars farthest from the galactic plane, with the total cluster velocity being maintained.

VizieR Online Data Catalog: Radial velocities in M3, M13, and M92 (Kamann+, 2014)

NASA Astrophysics Data System (ADS)

Kamann, S.; Wisotzki, L.; Roth, M. M.; Gerssen, J.; Husser, T.-O.; Sandin, C.; Weilbacher, P.

2014-04-01

Radial velocity data are presented for three Galactic globular clusters, M3, M13, and M92. The provided catalogues include several hundreds of stars in each cluster that cover a wide range of distances to the cluster centres. Besides the measured radial velocities, the catalogues contain measurement uncertainties, identifiers, world coordinates and variability information for each star. The velocities for stars near the centres of the clusters were obtained using PMAS integral field spectroscopy (IFS). Note that in order to facilitate future variability studies, for each star the individual velocity measurements are provided instead of a single combined velocity. The PMAS data are complemented with velocities reported in various literature studies for stars at larger distances to the centres. (6 data files).

A Study Of Anomalous Stars and Binary Populations Within Open Clusters: Tests Of Theoretical Models

NASA Astrophysics Data System (ADS)

Geller, Aaron M.; Mathieu, Robert D.; Braden, Ella; Latham, David W.

2008-08-01

``Anomalous'' stars, such as blue stragglers and more recently sub- subgiants, have been an enduring challenge for stellar evolution theory. Recently it has become clear that in star clusters these systems are closely linked to the binary star populations. Furthermore, through advances in N-body modeling, we have come to realize that stellar dynamical processes play a central role in the formation of such anomalous stars. Indeed, these stars trace the interface between the classical fields of stellar evolution and stellar dynamics. We propose a thesis study to directly probe this interface through high-precision radial-velocity measurements of the anomalous stars and the binary populations in four open clusters. We have selected NGC 188 (7 Gyr), M67 (NGC 2682; 4 Gyr), NGC 6819 (2.4 Gyr), and M35 (NGC 2168; 150 Myr), as these span a wide range in age, are rich enough to provide statistically significant conclusions, and already have an extensive base of kinematic, spectroscopic, and photometric observations from the WIYN Open Cluster Study. Our proposed observations will define the spectroscopic hard binary populations (fraction, frequency distributions of orbital parameters, mass ratios) for orbital periods approaching the hard-soft boundary. These observations will also provide a comprehensive survey for anomalous stars, including secure establishment of their cluster membership. These data will allow us to perform the first detailed comparison to predictions from open cluster simulations of the binary populations among normal and anomalous stars, and thereby to constrain the evolutionary paths from one to the other.

A Study Of Anomalous Stars and Binary Populations Within Open Clusters: Tests Of Theoretical Models

NASA Astrophysics Data System (ADS)

Geller, Aaron M.; Mathieu, Robert D.; Gosnell, Natalie; Latham, David W.

2009-02-01

``Anomalous'' stars, such as blue stragglers and more recently sub- subgiants, have been an enduring challenge for stellar evolution theory. Recently it has become clear that in star clusters these systems are closely linked to the binary star populations. Furthermore, through advances in N-body modeling, we have come to realize that stellar dynamical processes play a central role in the formation of such anomalous stars. Indeed, these stars trace the interface between the classical fields of stellar evolution and stellar dynamics. We propose a thesis study to directly probe this interface through high-precision radial-velocity measurements of the anomalous stars and the binary populations in four open clusters. We have selected NGC 188 (7 Gyr), M67 (NGC 2682; 4 Gyr), NGC 6819 (2.4 Gyr), and M35 (NGC 2168; 150 Myr), as these span a wide range in age, are rich enough to provide statistically significant conclusions, and already have an extensive base of kinematic, spectroscopic, and photometric observations from the WIYN Open Cluster Study. Our proposed observations will define the spectroscopic hard binary populations (fraction, frequency distributions of orbital parameters, mass ratios) for orbital periods approaching the hard-soft boundary. These observations will also provide a comprehensive survey for anomalous stars, including secure establishment of their cluster membership. These data will allow us to perform the first detailed comparison to predictions from open cluster simulations of the binary populations among normal and anomalous stars, and thereby to constrain the evolutionary paths from one to the other.

A Study Of Anomalous Stars and Binary Populations Within Open Clusters: Tests Of Theoretical Models

NASA Astrophysics Data System (ADS)

Geller, Aaron M.; Mathieu, Robert D.; Braden, Ella; Latham, David W.

2008-02-01

``Anomalous'' stars, such as blue stragglers and more recently sub- subgiants, have been an enduring challenge for stellar evolution theory. Recently it has become clear that in star clusters these systems are closely linked to the binary star populations. Furthermore, through advances in N-body modeling, we have come to realize that stellar dynamical processes play a central role in the formation of such anomalous stars. Indeed, these stars trace the interface between the classical fields of stellar evolution and stellar dynamics. We propose a thesis study to directly probe this interface through high-precision radial-velocity measurements of the anomalous stars and the binary populations in four open clusters. We have selected NGC 188 (7 Gyr), M67 (NGC 2682; 4 Gyr), NGC 6819 (2.4 Gyr), and M35 (NGC 2168; 150 Myr), as these span a wide range in age, are rich enough to provide statistically significant conclusions, and already have an extensive base of kinematic, spectroscopic, and photometric observations from the WIYN Open Cluster Study. Our proposed observations will define the spectroscopic hard binary populations (fraction, frequency distributions of orbital parameters, mass ratios) for orbital periods approaching the hard-soft boundary. These observations will also provide a comprehensive survey for anomalous stars, including secure establishment of their cluster membership. These data will allow us to perform the first detailed comparison to predictions from open cluster simulations of the binary populations among normal and anomalous stars, and thereby to constrain the evolutionary paths from one to the other.

On Iron Enrichment, Star Formation, and Type Ia Supernovae in Galaxy Clusters

NASA Technical Reports Server (NTRS)

Loewenstein, Michael

2006-01-01

The nature of star formation and Type Ia supernovae (SNIa) in galaxies in the field and in rich galaxy clusters are contrasted by juxtaposing the buildup of heavy metals in the universe inferred from observed star formation and supernovae rate histories with data on the evolution of Fe abundances in the intracluster medium (ICM). Models for the chemical evolution of Fe in these environments are constructed, subject to observational constraints, for this purpose. While models with a mean delay for SNIa of 3 Gyr and standard initial mass function (IMF) are fully consistent with observations in the field, cluster Fe enrichment immediately tracked a rapid, top-heavy phase of star formation - although transport of Fe into the ICM may have been more prolonged and star formation likely continued beyond redshift 1. The means of this prompt enrichment consisted of SNII yielding greater than or equal to 0.1 solar mass per explosion (if the SNIa rate normalization is scaled down from its value in the field according to the relative number of candidate progenitor stars in the 3 - 8 solar mass range) and/or SNIa with short delay times originating during the rapid star formation epoch. Star formation is greater than 3 times more efficient in rich clusters than in the field, mitigating the overcooling problem in numerical cluster simulations. Both the fraction of baryons cycled through stars, and the fraction of the total present-day stellar mass in the form of stellar remnants, are substantially greater in clusters than in the field.

A Database of Young Star Clusters for Five Hundred Galaxies

NASA Astrophysics Data System (ADS)

Evans, Jessica; Whitmore, B. C.; Lindsay, K.; Chandar, R.; Larsen, S.

2009-01-01

The study of young massive stellar clusters has faced a series of observational challenges, such as the use of inconsistent data sets and low number statistics. To rectify these shortcomings, this project will use the source lists developed as part of the Hubble Legacy Archive to obtain a large, uniform database of super star clusters in nearby star-forming galaxies in order to address two fundamental astronomical questions: 1) To what degree is the cluster luminosity (and mass) function of star clusters universal? 2) What fraction of super star clusters are "missing" in optical studies (i.e., are hidden by dust)? The archive's recent data release (Data Release 2 - September, 2008) will help us achieve the large sample necessary (N 50 galaxies for multi-wavelength, N 500 galaxies for ACS F814W). The uniform data set will comprise of ACS, WFPC2, and NICMOS data, with DAOphot used for object detection. This database will also support comparisons with new Monte-Carlo simulations that have independently been developed in the past few years, and will be used to test the Whitmore, Chandar, Fall (2007) framework designed to understand the demographics of star clusters in all star forming galaxies. The catalogs will increase the number of galaxies with measured mass and luminosity functions by an order of magnitude, and will provide a powerful new tool for comparative studies, both ours and the community's. The poster will describe our preliminary investigation for the first 30 galaxies in the sample.

A Comparison of Young Star Properties with Local Galactic Environment for LEGUS/LITTLE THINGS Dwarf Irregular Galaxies

NASA Astrophysics Data System (ADS)

Hunter, Deidre A.; Adamo, Angela; Elmegreen, Bruce G.; Gallardo, Samavarti; Lee, Janice C.; Cook, David O.; Thilker, David; Kayitesi, Bridget; Kim, Hwihyun; Kahre, Lauren; Ubeda, Leonardo; Bright, Stacey N.; Ryon, Jenna E.; Calzetti, Daniela; Tosi, Monica; Grasha, Kathryn; Messa, Matteo; Fumagalli, Michele; Dale, Daniel A.; Sabbi, Elena; Cignoni, Michele; Smith, Linda J.; Gouliermis, Dimitrios M.; Grebel, Eva K.; Aloisi, Alessandra; Whitmore, Bradley C.; Chandar, Rupali; Johnson, Kelsey E.

2018-07-01

We have explored the role environmental factors play in determining characteristics of young stellar objects in nearby dwarf irregular and blue compact dwarf galaxies. Star clusters are characterized by concentrations, masses, and formation rates; OB associations by mass and mass surface density; O stars by their numbers and near-ultraviolet absolute magnitudes; and H II regions by Hα surface brightnesses. These characteristics are compared to surrounding galactic pressure, stellar mass density, H I surface density, and star formation rate (SFR) surface density. We find no trend of cluster characteristics with environmental properties, implying that larger-scale effects are more important in determining cluster characteristics or that rapid dynamical evolution erases any memory of the initial conditions. On the other hand, the most massive OB associations are found at higher pressure and H I surface density, and there is a trend of higher H II region Hα surface brightness with higher pressure, suggesting that a higher concentration of massive stars and gas is found preferentially in regions of higher pressure. At low pressures we find massive stars but not bound clusters and OB associations. We do not find evidence for an increase of cluster formation efficiency as a function of SFR density. However, there is an increase in the ratio of the number of clusters to the number of O stars with increasing pressure, perhaps reflecting an increase in clustering properties with SFR.

The Gaia-ESO Survey and CSI 2264: Substructures, disks, and sequential star formation in the young open cluster NGC 2264

NASA Astrophysics Data System (ADS)

Venuti, L.; Prisinzano, L.; Sacco, G. G.; Flaccomio, E.; Bonito, R.; Damiani, F.; Micela, G.; Guarcello, M. G.; Randich, S.; Stauffer, J. R.; Cody, A. M.; Jeffries, R. D.; Alencar, S. H. P.; Alfaro, E. J.; Lanzafame, A. C.; Pancino, E.; Bayo, A.; Carraro, G.; Costado, M. T.; Frasca, A.; Jofré, P.; Morbidelli, L.; Sousa, S. G.; Zaggia, S.

2018-01-01

Context. Reconstructing the structure and history of young clusters is pivotal to understanding the mechanisms and timescales of early stellar evolution and planet formation. Recent studies suggest that star clusters often exhibit a hierarchical structure, possibly resulting from several star formation episodes occurring sequentially rather than a monolithic cloud collapse. Aims: We aim to explore the structure of the open cluster and star-forming region NGC 2264 ( 3 Myr), which is one of the youngest, richest and most accessible star clusters in the local spiral arm of our Galaxy; we link the spatial distribution of cluster members to other stellar properties such as age and evolutionary stage to probe the star formation history within the region. Methods: We combined spectroscopic data obtained as part of the Gaia-ESO Survey (GES) with multi-wavelength photometric data from the Coordinated Synoptic Investigation of NGC 2264 (CSI 2264) campaign. We examined a sample of 655 cluster members, with masses between 0.2 and 1.8 M⊙ and including both disk-bearing and disk-free young stars. We used Teff estimates from GES and g,r,i photometry from CSI 2264 to derive individual extinction and stellar parameters. Results: We find a significant age spread of 4-5 Myr among cluster members. Disk-bearing objects are statistically associated with younger isochronal ages than disk-free sources. The cluster has a hierarchical structure, with two main blocks along its latitudinal extension. The northern half develops around the O-type binary star S Mon; the southern half, close to the tip of the Cone Nebula, contains the most embedded regions of NGC 2264, populated mainly by objects with disks and ongoing accretion. The median ages of objects at different locations within the cluster, and the spatial distribution of disked and non-disked sources, suggest that star formation began in the north of the cluster, over 5 Myr ago, and was ignited in its southern region a few Myr later. Star formation is likely still ongoing in the most embedded regions of the cluster, while the outer regions host a widespread population of more evolved objects; these may be the result of an earlier star formation episode followed by outward migration on timescales of a few Myr. We find a detectable lag between the typical age of disk-bearing objects and that of accreting objects in the inner regions of NGC 2264: the first tend to be older than the second, but younger than disk-free sources at similar locations within the cluster. This supports earlier findings that the characteristic timescales of disk accretion are shorter than those of disk dispersal, and smaller than the average age of NGC 2264 (i.e., ≲3 Myr). At the same time, we note that disks in the north of the cluster tend to be shorter-lived ( 2.5 Myr) than elsewhere; this may reflect the impact of massive stars within the region (notably S Mon), that trigger rapid disk dispersal. Conclusions: Our results, consistent with earlier studies on NGC 2264 and other young clusters, support the idea of a star formation process that takes place sequentially over a prolonged span in a given region. A complete understanding of the dynamics of formation and evolution of star clusters requires accurate astrometric and kinematic characterization of its population; significant advance in this field is foreseen in the upcoming years thanks to the ongoing Gaia mission, coupled with extensive ground-based surveys like GES. Full Table B.1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/609/A10

X-Ray Binaries and Star Clusters in the Antennae: Optical Cluster Counterparts

NASA Astrophysics Data System (ADS)

Rangelov, Blagoy; Chandar, Rupali; Prestwich, Andrea; Whitmore, Bradley C.

2012-10-01

We compare the locations of 82 X-ray binaries (XRBs) detected in the merging Antennae galaxies by Zezas et al., based on observations taken with the Chandra X-Ray Observatory, with a catalog of optically selected star clusters presented by Whitmore et al., based on observations taken with the Hubble Space Telescope. Within the 2σ positional uncertainty of ≈0farcs8, we find 22 XRBs are coincident with star clusters, where only two to three chance coincidences are expected. The ages of the clusters were estimated by comparing their UBVI, Hα colors with predictions from stellar evolutionary models. We find that 14 of the 22 coincident XRBs (64%) are hosted by star clusters with ages of ≈6 Myr or less. All of the very young host clusters are fairly massive and have M >~ 3 × 104 M ⊙, with many having masses M ≈ 105 M ⊙. Five of the XRBs are hosted by young clusters with ages τ ≈ 10-100 Myr, while three are hosted by intermediate-age clusters with τ ≈ 100-300 Myr. Based on the results from recent N-body simulations, which suggest that black holes are far more likely to be retained within their parent clusters than neutron stars, we suggest that our sample consists primarily of black hole binaries with different ages.

SPECTROSCOPY OF LUMINOUS COMPACT BLUE GALAXIES IN DISTANT CLUSTERS. II. PHYSICAL PROPERTIES OF dE PROGENITOR CANDIDATES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crawford, S. M.; Wirth, Gregory D.; Bershady, M. A.

2016-02-01

Luminous Compact Blue Galaxies (LCBGs) are an extreme star-bursting population of galaxies that were far more common at earlier epochs than today. Based on spectroscopic and photometric measurements of LCBGs in massive (M > 10{sup 15} M{sub ⊙}), intermediate redshift (0.5 < z < 0.9) galaxy clusters, we present their rest-frame properties including star formation rate, dynamical mass, size, luminosity, and metallicity. The appearance of these small, compact galaxies in clusters at intermediate redshift helps explain the observed redshift evolution in the size–luminosity relationship among cluster galaxies. In addition, we find the rest-frame properties of LCBGs appearing in galaxy clusters are indistinguishable from field LCBGs atmore » the same redshift. Up to 35% of the LCBGs show significant discrepancies between optical and infrared indicators of star formation, suggesting that star formation occurs in obscured regions. Nonetheless, the star formation for LCBGs shows a decrease toward the center of the galaxy clusters. Based on their position and velocity, we estimate that up to 10% of cluster LCBGs are likely to merge with another cluster galaxy. Finally, the observed properties and distributions of the LCBGs in these clusters lead us to conclude that we are witnessing the quenching of the progenitors of dwarf elliptical galaxies that dominate the number density of present-epoch galaxy clusters.« less

Hubble Sees a Youthful Cluster

NASA Image and Video Library

2017-12-08

Shown here in a new image taken with the Advanced Camera for Surveys (ACS) on board the NASA/ESA Hubble Space Telescope is the globular cluster NGC 1783. This is one of the biggest globular clusters in the Large Magellanic Cloud, a satellite galaxy of our own galaxy, the Milky Way, in the southern hemisphere constellation of Dorado. First observed by John Herschel in 1835, NGC 1783 is nearly 160,000 light-years from Earth, and has a mass around 170,000 times that of the sun. Globular clusters are dense collections of stars held together by their own gravity, which orbit around galaxies like satellites. The image clearly shows the symmetrical shape of NGC 1783 and the concentration of stars towards the center, both typical features of globular clusters. By measuring the color and brightness of individual stars, astronomers can deduce an overall age for a cluster and a picture of its star formation history. NGC 1783 is thought to be less than one and a half billion years old — which is very young for globular clusters, which are typically several billion years old. During that time, it is thought to have undergone at least two periods of star formation, separated by 50 to 100 million years. This ebb and flow of star-forming activity is an indicator of how much gas is available for star formation at any one time. When the most massive stars created in the first burst of formation explode as supernovae they blow away the gas needed to form further stars, but the gas reservoir can later be replenished by less massive stars which last longer and shed their gas less violently. After this gas flows to the dense central regions of the star cluster, a second phase of star formation can take place and once again the short-lived massive stars blow away any leftover gas. This cycle can continue a few times, at which time the remaining gas reservoir is thought to be too small to form any new stars. Image credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

New Asteroseismic Scaling Relations Based on the Hayashi Track Relation Applied to Red Giant Branch Stars in NGC 6791 and NGC 6819

NASA Astrophysics Data System (ADS)

Wu, T.; Li, Y.; Hekker, S.

2014-01-01

Stellar mass M, radius R, and gravity g are important basic parameters in stellar physics. Accurate values for these parameters can be obtained from the gravitational interaction between stars in multiple systems or from asteroseismology. Stars in a cluster are thought to be formed coevally from the same interstellar cloud of gas and dust. The cluster members are therefore expected to have some properties in common. These common properties strengthen our ability to constrain stellar models and asteroseismically derived M, R, and g when tested against an ensemble of cluster stars. Here we derive new scaling relations based on a relation for stars on the Hayashi track (\\sqrt{T_eff} \\sim g^pR^q) to determine the masses and metallicities of red giant branch stars in open clusters NGC 6791 and NGC 6819 from the global oscillation parameters Δν (the large frequency separation) and νmax (frequency of maximum oscillation power). The Δν and νmax values are derived from Kepler observations. From the analysis of these new relations we derive: (1) direct observational evidence that the masses of red giant branch stars in a cluster are the same within their uncertainties, (2) new methods to derive M and z of the cluster in a self-consistent way from Δν and νmax, with lower intrinsic uncertainties, and (3) the mass dependence in the Δν - νmax relation for red giant branch stars.

Massive Stars in the MCs: What They Tell Us about the IMF, Stellar Evolution, and Upper Mass "Cutoffs"

NASA Astrophysics Data System (ADS)

Massey, P.

Massive stars in the Magellanic Clouds provide an instantaneous "snapshot" of star-formation. In this talk I will review what we have learned both about star formation, and stellar evolution. Studies over the past decade have shown that the initial mass function (IMF) is the same for massive stars born in OB associations in the LMC and SMC as in associations and clusters in the Milky Way; the slope of the IMF is essentially Salpeter (Gamma ~ -1.3), despite the factor of 10 difference in metallicity between these systems. Recent work on the R136 cluster (described in Hunter's review talk) suggest that there is no such thing as an upper mass cutoff to the IMF, at least not one that has been found observationally: for the youngest clusters (2 Myr and younger), the mass of the highest mass star present is simply dependent upon how populous the cluster is; i.e., the IMF is truncated by statistics, not physics. There does appear to be a significant population of massive stars that are born in the "field" (not part of a large OB association or cluster); the IMF of these stars is quite a bit steeper (Gamma ~ -4), although stars as massive as those found in associations are also found in the field. The mixed-age population of the MCs as a whole can be used to test stellar evolutionary models; the agreement with the work of the Geneva group is found to be excellent, for stars with masses >25 Mo, although the youngest stars may be missing in the HRD. The discovery that clusters born in associations are quite coeval (Delta tau <1-2 Myr) allows us to use the "turn-off masses" to determine what mass objects become Wolf-Rayet stars of various types, and new results will be reviewed.

Distances, Kinematics, And Structure Of Nearby Star-Forming Regions

NASA Astrophysics Data System (ADS)

Kounkel, Marina

2017-08-01

In this thesis I present an analysis of the structure and kinematics of the Orion Molecular Cloud Complex in an effort to better characterize the dynamical state of the closest region of the ongoing massive star formation and to provide a baseline for comparison of the upcoming results from the Gaia space telescope. In order to achieve this goal, I measured stellar parallax and proper motions, using very large baseline radio interferometry of non-thermally-emitting sources.. Based on these observations I measured the average distance in Orion A molecular cloud of 388±5 pc toward the Orion Nebula Cluster (ONC), 428±10 pc toward the southern portion of L1641, as well as the distance in Orion B of 388±10 pc toward NGC 2068, and roughly ˜420 pc toward NGC 2024. These are the first direct distance measurements with < 5% uncertainty to the regions within the Orion Complex outside of the ONC. Little can be said about the proper motions due to the sparcity of the sample size; however, I identified a number of binary systems and fitted their orbital motion, which allows for the direct measurement of the masses of the individual components. I also identified three stars that have been ejected from the ONC due to the gravitational interactions with its most massive stars.I complemented the parallax and proper motion measurements with the observations of radial velocities (RV) of the stars toward the Orion Complex, probing the histories of both dynamic evolution and star formation in the region. I found that in the Orion A cloud and in NGC 2024 there exists an asymmetry between the stellar RVs and those of the molecular gas, with a small fraction of the stars stars being preferentially blueshifted relative to the gas. Several possible explanations for this have been proposed, although presently there is not yet a definitive solution. I also analyzed the multiplicity fraction of the spectroscopic binaries in the ONC, and found that it is largely consistent to what is observed in the nearby field stars.Finally, I explored the substructure of the ONC by focusing on NGC 1980, a cluster that has previously been identified as foreground to and older than the ONC. I examined these claims to show that there is little evidence that there is a discrepancy in distance between the stellar populations of the ONC and NGC 1980. Additionally, while the stars of NGC 1980 are likely somewhat older than the ONC, their age is consistent with the stellar population of the rest of the Orion A molecular cloud.

THE INFLUENCE OF ORBITAL ECCENTRICITY ON TIDAL RADII OF STAR CLUSTERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Webb, Jeremy J.; Harris, William E.; Sills, Alison

2013-02-20

We have performed N-body simulations of star clusters orbiting in a spherically symmetric smooth galactic potential. The model clusters cover a range of initial half-mass radii and orbital eccentricities in order to test the historical assumption that the tidal radius of a cluster is imposed at perigalacticon. The traditional assumption for globular clusters is that since the internal relaxation time is larger than its orbital period, the cluster is tidally stripped at perigalacticon. Instead, our simulations show that a cluster with an eccentric orbit does not need to fully relax in order to expand. After a perigalactic pass, a clustermore » recaptures previously unbound stars, and the tidal shock at perigalacticon has the effect of energizing inner region stars to larger orbits. Therefore, instead of the limiting radius being imposed at perigalacticon, it more nearly traces the instantaneous tidal radius of the cluster at any point in the orbit. We present a numerical correction factor to theoretical tidal radii calculated at perigalacticon which takes into consideration both the orbital eccentricity and current orbital phase of the cluster.« less

Spitzer Telescope Sends Rose for Valentine Day

NASA Image and Video Library

2004-02-12

A cluster of newborn stars herald their birth in this interstellar Valentine Day commemorative picture obtained with NASA Spitzer Space Telescope. These bright young stars are found in a rosebud-shaped and rose-colored nebulosity known as NGC 7129. The star cluster and its associated nebula are located at a distance of 3300 light-years in the constellation Cepheus. A recent census of the cluster reveals the presence of 130 young stars. The stars formed from a massive cloud of gas and dust that contains enough raw materials to create a thousand Sun-like stars. In a process that astronomers still poorly understand, fragments of this molecular cloud became so cold and dense that they collapsed into stars. Most stars in our Milky Way galaxy are thought to form in such clusters. The Spitzer Space Telescope image was obtained with an infrared array camera that is sensitive to invisible infrared light at wavelengths that are about ten times longer than visible light. In this four-color composite, emission at 3.6 microns is depicted in blue, 4.5 microns in green, 5.8 microns in orange, and 8.0 microns in red. The image covers a region that is about one quarter the size of the full moon. As in any nursery, mayhem reigns. Within the astronomically brief period of a million years, the stars have managed to blow a large, irregular bubble in the molecular cloud that once enveloped them like a cocoon. The rosy pink hue is produced by glowing dust grains on the surface of the bubble being heated by the intense light from the embedded young stars. Upon absorbing ultraviolet and visible-light photons produced by the stars, the surrounding dust grains are heated and re-emit the energy at the longer infrared wavelengths observed by Spitzer. The reddish colors trace the distribution of molecular material thought to be rich in hydrocarbons. The cold molecular cloud outside the bubble is mostly invisible in these images. However, three very young stars near the center of the image are sending jets of supersonic gas into the cloud. The impact of these jets heats molecules of carbon monoxide in the cloud, producing the intricate green nebulosity that forms the stem of the rosebud. Not all stars are formed in clusters. Away from the main nebula and its young cluster are two smaller nebulae, to the left and bottom of the central 'rosebud,'each containing a stellar nursery with only a few young stars. Astronomers believe that our own Sun may have formed billions of years ago in a cluster similar to NGC 7129. Once the radiation from new cluster stars destroys the surrounding placental material, the stars begin to slowly drift apart. http://photojournal.jpl.nasa.gov/catalog/PIA05266

Effects of Combined Stellar Feedback on Star Formation in Stellar Clusters

NASA Astrophysics Data System (ADS)

Wall, Joshua Edward; McMillan, Stephen; Pellegrino, Andrew; Mac Low, Mordecai; Klessen, Ralf; Portegies Zwart, Simon

2018-01-01

We present results of hybrid MHD+N-body simulations of star cluster formation and evolution including self consistent feedback from the stars in the form of radiation, winds, and supernovae from all stars more massive than 7 solar masses. The MHD is modeled with the adaptive mesh refinement code FLASH, while the N-body computations are done with a direct algorithm. Radiation is modeled using ray tracing along long characteristics in directions distributed using the HEALPIX algorithm, and causes ionization and momentum deposition, while winds and supernova conserve momentum and energy during injection. Stellar evolution is followed using power-law fits to evolution models in SeBa. We use a gravity bridge within the AMUSE framework to couple the N-body dynamics of the stars to the gas dynamics in FLASH. Feedback from the massive stars alters the structure of young clusters as gas ejection occurs. We diagnose this behavior by distinguishing between fractal distribution and central clustering using a Q parameter computed from the minimum spanning tree of each model cluster. Global effects of feedback in our simulations will also be discussed.

A Photometric Survey of the Open Clusters NGC 7789 and M67

NASA Astrophysics Data System (ADS)

Janes, Kenneth

2010-01-01

Although there is strong evidence that stellar activity declines as a star ages, beyond about the age of the Hyades (600 Myr) there is little direct confirmation of this decline in stars of known age. This report is an update of an earlier report (Hayes-Gehrke, et al., 2004, AJ, 128, 2862) of a long-term project to explore stellar activity in old open clusters. I have now accumulated 12 years of photometry of the old clusters NGC 7789 (about 1.8 Gyr) and M 67 (about 4 Gyr). An analysis of these data has revealed a substantial number of low-amplitude variable stars in both clusters, including a number of previously-discovered eclipsing binary stars, and several stars near the main sequence turnoff of both clusters that exhibit apparently erratic variations. Some of the M 67 erratics are known X-ray sources. On the main sequence, the large majority of stars show little or no evidence for variability at the 0.1% - 0.2% level, consistent with a regular systematic decline in activity level with age.

Cluster of Stars in Kepler Sight

NASA Image and Video Library

2009-04-16

This image zooms into a small portion of NASA Kepler full field of view, an expansive, 100-square-degree patch of sky in our Milky Way galaxy. An eight-billion-year-old cluster of stars 13,000 light-years from Earth, called NGC 6791, can be seen in the image. Clusters are families of stars that form together out of the same gas cloud. This particular cluster is called an open cluster, because the stars are loosely bound and have started to spread out from each other. The area pictured is 0.2 percent of Kepler's full field of view, and shows hundreds of stars in the constellation Lyra. The image has been color-coded so that brighter stars appear white, and fainter stars, red. It is a 60-second exposure, taken on April 8, 2009, one day after the spacecraft's dust cover was jettisoned. Kepler was designed to hunt for planets like Earth. The mission will spend the next three-and-a-half years staring at the same stars, looking for periodic dips in brightness. Such dips occur when planets cross in front of their stars from our point of view in the galaxy, partially blocking the starlight. To achieve the level of precision needed to spot planets as small as Earth, Kepler's images are intentionally blurred slightly. This minimizes the number of saturated stars. Saturation, or "blooming," occurs when the brightest stars overload the individual pixels in the detectors, causing the signal to spill out into nearby pixels. http://photojournal.jpl.nasa.gov/catalog/PIA11986

Galactic Doppelgängers: The Chemical Similarity Among Field Stars and Among Stars with a Common Birth Origin

NASA Astrophysics Data System (ADS)

Ness, M.; Rix, H.-W.; Hogg, David W.; Casey, A. R.; Holtzman, J.; Fouesneau, M.; Zasowski, G.; Geisler, D.; Shetrone, M.; Minniti, D.; Frinchaboy, Peter M.; Roman-Lopes, Alexandre

2018-02-01

We explore to what extent stars within Galactic disk open clusters resemble each other in the high-dimensional space of their photospheric element abundances and contrast this with pairs of field stars. Our analysis is based on abundances for 20 elements, homogeneously derived from APOGEE spectra (with carefully quantified uncertainties of typically 0.03 dex). We consider 90 red giant stars in seven open clusters and find that most stars within a cluster have abundances in most elements that are indistinguishable (in a {χ }2-sense) from those of the other members, as expected for stellar birth siblings. An analogous analysis among pairs of > 1000 field stars shows that highly significant abundance differences in the 20 dimensional space can be established for the vast majority of these pairs, and that the APOGEE-based abundance measurements have high discriminating power. However, pairs of field stars whose abundances are indistinguishable even at 0.03 dex precision exist: ∼0.3% of all field star pairs and ∼1.0% of field star pairs at the same (solar) metallicity [Fe/H] = 0 ± 0.02. Most of these pairs are presumably not birth siblings from the same cluster, but rather doppelgängers. Our analysis implies that “chemical tagging” in the strict sense, identifying birth siblings for typical disk stars through their abundance similarity alone, will not work with such data. However, our approach shows that abundances have extremely valuable information for probabilistic chemo-orbital modeling, and combined with velocities, we have identified new cluster members from the field.

Initial Dynamical Evolution of Star Clusters with Tidal Field

NASA Astrophysics Data System (ADS)

Park, So-Myoung; Goodwin, Simon P.; Kim, Sungsoo S.

2017-03-01

Observations have been suggested that star clusters could form from the rapid collapse and violent relaxation of substructured distributions. We investigate the collapse of fractal stellar distributions in no, weak, and very strong tidal fields. We find that the rapid collapse of substructure into spherical clusters happens quickly with no or a weak tidal field, but very strong tidal fields prevent a cluster forming. However, we also find that dense Plummer spheres are also rapidly destroyed in strong tidal fields. We suggest that this is why the low-mass star clusters cannot survive near the galactic centre which has strong tidal field.

Intergalactic stellar populations in intermediate redshift clusters

NASA Astrophysics Data System (ADS)

Melnick, J.; Giraud, E.; Toledo, I.; Selman, F.; Quintana, H.

2012-11-01

A substantial fraction of the total stellar mass in rich clusters of galaxies resides in a diffuse intergalactic component usually referred to as the intracluster light (ICL). Theoretical models indicate that these intergalactic stars originate mostly from the tidal interaction of the cluster galaxies during the assembly history of the cluster, and that a significant fraction of these stars could have formed in situ from the late infall of cold metal-poor gas clouds on to the cluster. However, these models also overpredict the fraction of stellar mass in the ICL by a substantial margin, something that is still not well understood. The models also make predictions about the age distribution of the ICL stars, which may provide additional observational constraints. Here we present population synthesis models for the ICL of an intermediate redshift (z = 0.29) X-ray cluster that we have extensively studied in previous papers. The advantage of observing intermediate redshift clusters rather than nearby ones is that the former fit the field of view of multi-object spectrographs in 8-m telescopes and therefore permit us to encompass most of the ICL with only a few well-placed slits. In this paper we show that by stacking spectra at different locations within the ICL it is possible to reach sufficiently high signal-to-noise ratios to fit population synthesis models and derive meaningful results. The models provide ages and metallicities for the dominant populations at several different locations within the ICL and the brightest cluster galaxies (BCG) halo, as well as measures of the kinematics of the stars as a function of distance from the BCG. We thus find that the ICL in our cluster is dominated by old metal-rich stars, at odds with what has been found in nearby clusters where the stars that dominate the ICL are old and metal poor. While we see weak evidence of a young, metal-poor component, if real, these young stars would amount to less than 1 per cent of the total ICL mass, much less than the up to 30 per cent predicted by the models. We propose that the very metal-rich (i.e. 2.5× solar) stars in the ICL of our cluster, which comprise ˜40 per cent of the total mass, originate mostly from the central dumb-bell galaxy, while the remaining solar and metal-poor stars come from spiral, post-starburst (E+A) and metal-poor dwarf galaxies. About 16 per cent of the ICL stars are old and metal poor.

Photometric binary stars in Praesepe and the search for globular cluster binaries

NASA Technical Reports Server (NTRS)

Bolte, Michael

1991-01-01

A radial velocity study of the stars which are located on a second sequence above the single-star zero-age main sequence at a given color in the color-magnitude diagram of the open cluster Praesepe, (NGC 2632) shows that 10, and possibly 11, of 17 are binary systems. Of the binary systems, five have full amplitudes for their velocity variations that are greater than 50 km/s. To the extent that they can be applied to globular clusters, these results suggests that (1) observations of 'second-sequence' stars in globular clusters would be an efficient way of finding main-sequence binary systems in globulars, and (2) current instrumentation on large telescopes is sufficient for establishing unambiguously the existence of main-sequence binary systems in nearby globular clusters.

Photometric properties of stars clusters with young or mixed age stellar populations

NASA Astrophysics Data System (ADS)

Mollá, M.; García-Vargas, M. L.; Martín-Manjón, M. L.

2013-05-01

The main goal of this work is to present and discuss the synthetic photometrical properties of stellar clusters resulting from the PopStar code. Colors in Johnson and SDSS systems, Hα and Hβ luminosities and equivalent widths, and ionizing region size, have been computed for a wide range of metallicities Z = 0.0001, 0.0004, 0.004,0.008,0.02 and 0.05, and ages, from 0.1 Myr to 20 Gyr in Mollá, Garc{í}a-Vargas, & Bressan (2009, MNRAS, 398, 451). Emission lines are shown in Mart{í}n-Manj{ó}n et al. (2010, MNRAS, 403, 2012). Now we calculate colors with the emission lines contribution to the broad band color, so colors include stellar and nebular components, plus the emission lines following the evolution of the cluster and the region geometry in a consistent way. We compare the Single Stellar Populations contaminated and uncontaminated colors (in both Johnson and SDSS systems) and show the importance of emission lines contribution when photometry is used as a tool to characterize stellar populations. With these models we may determine the physical properties of young ionizing clusters when only photometrical observations are available and these correspond to the isolated star forming regions, subtracted the contribution of the underlying population In most cases, however, the ionizing population is usually embedded in a large and complex system, and the observed photometrical properties are the result of the combination of both the young star-forming burst and the host-underlying older population. The second objective of our work is therefore to provide a grid of models for nearby galaxies able to interpret mixed regions where the separation of young and old population is not possible or reliable enough. We obtain a set of PopStar Spectral Energy Distributions (available at PopStar site and also in VO) and derived colors for mixed populations where an underlying host population is combined in different mass ratios with a recent, metal-rich ionizing burst. These colors, together with other photometrical parameters, like Hα radius of the ionized region, and Balmer lines equivalent width and luminosity allow to infer the physical properties of star-forming regions without any spectroscopic information. For details and a complete set of tables and figures see Mollá, García-Vargas, & Martín-Manjón (2012, MNRAS, submitted).

X-ray insights into star and planet formation.

PubMed

Feigelson, Eric D

2010-04-20

Although stars and planets form in cold environments, X-rays are produced in abundance by young stars. This review examines the implications of stellar X-rays for star and planet formation studies, highlighting the contributions of NASA's (National Aeronautics and Space Administration) Chandra X-ray Observatory. Seven topics are covered: X-rays from protostellar outflow shocks, X-rays from the youngest protostars, the stellar initial mass function, the structure of young stellar clusters, the fate of massive stellar winds, X-ray irradiation of protoplanetary disks, and X-ray flare effects on ancient meteorites. Chandra observations of star-forming regions often show dramatic star clusters, powerful magnetic reconnection flares, and parsec-scale diffuse plasma. X-ray selected samples of premain sequence stars significantly advance studies of star cluster formation, the stellar initial mass function, triggered star-formation processes, and protoplanetary disk evolution. Although X-rays themselves may not play a critical role in the physics of star formation, they likely have important effects on protoplanetary disks by heating and ionizing disk gases.

X-ray insights into star and planet formation

PubMed Central

Feigelson, Eric D.

2010-01-01

Although stars and planets form in cold environments, X-rays are produced in abundance by young stars. This review examines the implications of stellar X-rays for star and planet formation studies, highlighting the contributions of NASA’s (National Aeronautics and Space Administration) Chandra X-ray Observatory. Seven topics are covered: X-rays from protostellar outflow shocks, X-rays from the youngest protostars, the stellar initial mass function, the structure of young stellar clusters, the fate of massive stellar winds, X-ray irradiation of protoplanetary disks, and X-ray flare effects on ancient meteorites. Chandra observations of star-forming regions often show dramatic star clusters, powerful magnetic reconnection flares, and parsec-scale diffuse plasma. X-ray selected samples of premain sequence stars significantly advance studies of star cluster formation, the stellar initial mass function, triggered star-formation processes, and protoplanetary disk evolution. Although X-rays themselves may not play a critical role in the physics of star formation, they likely have important effects on protoplanetary disks by heating and ionizing disk gases. PMID:20404197

Evidence for feedback and stellar-dynamically regulated bursty star cluster formation: the case of the Orion Nebula Cluster

NASA Astrophysics Data System (ADS)

Kroupa, Pavel; Jeřábková, Tereza; Dinnbier, František; Beccari, Giacomo; Yan, Zhiqiang

2018-04-01

A scenario for the formation of multiple co-eval populations separated in age by about 1 Myr in very young clusters (VYCs, ages less than 10 Myr) and with masses in the range 600-20 000 M⊙ is outlined. It rests upon a converging inflow of molecular gas building up a first population of pre-main sequence stars. The associated just-formed O stars ionise the inflow and suppress star formation in the embedded cluster. However, they typically eject each other out of the embedded cluster within 106 yr, that is before the molecular cloud filament can be ionised entirely. The inflow of molecular gas can then resume forming a second population. This sequence of events can be repeated maximally over the life-time of the molecular cloud (about 10 Myr), but is not likely to be possible in VYCs with mass <300 M⊙, because such populations are not likely to contain an O star. Stellar populations heavier than about 2000 M⊙ are likely to have too many O stars for all of these to eject each other from the embedded cluster before they disperse their natal cloud. VYCs with masses in the range 600-2000 M⊙ are likely to have such multi-age populations, while VYCs with masses in the range 2000-20 000 M⊙ can also be composed solely of co-eval, mono-age populations. More massive VYCs are not likely to host sub-populations with age differences of about 1 Myr. This model is applied to the Orion Nebula Cluster (ONC), in which three well-separated pre-main sequences in the colour-magnitude diagram of the cluster have recently been discovered. The mass-inflow history is constrained using this model and the number of OB stars ejected from each population are estimated for verification using Gaia data. As a further consequence of the proposed model, the three runaway O star systems, AE Aur, μ Col and ι Ori, are considered as significant observational evidence for stellar-dynamical ejections of massive stars from the oldest population in the ONC. Evidence for stellar-dynamical ejections of massive stars in the currently forming population is also discussed.

Star cluster formation history along the minor axis of the Large Magellanic Cloud

NASA Astrophysics Data System (ADS)

Piatti, Andrés E.; Cole, Andrew A.; Emptage, Bryn

2018-01-01

We analysed Washington CMT1 photometry of star clusters located along the minor axis of the Large Magellanic Cloud (LMC), from the LMC optical centre up to ∼39° outwards to the North-West. The data base was exploited in order to search for new star cluster candidates, to produce cluster CMDs cleaned from field star contamination and to derive age estimates for a statistically complete cluster sample. We confirmed that 146 star cluster candidates are genuine physical systems, and concluded that an overall ∼30 per cent of catalogued clusters in the surveyed regions are unlikely to be true physical systems. We did not find any new cluster candidates in the outskirts of the LMC (deprojected distance ≳ 8°). The derived ages of the studied clusters are in the range 7.2 < log(t yr-1) ≤ 9.4, with the sole exception of the globular cluster NGC 1786 (log(t yr-1) = 10.10). We also calculated the cluster frequency for each region, from which we confirmed previously proposed outside-in formation scenarios. In addition, we found that the outer LMC fields show a sudden episode of cluster formation (log(t yr-1) ∼7.8-7.9) which continued until log(t yr-1) ∼7.3 only in the outermost LMC region. We link these features to the first pericentre passage of the LMC to the Milky Way (MW), which could have triggered cluster formation due to ram pressure interaction between the LMC and MW halo.

Nuclear Rings in the IR: Hidden Super Star Clusters

NASA Astrophysics Data System (ADS)

Maoz, Dan

1997-07-01

We propose NICMOS broad-band {F160W, F187W} and Paschen Alpha {F187N} imaging of nuclear starburst rings in two nearby galaxies. We already have UV {F220W} FOC data, and are scheduled to obtain WFPC2 images in U, V, I, and Halpha+[NII] of these rings. The rings contain large populations of super star clusters similar to those recently discovered in other types of starburst systems. Nuclear rings contain large numbers of these clusters in relatively unobscured starburst environments. Measurement of the age, size, and stellar contents of the clusters can test the hypothesis that super star clusters are young globular clusters. Together with our UV and optical data, NICMOS images will provide the SED of numerous super star clusters over a decade in wavelength. Our already-approved observations will allow us to estimate, by comparison with evolutionary synthesis models, the masses and ages of the clusters. The proposed IR data will be sensitive to the number of supergiants {1.6 micron} and O-stars {Paschen Alpha} in each of the clusters. The observations will provide an independent determination of the reddening, mass, and age of each cluster. We expect to see in the IR numerous clusters that are obscured in the UV and optical. These clusters may be the younger ones, which are still embedded in their molecular clouds. By measuring the mass, age, and size of a large number of clusters, we can actually obtain an evolutionary picture of these objects at different stages in their lives.

Rotation periods of open-cluster stars, 3

NASA Technical Reports Server (NTRS)

Prosser, Charles F.; Shetrone, Matthew D.; Dasgupta, Amil; Backman, Dana E.; Laaksonen, Bentley D.; Baker, Shawn W.; Marschall, Laurence A.; Whitney, Barbara A.; Kuijken, Konrad; Stauffer, John R.

1995-01-01

We present the results from a photometric monitoring program of 15 open cluster stars and one weak-lined T Tauri star during late 1993/early 1994. Several show rotators which are members of the Alpha Persei, Pleiades, and Hyades open clusters have been monitored and period estimates derived. Using all available Pleiades stars with photometric periods together with current X-ray flux measurements, we illustrate the X-ray activity/rotation relation among Pleiades late-G/K dwarfs. The data show a clear break in the rotation-activity relation around P approximately 6-7 days -- in general accordance with previous results using more heterogeneous samples of G/K stars.

Early Results from Star Date: M83 - A Citizen Science Project to Age Date Star Clusters in the Southern Pinwheel Galaxy

NASA Astrophysics Data System (ADS)

Heartley, Jeremy; Whitmore, B. C.; Blair, W. P.; Christian, C. A.; Donaldson, T.; Hammer, D.; Smith, S.; Viana, A.

2014-01-01

The M83 Citizen Science Project is a collaborative effort currently in development between the Space Telescope Science Institute (STScI) and Zooniverse under the guidance of Dr. Brad Whitmore as part of Cy 19 proposal 12513 (PI - Dr. William Blair). This unique citizen science project will allow users to analyze individual star clusters within The Southern Pinwheel Galaxy, M83. The project will show users color-composite images taken with Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope (HST) and ask them to estimate the age of the star cluster. Through a multistage process, the project will educate and familiarize the user with the appearance of each age category based on the presence and shape of H-alpha emission, degree of resolution of the individual stars, and color of the cluster. (Whitmore et al. 2011). Additionally, the project will involve the actual measurement of the star cluster and H-alpha cloud radii to be used for further assessment and reinforcement of age. The data from this project and the statistics it yields will quantify these ages which can then be used to inform the debate between universal and environmental models of star cluster formation and destruction in galaxies. The tentative launch date is December 2013, therefore early results should be available at the time of the conference.

Rapidly rotating second-generation progenitors for the 'blue hook' stars of ω Centauri.

PubMed

Tailo, Marco; D'Antona, Francesca; Vesperini, Enrico; Di Criscienzo, Marcella; Ventura, Paolo; Milone, Antonino P; Bellini, Andrea; Dotter, Aaron; Decressin, Thibaut; D'Ercole, Annibale; Caloi, Vittoria; Capuzzo-Dolcetta, Roberto

2015-07-16

Horizontal branch stars belong to an advanced stage in the evolution of the oldest stellar galactic population, occurring either as field halo stars or grouped in globular clusters. The discovery of multiple populations in clusters that were previously believed to have single populations gave rise to the currently accepted theory that the hottest horizontal branch members (the 'blue hook' stars, which had late helium-core flash ignition, followed by deep mixing) are the progeny of a helium-rich 'second generation' of stars. It is not known why such a supposedly rare event (a late flash followed by mixing) is so common that the blue hook of ω Centauri contains approximately 30 per cent of the horizontal branch stars in the cluster, or why the blue hook luminosity range in this massive cluster cannot be reproduced by models. Here we report that the presence of helium core masses up to about 0.04 solar masses larger than the core mass resulting from evolution is required to solve the luminosity range problem. We model this by taking into account the dispersion in rotation rates achieved by the progenitors, whose pre-main-sequence accretion disk suffered an early disruption in the dense environment of the cluster's central regions, where second-generation stars form. Rotation may also account for frequent late-flash-mixing events in massive globular clusters.

Parameters of oscillation generation regions in open star cluster models

NASA Astrophysics Data System (ADS)

Danilov, V. M.; Putkov, S. I.

2017-07-01

We determine the masses and radii of central regions of open star cluster (OCL) models with small or zero entropy production and estimate the masses of oscillation generation regions in clustermodels based on the data of the phase-space coordinates of stars. The radii of such regions are close to the core radii of the OCL models. We develop a new method for estimating the total OCL masses based on the cluster core mass, the cluster and cluster core radii, and radial distribution of stars. This method yields estimates of dynamical masses of Pleiades, Praesepe, and M67, which agree well with the estimates of the total masses of the corresponding clusters based on proper motions and spectroscopic data for cluster stars.We construct the spectra and dispersion curves of the oscillations of the field of azimuthal velocities v φ in OCL models. Weak, low-amplitude unstable oscillations of v φ develop in cluster models near the cluster core boundary, and weak damped oscillations of v φ often develop at frequencies close to the frequencies of more powerful oscillations, which may reduce the non-stationarity degree in OCL models. We determine the number and parameters of such oscillations near the cores boundaries of cluster models. Such oscillations points to the possible role that gradient instability near the core of cluster models plays in the decrease of the mass of the oscillation generation regions and production of entropy in the cores of OCL models with massive extended cores.

Search for OB stars running away from young star clusters. I. NGC 6611

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.; Bomans, D. J.

2008-11-01

N-body simulations have shown that the dynamical decay of the young (~1 Myr) Orion Nebula cluster could be responsible for the loss of at least half of its initial content of OB stars. This result suggests that other young stellar systems could also lose a significant fraction of their massive stars at the very beginning of their evolution. To confirm this expectation, we used the Mid-Infrared Galactic Plane Survey (completed by the Midcourse Space Experiment satellite) to search for bow shocks around a number of young (⪉several Myr) clusters and OB associations. We discovered dozens of bow shocks generated by OB stars running away from these stellar systems, supporting the idea of significant dynamical loss of OB stars. In this paper, we report the discovery of three bow shocks produced by O-type stars ejected from the open cluster NGC 6611 (M16). One of the bow shocks is associated with the O9.5Iab star HD165319, which was suggested to be one of “the best examples for isolated Galactic high-mass star formation” (de Wit et al. 2005, A&A, 437, 247). Possible implications of our results for the origin of field OB stars are discussed.

Neutron stars and millisecond pulsars from accretion-induced collapse in globular clusters

NASA Technical Reports Server (NTRS)

Bailyn, Charles D.; Grindlay, Jonathan E.

1990-01-01

This paper examines the limits on the number of millisecond pulsars which could be formed in globular clusters by the generally accepted scenario (in which a neutron star is created by the supernova of an initially massive star and subsequently captures a companion to form a low-mass X-ray binary which eventually becomes a millisecond pulsar). It is found that, while the number of observed low-mass X-ray binaries can be adequately explained in this way, the reasonable assumption that the pulsar luminosity function in clusters extends below the current observational limits down to the luminosity of the faintest millisecond pulsars in the field suggests a cluster population of millisecond pulsars which is substantially larger than the standard model can produce. Alleviating this problem by postulating much shorter lifetimes for the X-ray binaries requires massive star populations sufficiently large that the mass loss resulting from their evolution would be likely to unbind the cluster. It is argued that neutron star formation in globular clusters by accretion-induced collapse of white dwarfs may resolve the discrepancy in birthrates.

Active galactic nuclei. IV - Supplying black hole clusters by tidal disruption and by tidal capture of stars

NASA Technical Reports Server (NTRS)

Stoeger, W. R.; Pacholczyk, A. G.; Stepinski, T. F.

1992-01-01

The extent to which individual holes in a cluster of black holes with a mass spectrum can liberate and accrete the resulting material by tidally disrupting stars they encounter, or by capturing stars as binary companions is studied. It is found that the smaller black holes in 'the halo' of such clusters can adequately supply themselves to the level M-dot sub h or greater than 0.0001(M-dot sub h) sub crit, and up to 0.05(M-dot sub h)sub crit for the smallest holes, by tidal disruption, as long as the cluster is embedded in a distribution of stars of relatively high density (not less than 0.1M sub cl/cu pc), and as long as the entire cluster of stars is not too compact (not less than 0.5 pc). Consideration is given to modifications this 'internal' mode of supply introduces in the spectrum emitted by such black hole clusters, and to the current status of their viability as models for AGN and QSOs in light of dynamical studies by Quinlan and Shapiro (1987, 1989).

The SILCC project - III. Regulation of star formation and outflows by stellar winds and supernovae

NASA Astrophysics Data System (ADS)

Gatto, Andrea; Walch, Stefanie; Naab, Thorsten; Girichidis, Philipp; Wünsch, Richard; Glover, Simon C. O.; Klessen, Ralf S.; Clark, Paul C.; Peters, Thomas; Derigs, Dominik; Baczynski, Christian; Puls, Joachim

2017-04-01

We study the impact of stellar winds and supernovae on the multiphase interstellar medium using three-dimensional hydrodynamical simulations carried out with FLASH. The selected galactic disc region has a size of (500 pc)2 × ±5 kpc and a gas surface density of 10 M⊙ pc-2. The simulations include an external stellar potential and gas self-gravity, radiative cooling and diffuse heating, sink particles representing star clusters, stellar winds from these clusters that combine the winds from individual massive stars by following their evolution tracks, and subsequent supernova explosions. Dust and gas (self-) shielding is followed to compute the chemical state of the gas with a chemical network. We find that stellar winds can regulate star (cluster) formation. Since the winds suppress the accretion of fresh gas soon after the cluster has formed, they lead to clusters that have lower average masses (102-104.3 M⊙) and form on shorter time-scales (10-3-10 Myr). In particular, we find an anticorrelation of cluster mass and accretion time-scale. Without winds, the star clusters easily grow to larger masses for ˜5 Myr until the first supernova explodes. Overall, the most massive stars provide the most wind energy input, while objects beginning their evolution as B-type stars contribute most of the supernova energy input. A significant outflow from the disc (mass loading ≳1 at 1 kpc) can be launched by thermal gas pressure if more than 50 per cent of the volume near the disc mid-plane can be heated to T > 3 × 105 K. Stellar winds alone cannot create a hot volume-filling phase. The models that are in best agreement with observed star formation rates drive either no outflows or weak outflows.

A YOUNG ECLIPSING BINARY AND ITS LUMINOUS NEIGHBORS IN THE EMBEDDED STAR CLUSTER Sh 2-252E

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lester, Kathryn V.; Gies, Douglas R.; Guo, Zhao, E-mail: lester@chara.gsu.edu, E-mail: gies@chara.gsu.edu, E-mail: guo@chara.gsu.edu

We present a photometric and light curve analysis of an eccentric eclipsing binary in the K2 Campaign 0 field, which resides in Sh 2-252E, a young star cluster embedded in an H ii region. We describe a spectroscopic investigation of the three brightest stars in the crowded aperture to identify which is the binary system. We find that none of these stars are components of the eclipsing binary system, which must be one of the fainter nearby stars. These bright cluster members all have remarkable spectra: Sh 2-252a (EPIC 202062176) is a B0.5 V star with razor sharp absorption lines, Sh 2-252b is amore » Herbig A0 star with disk-like emission lines, and Sh 2-252c is a pre-main-sequence star with very red color.« less

Interstellar Extinction in 20 Open Star Clusters

NASA Astrophysics Data System (ADS)

Rangwal, Geeta; Yadav, R. K. S.; Durgapal, Alok K.; Bisht, D.

2017-12-01

The interstellar extinction law in 20 open star clusters namely, Berkeley 7, Collinder 69, Hogg 10, NGC 2362, Czernik 43, NGC 6530, NGC 6871, Bochum 10, Haffner 18, IC 4996, NGC 2384, NGC 6193, NGC 6618, NGC 7160, Collinder 232, Haffner 19, NGC 2401, NGC 6231, NGC 6823, and NGC 7380 have been studied in the optical and near-IR wavelength ranges. The difference between maximum and minimum values of E(B - V) indicates the presence of non-uniform extinction in all the clusters except Collinder 69, NGC 2362, and NGC 2384. The colour excess ratios are consistent with a normal extinction law for the clusters NGC 6823, Haffner 18, Haffner 19, NGC 7160, NGC 6193, NGC 2401, NGC 2384, NGC 6871, NGC 7380, Berkeley 7, Collinder 69, and IC 4996. We have found that the differential colour-excess ΔE(B - V), which may be due to the occurrence of dust and gas inside the clusters, decreases with the age of the clusters. A spatial variation of colour excess is found in NGC 6193 in the sense that it decreases from east to west in the cluster region. For the clusters Berkeley 7, NGC 7380, and NGC 6871, a dependence of colour excess E(B - V) with spectral class and luminosity is observed. Eight stars in Collinder 232, four stars in NGC 6530, and one star in NGC 6231 have excess flux in near-IR. This indicates that these stars may have circumstellar material around them.

Color-magnitude diagrams for six metal-rich, low-latitude globular clusters

NASA Technical Reports Server (NTRS)

Armandroff, Taft E.

1988-01-01

Colors and magnitudes for stars on CCD frames for six metal-rich, low-latitude, previously unstudied globular clusters and one well-studied, metal-rich cluster (47 Tuc) have been derived and color-magnitude diagrams have been constructed. The photometry for stars in 47 Tuc are in good agreement with previous studies, while the V magnitudes of the horizontal-branch stars in the six program clusters do not agree with estimates based on secondary methods. The distances to these clusters are different from prior estimates. Redding values are derived for each program cluster. The horizontal branches of the program clusters all appear to lie entirely redwards of the red edge of the instability strip, as is normal for their metallicities.

New Halo Stars of the Galactic Globular Clusters M3 and M13 in the LAMOST DR1 Catalog

NASA Astrophysics Data System (ADS)

Navin, Colin A.; Martell, Sarah L.; Zucker, Daniel B.

2016-10-01

M3 and M13 are Galactic globular clusters with previous reports of surrounding stellar halos. We present the results of a search for members and extratidal cluster halo stars within and outside of the tidal radius of these clusters in the LAMOST Data Release 1. We find seven candidate cluster members (inside the tidal radius) of both M3 and M13, respectively. In M3 we also identify eight candidate extratidal cluster halo stars at distances up to ˜9.8 times the tidal radius, and in M13 we identify 12 candidate extratidal cluster halo stars at distances up to ˜13.8 times the tidal radius. These results support previous indications that both M3 and M13 are surrounded by extended stellar halos, and we find that the GC destruction rates corresponding to the observed mass loss are generally significantly higher than theoretical studies predict.

COCOA: Simulating Observations of Star Cluster Simulations

NASA Astrophysics Data System (ADS)

Askar, Abbas; Giersz, Mirek; Pych, Wojciech; Dalessandro, Emanuele

2017-03-01

COCOA (Cluster simulatiOn Comparison with ObservAtions) creates idealized mock photometric observations using results from numerical simulations of star cluster evolution. COCOA is able to present the output of realistic numerical simulations of star clusters carried out using Monte Carlo or N-body codes in a way that is useful for direct comparison with photometric observations. The code can simulate optical observations from simulation snapshots in which positions and magnitudes of objects are known. The parameters for simulating the observations can be adjusted to mimic telescopes of various sizes. COCOA also has a photometry pipeline that can use standalone versions of DAOPHOT (ascl:1104.011) and ALLSTAR to produce photometric catalogs for all observed stars.

Fluorine Abundances of AGB Stars in Stellar Clusters

NASA Astrophysics Data System (ADS)

Hren, A.; Lebzelter, T.; Aringer, B.; Hinkle, K. H.; Nowotny, W.

2015-08-01

We have measured the abundance of fluorine, [F/Fe], in a number of AGB stars in stellar clusters have correlated the results with their C/O ratios. This allows us to investigate the change in the fluorine abundance along the evolution on the giant branch. The target list includes primarily O-rich stars in three LMC globular clusters - NGC 1806, NGC 1846 and NGC 1978 - as well as Rup 106 and 47 Tuc in our Galaxy. The observational data were obtained with the PHOENIX spectrograph, and the COMA code was used for modelling the synthetic spectra. Within individual clusters, we find consistent [F/Fe] values at similar C/O for most of our target stars.

A Photometric Search for Planets in the Open Cluster NGC 7086

NASA Astrophysics Data System (ADS)

Rosvick, Joanne M.; Robb, Russell

2006-12-01

In an attempt to discover short-period, Jupiter-mass planets orbiting solar-type stars in open clusters, we searched for planetary transits in the populous and relatively unstudied open cluster NGC 7086. A color-magnitude diagram constructed from new B and V photometry is presented, along with revised estimates of the cluster's color excess, distance modulus, and age. Several turnoff stars were observed spectroscopically in order to determine a color excess of E(B-V)=0.83+/-0.02. Empirically fitting the main sequences of two young open clusters and the semiempirical zero-age main sequence of Vandenberg and Poll yielded a distance modulus of (V-MV)=13.4+/-0.3 mag. This corresponds to a true distance modulus of (m-M)0=10.8 mag or a distance of 1.5 kpc to NGC 7086. These values were used with isochrones from the Padova group to obtain a cluster age of 100 Myr. Eleven nights of R-band photometry were used to search for planetary transits. Differential magnitudes were constructed for each star in the cluster. Light curves for each star were produced on a night-to-night basis and inspected for variability. No planetary transits were apparent; however, some interesting variable stars were discovered: a pulsating variable that appears to be a member of the γ Dor class and four possible eclipsing binary stars, one of which actually may be a multiple system.

Astrometry in the globular cluster M13. II. Membership probabilities from old proper motions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cudworth, K.

Astrometric cluster membership probabilities have been derived from proper motions measured by other authors for stars in the region of the globular cluster M13. Several stars of individual interest are discussed.

Baryonic dark clusters in galactic halos and their observable consequences

NASA Technical Reports Server (NTRS)

Wasserman, Ira; Salpeter, Edwin E.

1994-01-01

We consider the possibility that approximately 10% of the mass of a typical galaxy halo is in the form of massive (approximately 10(exp 7) solar masses), compact (escape speeds approximately 100 km/s) baryonic clusters made of neutron stars (approximately 10% by mass), black holes (less than or approximately equal to 1%) and brown dwarfs, asteroids, and other low-mass debris (approximately 90%). These general properties are consistent with several different observational and phenomenological constraints on cluster properties subject to the condition that neutron stars comprise approximately 1% of the total halo mass. Such compact, dark clusters could be the sites of a variety of collisional phenomena involving neutron stars. We find that integrated out to the Hubble distance approximately one neutron star-neutron star or neutron star-black hole collision occurs daily. Of order 0.1-1 asteroid-neutron star collisions may also happen daily in the halo of the Milky Way if there is roughly equal cluster mass per logarithmic particle mass interval between asteroids and brown dwarfs. These event rates are comparable to the frequency of gamma-ray burst detections by the Burst and Transient Source Experiment (BATSE) on the Compton Observatory, implying that if dark halo clusters are the sites of most gamma-ray bursts, perhaps approximately 90% of all bursts are extragalactic, but approximately 10% are galactic. It is possible that dark clusters of the kind discussed here could be detected directly by the Infrared Space Observatory (ISO) or Space Infrared Telescope Facility (SIRTF). If the clusters considered in this paper exist, they should produce spatially correlated gravitational microlensing of stars in the Large Magellanic Cloud (LMC). If 10% of the halo is in the form of dark baryonic clusters, and the remaining 90% is in brown dwarfs and other dark objects which are either unclustered or collected into low-mass clusters, then we expect that two events within approximately 1 min of one another are likely to be seen after a total of order 20-30 microlenses have been detected.

The Difference Between Clusters and Groups: A Journey from Cluster Cores to Their Outskirts and Beyond

NASA Astrophysics Data System (ADS)

Bower, Richard G.; Balogh, Michael L.

In this review, we take the reader on a journey. We start by looking at the properties of galaxies in the cores of rich clusters. We have focused on the overall picture: star formation in clusters is strongly suppressed relative to field galaxies at the same redshift. We will argue that the increasing activity and blue populations of clusters with redshift results from a greater level of activity in field galaxies rather than a change in the transformation imposed by the cluster environment. With this in mind, we travel out from the cluster, focusing first on the properties of galaxies in the outskirts of clusters and then on galaxies in isolated groups. At low redshift, we are able to efficiently probe these environments using the Sloan Digital Sky Survey and 2dF redshift surveys. These allow an accurate comparison of galaxy star formation rates in different regions. The current results show a strong suppression of star formation above a critical threshold in local density. The threshold seems similar regardless of the overall mass of the system. At low redshift at least, only galaxies in close, isolated pairs have their star formation rate boosted above the global average. At higher redshift, work on constructing homogeneous catalogs of galaxies in groups and in the infall regions of clusters is still at an early stage. In the final section, we draw these strands together, summarizing what we can deduce about the mechanisms that transform star-forming field galaxies into their quiescent cluster counterparts. We discuss what we can learn about the impact of environment on the global star formation history of the Universe.

The contribution of dissolving star clusters to the population of ultra faint objects in the outer halo of the Milky Way

NASA Astrophysics Data System (ADS)

Contenta, Filippo; Gieles, Mark; Balbinot, Eduardo; Collins, Michelle L. M.

2017-04-01

In the last decade, several ultra faint objects (UFOs, MV ≳ -3.5) have been discovered in the outer halo of the Milky Way. For some of these objects, it is not clear whether they are star clusters or (ultra faint) dwarf galaxies. In this work, we quantify the contribution of star clusters to the population of UFOs. We extrapolated the mass and Galactocentric radius distribution of the globular clusters using a population model, finding that the Milky Way contains about 3.3^{+7.3}_{-1.6} star clusters with MV ≳ -3.5 and Galactocentric radius ≥20 kpc. To understand whether dissolving clusters can appear as UFOs, we run a suite of direct N-body models, varying the orbit, the Galactic potential, the binary fraction and the black hole (BH) natal kick velocities. In the analyses, we consider observational biases such as luminosity limit, field stars and line-of-sight projection. We find that star clusters contribute to both the compact and the extended population of UFOs: clusters without BHs appear compact with radii ˜5 pc, while clusters that retain their BHs after formation have radii ≳ 20 pc. The properties of the extended clusters are remarkably similar to those of dwarf galaxies: high-inferred mass-to-light ratios due to binaries, binary properties mildly affected by dynamical evolution, no observable mass segregation and flattened stellar mass function. We conclude that the slope of the stellar mass function as a function of Galactocentric radius and the presence/absence of cold streams can discriminate between dark matter-free and dark matter-dominated UFOs.

Suppressed star formation by a merging cluster system

DOE PAGES

Mansheim, A. S.; Lemaux, B. C.; Tomczak, A. R.; ...

2017-03-24

We examine the effects of an impending cluster merger on galaxies in the large scale structure (LSS) RX J0910 at z =1.105. Using multi-wavelength data, including 102 spectral members drawn from the Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey and precise photometric redshifts, we calculate star formation rates and map the specific star formation rate density of the LSS galaxies. These analyses along with an investigation of the color-magnitude properties of LSS galaxies indicate lower levels of star formation activity in the region between the merging clusters relative to the outskirts of the system. We suggest thatmore » gravitational tidal forces due to the potential of the merging halos may be the physical mechanism responsible for the observed suppression of star formation in galaxies caught between the merging clusters.« less

THE VERY MASSIVE STAR CONTENT OF THE NUCLEAR STAR CLUSTERS IN NGC 5253

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smith, L. J.; Crowther, P. A.; Calzetti, D.

2016-05-20

The blue compact dwarf galaxy NGC 5253 hosts a very young starburst containing twin nuclear star clusters, separated by a projected distance of 5 pc. One cluster (#5) coincides with the peak of the H α emission and the other (#11) with a massive ultracompact H ii region. A recent analysis of these clusters shows that they have a photometric age of 1 ± 1 Myr, in apparent contradiction with the age of 3–5 Myr inferred from the presence of Wolf-Rayet features in the cluster #5 spectrum. We examine Hubble Space Telescope ultraviolet and Very Large Telescope optical spectroscopy ofmore » #5 and show that the stellar features arise from very massive stars (VMSs), with masses greater than 100 M {sub ⊙}, at an age of 1–2 Myr. We further show that the very high ionizing flux from the nuclear clusters can only be explained if VMSs are present. We investigate the origin of the observed nitrogen enrichment in the circumcluster ionized gas and find that the excess N can be produced by massive rotating stars within the first 1 Myr. We find similarities between the NGC 5253 cluster spectrum and those of metal-poor, high-redshift galaxies. We discuss the presence of VMSs in young, star-forming galaxies at high redshift; these should be detected in rest-frame UV spectra to be obtained with the James Webb Space Telescope . We emphasize that population synthesis models with upper mass cutoffs greater than 100 M {sub ⊙} are crucial for future studies of young massive star clusters at all redshifts.« less

A model for the infrared emission from an OB star cluster environment

NASA Technical Reports Server (NTRS)

Leisawitz, D.

1991-01-01

A model for the infrared emission from the neighborhood of an OB star cluster is described. The distribution of gas and dust around the stars, properties of the dust, and the cluster and interstellar radiation fields are variable. The model can be applied to regions around clusters embedded to various degrees in their parental molecular clouds (i.e., compact H II regions, blister-type H II regions, and the tenuous H II regions ionized by naked O stars). The model is used to simulate IRAS observations of a typical blister H II region. Infrared surface brightness and spectral energy distributions are predicted and the impact of limited spatial resolution is illustrated. The model results are shown to be consistent with observations of the exemplary outer Galaxy OB cluster NGC 7380. It is planned to use the model as a diagnostic tool to probe the physical conditions and dust properties in star-formation regions and, ultimately, in an interpretation of the spectral energy distributions of spiral galaxies.

Open clusters. II. Fundamental parameters of B stars in Collinder 223, Hogg 16, NGC 2645, NGC 3114, and NGC 6025

NASA Astrophysics Data System (ADS)

Aidelman, Y.; Cidale, L. S.; Zorec, J.; Panei, J. A.

2015-05-01

Context. The knowledge of accurate values of effective temperature, surface gravity, and luminosity of stars in open clusters is very important not only to derive cluster distances and ages but also to discuss the stellar structure and evolution. Unfortunately, stellar parameters are still very scarce. Aims: Our goal is to study five open clusters to derive stellar parameters of the B and Be star population and discuss the cluster properties. In a near future, we intend to gather a statistically relevant samples of Be stars to discuss their origin and evolution. Methods: We use the Barbier-Chalonge-Divan spectrophotometric system, based on the study of low-resolution spectra around the Balmer discontinuity, since it is independent of the interstellar and circumstellar extinction and provides accurate Hertzsprung-Russell diagrams and stellar parameters. Results: We determine stellar fundamental parameters, such as effective temperatures, surface gravities, spectral types, luminosity classes, absolute and bolometric magnitudes and colour gradient excesses of the stars in the field of Collinder 223, Hogg 16, NGC 2645, NGC 3114, and NGC 6025. Additional information, mainly masses and ages of cluster stellar populations, is obtained using stellar evolution models. In most cases, stellar fundamental parameters have been derived for the first time. We also discuss the derived cluster properties of reddening, age and distance. Conclusions: Collinder 223 cluster parameters are overline{E(B-V) = 0.25 ± 0.03} mag and overline{(mv - M_v)0 = 11.21 ± 0.25} mag. In Hogg 16, we clearly distinguish two groups of stars (Hogg 16a and Hogg 16b) with very different mean true distance moduli (8.91 ± 0.26 mag and 12.51 ± 0.38 mag), mean colour excesses (0.26 ± 0.03 mag and 0.63 ± 0.08 mag), and spectral types (B early-type and B late-/A-type stars, respectively). The farthest group could be merged with Collinder 272. NGC 2645 is a young cluster (<14 Myr) with overline{E(B-V) = 0.58 ± 0.05} mag and overline{(mv - M_v)0 = 12.18 ± 0.30} mag. The cluster parameters of NGC 3114 are overline{E(B-V) = 0.10 ± 0.01} mag and overline{(mv - M_v)0 = 9.20 ± 0.15} mag. This cluster presents an important population of Be star, but it is difficult to define the cluster membership of stars because of the high contamination by field stars or the possible overlapping with a nearby cluster. Finally, we derive the following cluster parameters of NGC 6025: overline{E(B-V) = 0.34 ± 0.02} mag, overline{(mv - M_v)0 = 9.25 ± 0.17} mag, and an age between 40 Myr and 69 Myr. In all the cases, new Be candidate stars are reported based on the appearance of a second Balmer discontinuity. Observations taken at CASLEO, operating under agreement of CONICET and the Universities of La Plata, Córdoba and San Juan, Argentina.

Extreme magnification of an individual star at redshift 1.5 by a galaxy-cluster lens

NASA Astrophysics Data System (ADS)

Kelly, Patrick L.; Diego, Jose M.; Rodney, Steven; Kaiser, Nick; Broadhurst, Tom; Zitrin, Adi; Treu, Tommaso; Pérez-González, Pablo G.; Morishita, Takahiro; Jauzac, Mathilde; Selsing, Jonatan; Oguri, Masamune; Pueyo, Laurent; Ross, Timothy W.; Filippenko, Alexei V.; Smith, Nathan; Hjorth, Jens; Cenko, S. Bradley; Wang, Xin; Howell, D. Andrew; Richard, Johan; Frye, Brenda L.; Jha, Saurabh W.; Foley, Ryan J.; Norman, Colin; Bradac, Marusa; Zheng, Weikang; Brammer, Gabriel; Benito, Alberto Molino; Cava, Antonio; Christensen, Lise; de Mink, Selma E.; Graur, Or; Grillo, Claudio; Kawamata, Ryota; Kneib, Jean-Paul; Matheson, Thomas; McCully, Curtis; Nonino, Mario; Pérez-Fournon, Ismael; Riess, Adam G.; Rosati, Piero; Schmidt, Kasper Borello; Sharon, Keren; Weiner, Benjamin J.

2018-04-01

Galaxy-cluster gravitational lenses can magnify background galaxies by a total factor of up to 50. Here we report an image of an individual star at redshift z = 1.49 (dubbed MACS J1149 Lensed Star 1) magnified by more than ×2,000. A separate image, detected briefly 0.26″ from Lensed Star 1, is probably a counterimage of the first star demagnified for multiple years by an object of ≳3 solar masses in the cluster. For reasonable assumptions about the lensing system, microlensing fluctuations in the stars' light curves can yield evidence about the mass function of intracluster stars and compact objects, including binary fractions and specific stellar evolution and supernova models. Dark-matter subhaloes or massive compact objects may help to account for the two images' long-term brightness ratio.

Chemical Abundances of Two Stars in the Large Magellanic Cloud Globular Cluster NGC 1718

NASA Astrophysics Data System (ADS)

Sakari, Charli M.; McWilliam, Andrew; Wallerstein, George

2017-05-01

Detailed chemical abundances of two stars in the intermediate-age Large Magellanic Cloud (LMC) globular cluster NGC 1718 are presented, based on high-resolution spectroscopic observations with the MIKE spectrograph. The detailed abundances confirm NGC 1718 to be a fairly metal-rich cluster, with an average [Fe/H] ˜ -0.55 ± 0.01. The two red giants appear to have primordial O, Na, Mg and Al abundances, with no convincing signs of a composition difference between the two stars - hence, based on these two stars, NGC 1718 shows no evidence for hosting multiple populations. The Mg abundance is lower than Milky Way field stars, but is similar to LMC field stars at the same metallicity. The previous claims of very low [Mg/Fe] in NGC 1718 are therefore not supported in this study. Other abundances (Si, Ca, Ti, V, Mn, Ni, Cu, Rb, Y, Zr, La and Eu) all follow the LMC field star trend, demonstrating yet again that (for most elements) globular clusters trace the abundances of their host galaxy's field stars. Similar to the field stars, NGC 1718 is found to be mildly deficient in explosive α-elements, but moderately to strongly deficient in O, Na, Mg, Al and Cu, elements that form during hydrostatic burning in massive stars. NGC 1718 is also enhanced in La, suggesting that it was enriched in ejecta from metal-poor asymptotic giant branch stars.

THE PROPERTIES OF DYNAMICALLY EJECTED RUNAWAY AND HYPER-RUNAWAY STARS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perets, Hagai B.; Subr, Ladislav

2012-06-01

Runaway stars are stars observed to have large peculiar velocities. Two mechanisms are thought to contribute to the ejection of runaway stars, both of which involve binarity (or higher multiplicity). In the binary supernova scenario, a runaway star receives its velocity when its binary massive companion explodes as a supernova (SN). In the alternative dynamical ejection scenario, runaway stars are formed through gravitational interactions between stars and binaries in dense, compact clusters or cluster cores. Here we study the ejection scenario. We make use of extensive N-body simulations of massive clusters, as well as analytic arguments, in order to characterizemore » the expected ejection velocity distribution of runaway stars. We find that the ejection velocity distribution of the fastest runaways (v {approx}> 80 km s{sup -1}) depends on the binary distribution in the cluster, consistent with our analytic toy model, whereas the distribution of lower velocity runaways appears independent of the binaries' properties. For a realistic log constant distribution of binary separations, we find the velocity distribution to follow a simple power law: {Gamma}(v){proportional_to}v{sup -8/3} for the high-velocity runaways and v{sup -3/2} for the low-velocity ones. We calculate the total expected ejection rates of runaway stars from our simulated massive clusters and explore their mass function and their binarity. The mass function of runaway stars is biased toward high masses and strongly depends on their velocity. The binarity of runaways is a decreasing function of their ejection velocity, with no binaries expected to be ejected with v > 150 km s{sup -1}. We also find that hyper-runaways with velocities of hundreds of km s{sup -1} can be dynamically ejected from stellar clusters, but only at very low rates, which cannot account for a significant fraction of the observed population of hyper-velocity stars in the Galactic halo.« less

Gravitational Conundrum? Dynamical Mass Segregation versus Disruption of Binary Stars in Dense Stellar Systems

NASA Astrophysics Data System (ADS)

de Grijs, Richard; Li, Chengyuan; Zheng, Yong; Deng, Licai; Hu, Yi; Kouwenhoven, M. B. N.; Wicker, James E.

2013-03-01

Upon their formation, dynamically cool (collapsing) star clusters will, within only a few million years, achieve stellar mass segregation for stars down to a few solar masses, simply because of gravitational two-body encounters. Since binary systems are, on average, more massive than single stars, one would expect them to also rapidly mass segregate dynamically. Contrary to these expectations and based on high-resolution Hubble Space Telescope observations, we show that the compact, 15-30 Myr old Large Magellanic Cloud cluster NGC 1818 exhibits tantalizing hints at the >~ 2σ level of significance (>3σ if we assume a power-law secondary-to-primary mass-ratio distribution) of an increasing fraction of F-star binary systems (with combined masses of 1.3-1.6 M ⊙) with increasing distance from the cluster center, specifically between the inner 10''-20'' (approximately equivalent to the cluster's core and half-mass radii) and the outer 60''-80''. If confirmed, then this will offer support for the theoretically predicted but thus far unobserved dynamical disruption processes of the significant population of "soft" binary systems—with relatively low binding energies compared to the kinetic energy of their stellar members—in star clusters, which we have access to here by virtue of the cluster's unique combination of youth and high stellar density.

Westerlund 1: monolithic formation of a starburst cluster

NASA Astrophysics Data System (ADS)

Negueruela, Ignacio; Clark, J. Simon; Ritchie, Ben; Goodwin, Simon

2015-08-01

Westerlund 1 is in all likelihood the most massive young cluster in the Milky Way, with a mass on the order of 105 Msol. We have been observing its massive star population for ten years, measuring radial velocity changes for a substantial fraction of its OB stars and evolved supergiants. The properties of the evolved population are entirely consisting with a single burst of star formation, in excellent agreement with the results of studies based on the lower-mass population.Here we will present two new studies of the cluster: 1) A direct measurement of its average radial velocity and velocity dispersion based on individual measurements for several dozen stars with constant radial velocity and 2) A search for massive stars in its immediate neighbourhood using multi-object spectroscopy.The results of these two studies show that Westerlund 1 is decidedly subvirial and has a systemic radial velocity significantly different from that of nearby gas, which was assumed to provide a dynamical distance by previous authors. Moreover, the dynamical distance is inconsistent with the properties of the high-mass stellar population. In addition, we find that the cluster is completely isolated, with hardly any massive star in its vicinity that could be associated in terms of distance modulus or radial velocity. The cluster halo does not extend much further than five parsec away from the centre. All these properties are very unusual among starburst clusters in the Local Universe, which tend to form in the context of large star-forming regions.Westerlund 1 is thus the best example we have of a starburst cluster formed monolithically.

The Solar-Type Hard-Binary Frequency and Distributions of Orbital Parameters in the Open Cluster M37

NASA Astrophysics Data System (ADS)

Geller, Aaron M.; Meibom, Soren; Barnes, Sydney A.; Mathieu, Robert D.

2014-02-01

Binary stars, and particularly the short-period ``hard'' binaries, govern the dynamical evolution of star clusters and determine the formation rates and mechanisms for exotic stars like blue stragglers and X-ray sources. Understanding the near-primordial hard-binary population of star clusters is of primary importance for dynamical models of star clusters, which have the potential to greatly advance our understanding of star cluster evolution. Yet the binary frequencies and distributions of binary orbital parameters (period, eccentricity, etc.) for young coeval stellar populations are poorly known, due to a lack of necessary observations. The young (~540 Myr) open cluster M37 hosts a rich binary population that can be used to empirically define these initial conditions. Importantly, this cluster has been the target of a comprehensive WIYN/Hydra radial-velocity (RV) survey, from which we have already identified a nearly complete sample of 329 solar-type (1.5 <=M [M_⊙] <=1.0) members in M37. Of these stars, 82 show significant RV variability, indicative of a binary companion. We propose to build upon these data with a multi-epoch RV survey using WIYN/Hydra to derive kinematic orbital solutions for these 82 binaries in M37. This project was granted time in 2013B and scheduled for later this year. We anticipate that about half of the detected binaries in M37 will acquire enough RV measurements (>=10) in 2013B to begin searching for orbital solutions. With this proposal and perhaps one additional semester we should achieve >=10 RV measurements for the remaining binaries.

X-ray studies of coeval star samples. II. The Pleiades cluster as observed with the Einstein Observatory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Micela, G.; Sciortino, S.; Vaiana, G.S.

1990-01-01

Coronal X-ray emission of the Pleiades stars is investigated, and maximum likelihood, integral X-ray luminosity functions are computed for Pleiades members in selected color-index ranges. A detailed search is conducted for long-term variability in the X-ray emission of those stars observed more than once. An overall comparison of the survey results with those of previous surveys confirms the ubiquity of X-ray emission in the Pleiades cluster stars and its higher rate of emission with respect to older stars. It is found that the X-ray emission from dA and early dF stars cannot be proven to be dissimilar to that ofmore » Hyades and field stars of the same spectral type. The Pleiades cluster members show a real rise of the X-ray luminosity from dA stars to early dF stars. X-ray emission for the young, solarlike Pleiades stars is about two orders of magnitude more intense than for the nearby solarlike stars. 77 refs.« less

Wide-Field Variability Survey of the Globular Cluster M 79 and a New Period-Luminosity Relation for SX Phe Stars

NASA Astrophysics Data System (ADS)

Kopacki, G.

2015-03-01

We present the results of a search for variable stars in a 26×39 arcmin2 field around globular cluster M 79 (NGC 1904). The search was made by means of an extended version of image subtraction, which allows us to analyze in a uniform manner CCD frames obtained with different telescopes and cameras of different sizes and resolutions. The search resulted in finding 20 new variable stars, among which 13 are cluster members. The members include one new RR Lyr star of subtype c, three SX Phe stars, and nine variable red giants. We also show that V7 is a W Vir star with a period of 13.985 d. Revised mean periods of RRab and RRc stars, ab=0.71 d and c=0.34 d, respectively, and relative percentage of RRc stars, Nc/(Nab+Nc)=45% confirm that M 79 belongs to the Oosterhoff II group of globular clusters. The mean V magnitude of the horizontal branch of M 79 based on ten RR Lyr stars has been estimated to be VHB=RR=16.11±0.03 mag. In one RRc star, V9, light changes with three close frequencies were detected, indicating excitation of nonradial modes. An SX Phe star, V18, is a double-mode pulsator with two radial modes excited, fundamental and first overtone. Moreover, we have discovered two SX Phe or δ Sct stars and one W UMa type system, all likely field objects. We also studied the period-luminosity relation for SX Phe stars. Using 62 fundamental and fundamentalized periods of radial double-mode and high-amplitude SX Phe stars known in Galactic globular clusters, we have derived the slope and zero point of this relation to be, -3.3±0.27 and 2.68±0.03 mag (at log(P/d)=-1.24), respectively.

From the sun to the Galactic Center: dust, stars and black hole(s)

NASA Astrophysics Data System (ADS)

Fritz, Tobias

2013-07-01

The centers of galaxies are their own ultimate gravitational sinks. Massive black holes and star clusters as well as gas are especially likely to fall into the centers of galaxies by dynamical friction or dissipation. Many galactic centers harbor supermassive black holes (SMBH) and dense nuclear (star) clusters which possibly arrived there by these processes. Nuclear clusters can be formed in situ from gas, or from smaller star clusters which fall to the center. Since the Milky Way harbors both an SMBH and a nuclear cluster, both can be studied best in the Galactic Center (GC), which is the closest galactic nucleus to us. In Chapter 1, I introduce the different components of the Milky Way, and put these into the context of the GC. I then give an overview of relevant properties (e.g. star content and distribution) of the GC. Afterwards, I report the results of four different studies about the GC. In Chapter 2, I analyze the limitations of astrometry, one of the most useful methods for the study of the GC. Thanks to the high density of stars and its relatively small distance from us it is possible to measure the motions of thousands of stars in the GC with images, separated by few years only. I find two main limitations to this method: (1) for bright stars the not perfectly correctable distortion of the camera limits the accuracy, and (2) for the majority of the fainter stars, the main limitation is crowding from the other stars in the GC. The position uncertainty of faint stars is mainly caused by the seeing halos of bright stars. In the very center faint unresolvable stars are also important for the position uncertainty. In Chapter 3, I evaluate the evidence for an intermediate mass black hole in the small candidate cluster IRS13E within the GC. Intermediate mass black holes (IMBHs) have a mass between the two types of confirmed black hole: the stellar remnants and the supermassive black holes in the centers of galaxies. One possibility for! their formation is the collision of stars in a dense young st! ar cluster. Such a cluster could sink to the GC by dynamical friction. There it would consist of few bright stars like IRS13E. Firstly, I analyze the SEDs of the objects in IRS13E. The SEDs of most objects can be explained by pure dust emission. Thus, most objects in IRS13E are pure dust clumps and only three young stars. This reduces the significance of the 'cluster' IRS13E compared to the stellar background. Secondly, I obtain acceleration limits for these three stars. The non-detection of accelerations makes an IMBH an unlikely scenario in IRS13E. However, since its three stars form a comoving association, which is unlikely to form by chance, the nature of IRS13E is not yet settled. In the third study (Chapter 4) I measure and analyze the extinction curve toward the GC. The extinction is a contaminant for GC observations and therefore it is necessary to know the extinction toward the GC to determine the luminosity properties of its stars. I obtain the extinction curve by measuring the flux of the HII region in the GC in several infrared HII lines and in the unextincted radio continuum. I compare these ratios with the ratios expected from recombination physics and obtain extinctions at 22 different lines between 1 and 19 micron. For the K-band I derive A_Ks=2.62+/-0.11. The extinction curve follows a power law with a steep slope of -2.11+/-0.06 shortward of 2.8 micron. At longer wavelengths the extinction is grayer and there are absorption features from ices. The extinction curve is a tool to constrain the properties of cosmic dust between the sun and the GC. The extinction curve cannot be explained by dust grains consisting of carbonaceous and silicate grains only. In addition composite particles, which also contain ices are necessary to fit the extinction curve. In the final part of this thesis (Chapter 5) I look at the properties of most of the stars in the GC. These are the old stars that form the nuclear cluster of the Milky Way. I obtain the mass distribution and the light distribution of these stars. I ! find that the flattening of the stellar distribution increases outside 70''. This indicates that inside a nearly spherical nuclear cluster dominates and that the surrounding light belongs mostly to the nuclear disk. I dissect the light in two components and obtain for the nuclear cluster L_Ks=2.7*10^7 L_sun. I obtain proper motions for more than 10000 stars and radial velocities for more than 2400 stars. Using Jeans modeling I combine velocities and the radial profile to obtain within 100'' (4 pc) a mass of 6.02*10^6 M_sun and a total nuclear cluster mass of 12.88*10^6 M_sun. The Jeans modeling and various other evidence weakly favor a core in the extended mass compared to a cusp. The old star light shows a similar core. The mass to light ratio of the old stars of the nuclear cluster is consistent with the usual initial mass function in the Galaxy. This suggests that most stars in GC formed in the usual way, in a mode different from the origin of the youngest stars there.

The fate of NGC602, an intense region of star-formation in the Wing of the SMC

NASA Astrophysics Data System (ADS)

Sabbi, Elena

2017-08-01

This is a small 2 orbit proposal designed to measure the internal dynamics of NGC602, a small region of intense star formation in the Wing of the SMC, with a low gas and dust density that has been often considered an unfavorable place for star formation. Small regions of massive star formation are important to study for our understanding of the process of star and cluster formation, the ionization of the interstellar medium, and the injection of energy and momentum into their host galaxy. By combining our new observations with archival ACS/WFC data acquired in July 2004, we will be able to measure the relative proper motions of the NGC602 sub-structures better than 2.3 km/s and investigate the nature of the apparently isolated massive stars found around NGC602. This study will provide unique observational data to characterize the early phase of cluster evolution and test cluster formation theories. It will also address significant open issues in star formation, cluster dynamics and the origin of isolated supernovae and GRBs.

New red giant star in the Kepler open cluster NGC 6819

NASA Astrophysics Data System (ADS)

Komucyeya, E.; Abedigamba, O. P.; Jurua, E.; Anguma, S. K.

2018-05-01

A recent study indicated that 39 red giant stars showing solar-like oscillations were discovered in the field of Kepleropen cluster NGC 6819. The study was based on photometric distance estimates of 27 stars out of the 39. Using photometric method alone may not be adequate to confirm the membership of these stars. The stars were not previously known in literature to belong to the open cluster NGC 6819. In this study, Kepler data was used to study the membership of the 27 stars. A plot of apparent magnitude as a function of the large frequency separation, supplemented with the proper motion and radial velocity values from literature revealed KIC 5112840 to lie on the same plane with the well known members of the cluster. Echelle diagram was constructed, and the median gravity-mode period spacings (ΔP) calculated for KIC 5112840. A value of ΔP = 66.3 s was obtained, thus placing the red giant star KIC 5112840 on the Red Giant Branch stage of evolution. Our evolutionary status result using the approach in this paper is in agreement with what is in the available literature.

A heavy-metal home

NASA Image and Video Library

2016-05-30

This 10.5-billion-year-old globular cluster, NGC 6496, is home to heavy-metal stars of a celestial kind! The stars comprising this spectacular spherical cluster are enriched with much higher proportions of metals — elements heavier than hydrogen and helium, are in astronomy curiously known as metals — than stars found in similar clusters. A handful of these high-metallicity stars are also variable stars, meaning that their brightness fluctuates over time. NGC 6496 hosts a selection of long-period variables — giant pulsating stars whose brightness can take up to, and even over, a thousand days to change — and short-period eclipsing binaries, which dim when eclipsed by a stellar companion. The nature of the variability of these stars can reveal important information about their mass, radius, luminosity, temperature, composition, and evolution, providing astronomers with measurements that would be difficult or even impossible to obtain through other methods. NGC 6496 was discovered in 1826 by Scottish astronomer James Dunlop. The cluster resides at about 35 000 light-years away in the southern constellation of Scorpius (The Scorpion).

Star formation in globular clusters and dwarf galaxies and implications for the early evolution of galaxies

NASA Technical Reports Server (NTRS)

Lin, Douglas N. C.; Murray, Stephen D.

1991-01-01

Based upon the observed properties of globular clusters and dwarf galaxies in the Local Group, we present important theoretical constraints on star formation in these systems. These constraints indicate that protoglobular cluster clouds had long dormant periods and a brief epoch of violent star formation. Collisions between protocluster clouds triggered fragmentation into individual stars. Most protocluster clouds dispersed into the Galactic halo during the star formation epoch. In contrast, the large spread in stellar metallicity in dwarf galaxies suggests that star formation in their pregenitors was self-regulated: we propose the protocluster clouds formed from thermal instability in the protogalactic clouds and show that a population of massive stars is needed to provide sufficient UV flux to prevent the collapsing protogalactic clouds from fragmenting into individual stars. Based upon these constraints, we propose a unified scenario to describe the early epochs of star formation in the Galactic halo as well as the thick and thin components of the Galactic disk.

X-RAY BINARIES AND STAR CLUSTERS IN THE ANTENNAE: OPTICAL CLUSTER COUNTERPARTS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rangelov, Blagoy; Chandar, Rupali; Prestwich, Andrea

2012-10-20

We compare the locations of 82 X-ray binaries (XRBs) detected in the merging Antennae galaxies by Zezas et al., based on observations taken with the Chandra X-Ray Observatory, with a catalog of optically selected star clusters presented by Whitmore et al., based on observations taken with the Hubble Space Telescope. Within the 2{sigma} positional uncertainty of Almost-Equal-To 0.''8, we find 22 XRBs are coincident with star clusters, where only two to three chance coincidences are expected. The ages of the clusters were estimated by comparing their UBVI, H{alpha} colors with predictions from stellar evolutionary models. We find that 14 ofmore » the 22 coincident XRBs (64%) are hosted by star clusters with ages of Almost-Equal-To 6 Myr or less. All of the very young host clusters are fairly massive and have M {approx}> 3 Multiplication-Sign 10{sup 4} M {sub Sun }, with many having masses M Almost-Equal-To 10{sup 5} M {sub Sun }. Five of the XRBs are hosted by young clusters with ages {tau} Almost-Equal-To 10-100 Myr, while three are hosted by intermediate-age clusters with {tau} Almost-Equal-To 100-300 Myr. Based on the results from recent N-body simulations, which suggest that black holes are far more likely to be retained within their parent clusters than neutron stars, we suggest that our sample consists primarily of black hole binaries with different ages.« less

Integrated Light Chemical Abundance Analyses of 7 M31 Outer Halo Globular Clusters from the Pan-Andromeda Archaeological Survey

NASA Astrophysics Data System (ADS)

Sakari, Charli; Venn, Kim; Mackey, Dougal; Shetrone, Matthew D.; Dotter, Aaron L.; Wallerstein, George

2015-01-01

Detailed chemical abundances of globular clusters provide insight into the formation and evolution of galaxies and their globular cluster systems. This talk presents detailed chemical abundances for seven M31 outer halo globular clusters (with projected radii greater than 30 kpc), as derived from high resolution integrated light spectra. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS). The integrated abundances show that 4 of these clusters are metal-poor ([Fe/H] < -1.5) while the other 3 are more metal-rich. The most metal-poor globular clusters are α-enhanced, though 3 of the 4 are possibly less α-enhanced than MW stars (at the 1σ level). Other chemical abundance ratios ([Ba/Eu], [Eu/Ca], and [Ni/Fe]) are consistent with origins in low mass dwarf galaxies (similar to Fornax). The most metal-rich cluster ([Fe/H] ~ -1) stands out as being chemically distinct from Milky Way field stars of the same metallicity---its chemical abundance ratios agree best with the stars and clusters in the Large Magellanic Cloud (LMC) and the Sagittarius dwarf spheroidal (Sgr) than with the Milky Way field stars. The other metal-rich clusters, H10 and H23, look similar to the LMC and Milky Way field stars in all abundance ratios. These results indicate that M31's outer halo is being at least partially built up by the accretion of dwarf satellites, in agreement with previous observations.

New asteroseismic scaling relations based on the Hayashi track relation applied to red giant branch stars in NGC 6791 and NGC 6819

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, T.; Li, Y.; Hekker, S., E-mail: wutao@ynao.ac.cn, E-mail: ly@ynao.ac.cn, E-mail: hekker@mps.mpg.de

2014-01-20

Stellar mass M, radius R, and gravity g are important basic parameters in stellar physics. Accurate values for these parameters can be obtained from the gravitational interaction between stars in multiple systems or from asteroseismology. Stars in a cluster are thought to be formed coevally from the same interstellar cloud of gas and dust. The cluster members are therefore expected to have some properties in common. These common properties strengthen our ability to constrain stellar models and asteroseismically derived M, R, and g when tested against an ensemble of cluster stars. Here we derive new scaling relations based on amore » relation for stars on the Hayashi track (√(T{sub eff})∼g{sup p}R{sup q}) to determine the masses and metallicities of red giant branch stars in open clusters NGC 6791 and NGC 6819 from the global oscillation parameters Δν (the large frequency separation) and ν{sub max} (frequency of maximum oscillation power). The Δν and ν{sub max} values are derived from Kepler observations. From the analysis of these new relations we derive: (1) direct observational evidence that the masses of red giant branch stars in a cluster are the same within their uncertainties, (2) new methods to derive M and z of the cluster in a self-consistent way from Δν and ν{sub max}, with lower intrinsic uncertainties, and (3) the mass dependence in the Δν - ν{sub max} relation for red giant branch stars.« less

Deep CO(1-0) Observations of z = 1.62 Cluster Galaxies with Substantial Molecular Gas Reservoirs and Normal Star Formation Efficiencies

NASA Astrophysics Data System (ADS)

Rudnick, Gregory; Hodge, Jacqueline; Walter, Fabian; Momcheva, Ivelina; Tran, Kim-Vy; Papovich, Casey; da Cunha, Elisabete; Decarli, Roberto; Saintonge, Amelie; Willmer, Christopher; Lotz, Jennifer; Lentati, Lindley

2017-11-01

We present an extremely deep CO(1-0) observation of a confirmed z = 1.62 galaxy cluster. We detect two spectroscopically confirmed cluster members in CO(1-0) with signal-to-noise ratio > 5. Both galaxies have log({{ M }}\\star /{{ M }}⊙ ) > 11 and are gas rich, with {{ M }}{mol}/({{ M }}\\star +{{ M }}{mol}) ˜ 0.17-0.45. One of these galaxies lies on the star formation rate (SFR)-{{ M }}\\star sequence, while the other lies an order of magnitude below. We compare the cluster galaxies to other SFR-selected galaxies with CO measurements and find that they have CO luminosities consistent with expectations given their infrared luminosities. We also find that they have gas fractions and star formation efficiencies (SFE) comparable to what is expected from published field galaxy scaling relations. The galaxies are compact in their stellar light distribution, at the extreme end for all high-redshift star-forming galaxies. However, their SFE is consistent with other field galaxies at comparable compactness. This is similar to two other sources selected in a blind CO survey of the HDF-N. Despite living in a highly quenched protocluster core, the molecular gas properties of these two galaxies, one of which may be in the process of quenching, appear entirely consistent with field scaling relations between the molecular gas content, stellar mass, star formation rate, and redshift. We speculate that these cluster galaxies cannot have any further substantive gas accretion if they are to become members of the dominant passive population in z< 1 clusters.

FIRST OBSERVATIONAL SIGNATURE OF ROTATIONAL DECELERATION IN A MASSIVE, INTERMEDIATE-AGE STAR CLUSTER IN THE MAGELLANIC CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Xiaohan; Li, Chengyuan; De Grijs, Richard

While the extended main-sequence turnoffs (eMSTOs) found in almost all 1–2 Gyr old star clusters in the Magellanic Clouds are often explained by postulating extended star formation histories (SFHs), the tight subgiant branches (SGBs) seen in some clusters challenge this popular scenario. Puzzlingly, the SGB of the eMSTO cluster NGC 419 is significantly broader at bluer than at redder colors. We carefully assess and confirm the reality of this observational trend. If we would assume that the widths of the features in color–magnitude space were entirely owing to a range in stellar ages, the SFHs of the eMSTO stars andmore » the blue SGB region would be significantly more prolonged than that of the red part of the SGB. This cannot be explained by assuming an internal age spread. We show that rotational deceleration of a population of rapidly rotating stars, a currently hotly debated alternative scenario, naturally explains the observed trend along the SGB. Our analysis shows that a “converging” SGB could be produced if the cluster is mostly composed of rapidly rotating stars that slow down over time owing to the conservation of angular momentum during their evolutionary expansion from main-sequence turnoff stars to red giants.« less

On the Stellar Population and Star-Forming History of the Orion Nebula Cluster

NASA Astrophysics Data System (ADS)

Hillenbrand, Lynne A.

1997-05-01

We report on the first phase of a study of the stellar population comprising the Orion Nebula Cluster (ONC). Approximately 50% of the ~ 3500 stars identified to date within ~ 2.5 pc of the namesake Trapezium stars are optically visible, and in this paper we focus on that sample with I < 17.5 mag. The large number and number density (npeak > 10(4) pc(-3) ) of stars, the wide range in stellar mass ( ~ 0.1-50 M_⊙), and the extreme youth (< 1-2 Myr) of the stellar population, make the ONC the best site for investigating: 1) the detailed shape of a truly ``initial'' mass spectrum; 2) the apparent age spread in a region thought to have undergone triggered star formation; 3) the time sequence of star formation as a function of stellar mass; and 4) trends of all of the above with cluster radius. Nearly 60% of the ~ 1600 optical stars have sufficient data (spectroscopy and photometry) for placement on a theoretical HR diagram; this subsample is unbiased with respect to apparent brightness or cluster radius, complete down to ~ 1 M_⊙, and representative of the total optical sample below ~ 1 M_⊙ for the age and extinction ranges characteristic of the cluster. Comparison of the derived HR diagram with traditional pre-main sequence evolutionary calculations shows a trend of increasing stellar age with increasing stellar mass. To avoid the implication of earlier characteristic formation times for higher-mass stars than for lower-mass stars, refinement of early evolutionary theory in a manner similar to the birthline hypothesis of Palla & Stahler (1993), is required. Subject to uncertainties in the tracks and isochrones, we can still investigate stellar mass and age distributions in the ONC. We find the ONC as a whole to be characterized by a mass spectrum which is not grossly inconsistent with ``standard'' stellar mass spectra. In particular, although there are structural differences between the detailed ONC mass spectrum and various models constructed from solar neighborhood data, the observed mass spectrum appears to a peak at ~ 0.2 M_⊙ and to fall off rapidly towards lower masses; several substellar objects are present. The abundance of low-mass stars relative to high-mass stars suggests that there is no bi-modal star formation mode; somewhat ironically, the ONC probably contains fractionally more low-mass stars than the solar neighborhood since the population not yet located on the HR diagram is dominated by sub-solar-mass stars. Nonetheless, the ONC mass spectrum is biased towards higher-mass stars within the innermost cluster radii (rprojected < 0.3 pc). We find the ONC as a whole to be characterized by a mean age of < 1 Myr and an age spread which is probably less than 2 Myr, but also by a bias towards younger stars at smaller projected cluster radii. Although the most massive stars and the youngest stars are found preferentially towards the center of the ONC it does not follow that the most massive stars are the youngest stars. A lower limit to the total cluster mass in stars is Mstars ~ 900 M_⊙ (probably a factor of < 2 underestimate). A lower limit to the recent star formation rate is ~ 10(-4) M_⊙ yr(-1) . All observational data in this study as well as stellar parameters derived from them are available in electronic format.

HUBBLE'S PANORAMIC PORTRAIT OF A VAST STAR-FORMING REGION

NASA Technical Reports Server (NTRS)

2002-01-01

NASA's Hubble Space Telescope has snapped a panoramic portrait of a vast, sculpted landscape of gas and dust where thousands of stars are being born. This fertile star-forming region, called the 30 Doradus Nebula, has a sparkling stellar centerpiece: the most spectacular cluster of massive stars in our cosmic neighborhood of about 25 galaxies. The mosaic picture shows that ultraviolet radiation and high-speed material unleashed by the stars in the cluster, called R136 [the large blue blob left of center], are weaving a tapestry of creation and destruction, triggering the collapse of looming gas and dust clouds and forming pillar-like structures that are incubators for nascent stars. The photo offers an unprecedented, detailed view of the entire inner region of 30 Doradus, measuring 200 light-years wide by 150 light-years high. The nebula resides in the Large Magellanic Cloud (a satellite galaxy of the Milky Way), 170,000 light-years from Earth. Nebulas like 30 Doradus are the 'signposts' of recent star birth. High-energy ultraviolet radiation from the young, hot, massive stars in R136 causes the surrounding gaseous material to glow. Previous Hubble telescope observations showed that R136 contains several dozen of the most massive stars known, each about 100 times the mass of the Sun and about 10 times as hot. These stellar behemoths all formed at the same time about 2 million years ago. The stars in R136 are producing intense 'stellar winds' (streams of material traveling at several million miles an hour), which are wreaking havoc on the gas and dust in the surrounding neighborhood. The winds are pushing the gas away from the cluster and compressing the inner regions of the surrounding gas and dust clouds [the pinkish material]. The intense pressure is triggering the collapse of parts of the clouds, producing a new generation of star formation around the central cluster. The new stellar nursery is about 30 to 50 light-years from R136. Most of the stars in the nursery are not visible because they are still encased in their cocoons of gas and dust. Some of the nascent stars are forming in long columns of gas and dust. Previous Hubble observations revealed that the process of 'triggered' star formation often involves massive pillars of material that point toward the central cluster. Such pillars form when particularly dense clouds of gas and dust shield columns of material behind them from the blistering radiation and strong winds released by massive stars, like the stars in R136. This protected material becomes the pillars where stars can form and grow. The Hubble telescope first spied these pillars of stellar creation when it captured close-up views of the Eagle Nebula. The new image of 30 Doradus shows numerous pillars -- each about several light-years long -- oriented toward the central cluster. These pillars, which resemble tiny fingers, are similar in size to those in the Eagle Nebula. Without Hubble's resolution, they would not be visible. One pillar is visible within the oval-shaped structure to the left of the cluster. Two [one dark and one bright] are next to each other below and to the right of the cluster. One pillar is at upper right, and still another is just above the cluster. Newborn stars within most of these pillars already have been discovered in pictures taken by Hubble's infrared camera, the Near Infrared Camera and Multi-Object Spectrometer, which can penetrate the dust to detect embryonic stars. Eventually, intense radiation and stellar winds from the developing stars will blow off the tops of the pillars. The Hubble image shows that one such eruption already has occurred in 30 Doradus. A trio of young stars has just been 'born' by breaking out of its natal pillar. These new stars are just a few hundred thousand years old. In another 2 million years, the new generation of stars will be in full bloom. But the massive stars in R136 will have burned themselves out. And the nebula's central region will be a giant shell, devoid of gas and dust. Still later, all of the most massive stars and gas will have disappeared from the entire region. Only older, less massive stars will remain in a region cleared of gas and dust. The mosaic image of 30 Doradus consists of five overlapping pictures taken between January 1994 and September 2000 by Hubble's Wide Field and Planetary Camera 2. Several color filters were used to enhance important details in the stars and the nebula. Blue corresponds to the hot stars. The greenish color denotes hot gas energized by the central cluster of stars. Pink depicts the glowing edges of the gas and dust clouds facing the cluster, which are being bombarded by winds and radiation. Reddish-brown represents the cooler surfaces of the clouds, which are not receiving direct radiation from the central cluster. Credits: NASA, N. Walborn and J. Ma`iz-Apell`aniz (Space Telescope Science Institute, Baltimore, MD), R. Barb`a (La Plata Observatory, La Plata, Argentina)

Red giants and yellow stragglers in the young open cluster NGC 2447

NASA Astrophysics Data System (ADS)

da Silveira, M. D.; Pereira, C. B.; Drake, N. A.

2018-06-01

In this work we analysed, using high-resolution spectroscopy, a sample of 12 single and 4 spectroscopic binary stars of the open cluster NGC 2447. For the single stars, we obtained atmospheric parameters and chemical abundances of Li, C, N, O, Na, Mg, Al, Ca, Si, Ti, Ni, Cr, Y, Zr, La, Ce, Nd, Eu. Rotational velocities were obtained for all the stars. The abundances of the light elements and Eu and the rotational velocities were derived using spectral synthesis technique. We obtained a mean metallicity of [Fe/H] = -0.17 ± 0.05. We found that the abundances of all elements are similar to field giants and/or giants of open clusters, even for the s-process elements, which are enhanced as in other young open clusters. We show that the spectroscopic binaries NGC 2447-26, 38, and 42 are yellow-straggler stars, of which the primary is a giant star and the secondary a main-sequence A-type star.

Starburst Cluster Shows Celestial Fireworks

NASA Image and Video Library

2017-12-08

NASA image release June 6, 2010 Like a July 4 fireworks display a young, glittering collection of stars looks like an aerial burst. The cluster is surrounded by clouds of interstellar gas and dust - the raw material for new star formation. The nebula, located 20,000 light-years away in the constellation Carina, contains a central cluster of huge, hot stars, called NGC 3603. This environment is not as peaceful as it looks. Ultraviolet radiation and violent stellar winds have blown out an enormous cavity in the gas and dust enveloping the cluster, providing an unobstructed view of the cluster. Most of the stars in the cluster were born around the same time but differ in size, mass, temperature, and color. The course of a star's life is determined by its mass, so a cluster of a given age will contain stars in various stages of their lives, giving an opportunity for detailed analyses of stellar life cycles. NGC 3603 also contains some of the most massive stars known. These huge stars live fast and die young, burning through their hydrogen fuel quickly and ultimately ending their lives in supernova explosions. Star clusters like NGC 3603 provide important clues to understanding the origin of massive star formation in the early, distant universe. Astronomers also use massive clusters to study distant starbursts that occur when galaxies collide, igniting a flurry of star formation. The proximity of NGC 3603 makes it an excellent lab for studying such distant and momentous events. This Hubble Space Telescope image was captured in August 2009 and December 2009 with the Wide Field Camera 3 in both visible and infrared light, which trace the glow of sulfur, hydrogen, and iron. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI) conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc. in Washington, D.C. Credit: NASA, ESA, R. O'Connell (University of Virginia), F. Paresce (National Institute for Astrophysics, Bologna, Italy), E. Young (Universities Space Research Association/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA) NASA Goddard Space Flight Center is home to the nation's largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

Kinematic evidence for feedback-driven star formation in NGC 1893

NASA Astrophysics Data System (ADS)

Lim, Beomdu; Sung, Hwankyung; Bessell, Michael S.; Lee, Sangwoo; Lee, Jae Joon; Oh, Heeyoung; Hwang, Narae; Park, Byeong-Gon; Hur, Hyeonoh; Hong, Kyeongsoo; Park, Sunkyung

2018-06-01

OB associations are the prevailing star-forming sites in the Galaxy. Up to now, the process of how OB associations were formed remained a mystery. A possible process is self-regulating star formation driven by feedback from massive stars. However, although a number of observational studies uncovered various signposts of feedback-driven star formation, the effectiveness of such feedback has been questioned. Stellar and gas kinematics is a promising tool to capture the relative motion of newborn stars and gas away from ionizing sources. We present high-resolution spectroscopy of stars and gas in the young open cluster NGC 1893. Our findings show that newborn stars and the tadpole nebula Sim 130 are moving away from the central cluster containing two O-type stars, and that the time-scale of sequential star formation is about 1 Myr within a 9 pc distance. The newborn stars formed by feedback from massive stars account for at least 18 per cent of the total stellar population in the cluster, suggesting that this process can play an important role in the formation of OB associations. These results support the self-regulating star formation model.

The mass-ratio and eccentricity distributions of barium and S stars, and red giants in open clusters

NASA Astrophysics Data System (ADS)

Van der Swaelmen, M.; Boffin, H. M. J.; Jorissen, A.; Van Eck, S.

2017-01-01

Context. A complete set of orbital parameters for barium stars, including the longest orbits, has recently been obtained thanks to a radial-velocity monitoring with the HERMES spectrograph installed on the Flemish Mercator telescope. Barium stars are supposed to belong to post-mass-transfer systems. Aims: In order to identify diagnostics distinguishing between pre- and post-mass-transfer systems, the properties of barium stars (more precisely their mass-function distribution and their period-eccentricity (P-e) diagram) are compared to those of binary red giants in open clusters. As a side product, we aim to identify possible post-mass-transfer systems among the cluster giants from the presence of s-process overabundances. We investigate the relation between the s-process enrichment, the location in the (P-e) diagram, and the cluster metallicity and turn-off mass. Methods: To invert the mass-function distribution and derive the mass-ratio distribution, we used the method pioneered by Boffin et al. (1992) that relies on a Richardson-Lucy deconvolution algorithm. The derivation of s-process abundances in the open-cluster giants was performed through spectral synthesis with MARCS model atmospheres. Results: A fraction of 22% of post-mass-transfer systems is found among the cluster binary giants (with companion masses between 0.58 and 0.87 M⊙, typical for white dwarfs), and these systems occupy a wider area than barium stars in the (P-e) diagram. Barium stars have on average lower eccentricities at a given orbital period. When the sample of binary giant stars in clusters is restricted to the subsample of systems occupying the same locus as the barium stars in the (P-e) diagram, and with a mass function compatible with a WD companion, 33% (=4/12) show a chemical signature of mass transfer in the form of s-process overabundances (from rather moderate - about 0.3 dex - to more extreme - about 1 dex). The only strong barium star in our sample is found in the cluster with the lowest metallicity in the sample (I.e. star 173 in NGC 2420, with [Fe/H] = -0.26), whereas the barium stars with mild s-process abundance anomalies (from 0.25 to 0.6 dex) are found in the clusters with slightly subsolar metallicities. Our finding confirms the classical prediction that the s-process nucleosynthesis is more efficient at low metallicities, since the s-process overabundance is not clearly correlated with the cluster turn-off (TO) mass; such a correlation would instead hint at the importance of the dilution factor. We also find a mild barium star in NGC 2335, a cluster with a large TO mass of 4.3 M⊙, which implies that asymptotic giant branch stars that massive still operate the s-process and the third dredge-up. Based on observations made with the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, and on observations made with the HARPS spectrograph installed on the 3.6 m telescope at the European Southern Observatory.

The Formation of Filamentary Structures in Radiative Cluster Winds

NASA Astrophysics Data System (ADS)

Rodríguez-González, Ary; Esquivel, Alejandro; Raga, Alejandro C.; Cantó, Jorge

We explore the dynamics of a "cluster wind" flow in the regime in which the shocks resulting from the interaction of winds from nearby stars are radiative. We show that for a cluster with low-intermedia mass stars, the wind interactions are indeed likely to be radiative. We then compute three dimensional, radiative simulations of a cluster of 75 young stars, exploring the effects of varying the wind parameters and the density of the initial ISM that permeates the volume of the cluster. These simulations show that the ISM is compressed by the action of the winds into a structure of dense knots and filaments.

Massive Stars and Star Clusters in the Era of JWST

NASA Astrophysics Data System (ADS)

Klein, Richard

Massive stars lie at the center of the web of physical processes that has shaped the universe as we know it, governing the evolution of the interstellar medium of galaxies, producing a majority of the heavy elements, and thereby determining the evolution of galaxies. Massive stars are also important as signposts, since they produce most of the light and almost all the ionizing radiation in regions of active star formation. A significant fraction of all stars form in massive clusters, which will be observable throughout the visible universe with JWST. Their luminosities are so high that the pressure of their light on interstellar dust grains is likely the dominant feedback mechanism regulating their formation. While this process has been studied in the local Universe, much less attention has been focused on how it behaves at high redshift, where the dust abundance is much lower due to the overall lower abundance of heavy elements. The high redshift Universe also differs from the nearby one in that observations imply that high redshift star formation occurs at significantly higher densities than are typically found locally. We propose to simulate the formation of individual massive stars from the high redshift universe to the present day universe spanning metallicities ranging from 0.001 to 1.0 and column densities from 0.1to 30.0 g/cm2 focusing on how the process depends on both the dust abundance and on the density of the star-forming gas. These simulations will be among the first to treat the formation of Population II stars, which form in regions of low metallicity. Based on these results, we shall then simulate the formation of clusters of stars across also cosmic time, both of moderate mass, such as the Orion Nebula Cluster, and of high mass, such as the super star clusters seen in starburst galaxies. These state-of-the-art simulations will be carried out using our newly developed advanced techniques in our radiation-magneto-hydrodynamic AMR code ORION, for radiative transfer with both ionizing and non-ionizing radiation that accurately handle both the direct radiation from stars and the diffuse infrared radiation field that builds up when direct radiation is reprocessed by dust grains. Our simulations include all of the relevant feedback effects such as radiative heating, radiation pressure, photodissociation and photoionization, protostellar outflows and stellar winds. The challenge in simulating the formation of massive stars and massive clusters is to include all these feedback effects self-consistently as they occur collectively. We are in an excellent position to do so. The results of these simulations will be directly relevant to the interpretation of observations with JWST, which will probe cluster formation in both the nearby and distant universe, and with SOFIA, which can observe high-mass star formation in the Galaxy. We shall make direct comparison with observations of massive protostars in the Galactic disk. We shall also compare with observations of star clusters that form in dense environments, such as the Galactic Center and in merging galaxies (e.g., the Antennae), and in low metallicity environments, such as the dwarf starburst galaxy I Zw 18. Once our simulations have been benchmarked with observations of massive protostars in the Galaxy and massive protoclusters in the local universe, they will provide the theoretical basis for interpreting observations of the formation of massive star clusters at high redshift with JWST. What determines the maximum mass of a star? How does stellar feedback affect the formation of individual stars and the formation of massive star clusters and how the answers to these questions evolve with cosmic time. The proposed research will provide high-resolution input to the study of stellar feedback on galaxy formation with a significantly more accurate treatment of the physics, particularly the radiative transfer that is so important for feedback.

On star formation in stellar systems. I - Photoionization effects in protoglobular clusters

NASA Technical Reports Server (NTRS)

Tenorio-Tagle, G.; Bodenheimer, P.; Lin, D. N. C.; Noriega-Crespo, A.

1986-01-01

The progressive ionization and subsequent dynamical evolution of nonhomogeneously distributed low-metal-abundance diffuse gas after star formation in globular clusters are investigated analytically, taking the gravitational acceleration due to the stars into account. The basic equations are derived; the underlying assumptions, input parameters, and solution methods are explained; and numerical results for three standard cases (ionization during star formation, ionization during expansion, and evolution resulting in a stable H II region at its equilibrium Stromgren radius) are presented in graphs and characterized in detail. The time scale of residual-gas loss in typical clusters is found to be about the same as the lifetime of a massive star on the main sequence.

Characterizing the Protostars in the Herschel Survey of Cygnus-X

NASA Astrophysics Data System (ADS)

Kirk, James; Hora, J. L.; Smith, H. A.; Herschel Cygnus-X Group

2014-01-01

The Cygnus-X complex is an extremely active region of massive star formation at a distance of ~1.4 kpc which can be studied with higher sensitivity and less confusion than more distant regions. The study of this region is important in improving our understanding of the formation processes and protostellar phases of massive stars. A previous Spitzer Legacy survey of Cygnus-X mapped the distributions of Class I and Class II YSOs within the region and studied the interaction between massive young stars and clusters of YSOs. Using data from the recent Herschel survey of the region, taken with the PACS and SPIRE instrument (70-500 microns), we are expanding this study of star formation to the youngest and most deeply embedded objects. Using these data we will expand the sample of massive protostars and YSOs in Cygnus-X, analyze the population of infrared dark clouds and their embedded objects, construct Spectral Energy Distributions (SEDs) using pre-existing Spitzer and near-IR data sets (1-500 microns), and fit these sources with models of protostars to derive luminosities and envelope masses. The derived luminosities and masses will enable us to create evolutionary diagrams and test models of high-mass star formation. We will also investigate what role OB associations, such as Cyg OB2, play in causing subsequent star formation in neighboring clouds, providing us with a comprehensive picture of star formation within this extremely active complex.

Hubble Friday - Heavy Metal Stars

NASA Image and Video Library

2017-12-08

Hubble rocks out with heavy metal stars! This 10.5-billion-year-old globular cluster, NGC 6496, is home to heavy-metal stars of a celestial kind! The stars comprising this spectacular spherical cluster are enriched with much higher proportions of metals — elements heavier than hydrogen and helium are curiously known as metals in astronomy — than stars found in similar clusters. A handful of these high-metallicity stars are also variable stars, meaning that their brightness fluctuates over time. NGC 6496 hosts a selection of long-period variables — giant pulsating stars whose brightness can take up to, and even over, a thousand days to change — and short-period eclipsing binaries, which dim when eclipsed by a stellar companion. The nature of the variability of these stars can reveal important information about their mass, radius, luminosity, temperature, composition, and evolution, providing astronomers with measurements that would be difficult or even impossible to obtain through other methods. NGC 6496 was discovered in 1826 by Scottish astronomer James Dunlop. The cluster resides at about 35,000 light-years away in the southern constellation of Scorpius (The Scorpion). Image credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt Text credit: European Space Agency Read more: go.nasa.gov/1U2wqGW

X-Ray source populations in old open clusters: Collinder 261

NASA Astrophysics Data System (ADS)

Vats, Smriti; van den Berg, Maureen; Wijnands, Rudy

2014-09-01

We are carrying out an X-ray survey of old open clusters with the Chandra X-ray Observatory. Single old stars, being slow rotators, are very faint in X-rays (L_X < 1×10^27 erg/s). Hence, X-rays produced by mass transfer in cataclysmic variables (CVs) or by rapid rotation of the stars in tidally locked, detached binaries (active binaries; ABs) can be detected, without contamination from single stars. By comparing the properties of various types of interacting binaries in different environments (the Galactic field, old open clusters, globular clusters), we aim to study binary evolution and how it may be affected by dynamical encounters with other cluster stars. Stellar clusters are good targets to study binaries, as age, distance, chemical composition, are well constrained. Collinder (Cr) 261 is an old open cluster (age ~ 7 Gyr), with one of the richest populations inferred of close binaries and blue stragglers of all open clusters and is therefore an obvious target to study the products of close encounters in open clusters. We will present the first results of this study, detailing the low-luminosity X-ray population of Cr 261, in conjunction with other open clusters in our survey (NGC 188, Berkeley 17, NGC 6253, M67, NGC 6791) and in comparison with populations in globular clusters.

Eating a planet and spinning up

NASA Astrophysics Data System (ADS)

Qureshi, Ahmed; Naoz, Smadar; Shkolnik, Evgenya L.

2018-01-01

One of the predictions of high eccentricity planetary migration is that many planets will end up plunging into their host stars. We investigate the consequence of planetary mergers on their stellar hosts’ spin-period. Energy and angular momentum conservation yield that a planet consumption by a star will spin-up of the star. We find that our calculations align with the observed bifurcation in the stellar spin-period in young clusters. After a Sun-like star has eaten a planet, it will then, spin down due to magnetic braking, consistent with the observed lack of fast rotators in old clusters. The agreement between the calculations presented here and the observed spin-period of stars in young clusters provides circumstantial evidence that planetary accretion onto their host stars is a generic feature in planetary-system evolution.

NGC 6273: Towards Defining A New Class of Galactic Globular Clusters?

NASA Astrophysics Data System (ADS)

Johnson, Christian I.; Rich, Robert Michael; Pilachowski, Catherine A.; Caldwell, Nelson; Mateo, Mario L.; Ira Bailey, John; Crane, Jeffrey D.

2016-01-01

A growing number of observations have found that several Galactic globular clusters exhibit abundance dispersions beyond the well-known light element (anti-)correlations. These clusters tend to be very massive, have >0.1 dex intrinsic metallicity dispersions, have complex sub-giant branch morphologies, and have correlated [Fe/H] and s-process element enhancements. Interestingly, nearly all of these clusters discovered so far have [Fe/H]~-1.7. In this context, we have examined the chemical composition of 18 red giant branch (RGB) stars in the massive, metal-poor Galactic bulge globular cluster NGC 6273 using high signal-to-noise, high resolution (R~27,000) spectra obtained with the Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph mounted on the Magellan-Clay 6.5m telescope at Las Campanas Observatory. We find that the cluster exhibits a metallicity range from [Fe/H]=-1.80 to -1.30 and is composed of two dominant populations separated in [Fe/H] and [La/Fe] abundance. The increase in [La/Eu] as a function of [La/H] suggests that the increase in [La/Fe] with [Fe/H] is due to almost pure s-process enrichment. The most metal-rich star in our sample is not strongly La-enhanced, but is α-poor and may belong to a third "anomalous" stellar population. The two dominant populations exhibit the same [Na/Fe]-[Al/Fe] correlation found in other "normal" globular clusters. Therefore, NGC 6273 joins ω Centauri, M 22, M 2, and NGC 5286 as a possible new class of Galactic globular clusters.

Deep, wide-field, multi-band imaging of z approximately equal to 0.4 clusters and their environs

NASA Technical Reports Server (NTRS)

Silva, David R.; Pierce, Michael J.

1993-01-01

The existence of an excess population of blue galaxies in the cores of distant, rich clusters of galaxies, commonly referred to as the 'Butcher-Oemler' effect is now well established. Spectroscopy of clusters at z = 0.2-0.4 has confirmed that the luminous blue populations comprise as much as 20 percent of these clusters. This fraction is much higher that the 2 percent blue fraction found for nearby rich clusters, such as Coma, indicating that rapid galaxy evolution has occurred on a relatively short time scale. Spectroscopy has also shown that the 'blue' galaxies can basically be divided into three classes: 'starburst' galaxies with large (O II) equivalent widths, 'post-starburst' E+A galaxies (i.e. galaxies with strong Balmer lines shortward of 4000A but elliptical-like colors, and normal spiral/irregulars. Unfortunately, it is difficult to obtain enough spectra of individual galaxies in these intermediate redshift clusters to say anything statistically meaningful. Thus, limited information is available about the relative numbers of these three classes of 'blue' galaxies and the associated E/SO population in these intermediate redshift clusters. More statistically meaningful results can be derived from deep imaging of these clusters. However, the best published data to date (e.g. MacLaren et al. 1988; Dressler & Gunn 1992) are limited to the cluster cores and do not sample the galaxy luminosity functions very deeply at the bluest wavelengths. Furthermore, only limited spectro-energy distribution data is available below 4000A in the observed cluster rest frame providing limited sensitivity to 'recent' star formation activity. To improve this situation, we are currently obtaining deep, wide-field UBRI images of all known rich clusters at z approx. equals 0.4. Our main objective is to obtain the necessary color information to distinguish between the E+SO, 'E+A', and spiral/irregular galaxy populations throughout the cluster/supercluster complex. At this redshift, UBRI correspond to rest-frame 2500A/UVR bandpasses. The rest-frame UVR system provides a powerful 'blue' galaxy discriminate given the expected color distribution. Moreover, since 'hot' stars peak near 2500A, that bandpass is a powerful probe of recent star formation activity in all classes of galaxies. In particular, it is sensitive to ellipticals with 'UV excess' populations (MacLaren et al. 1988).

VizieR Online Data Catalog: Spatial structure of young stellar clusters. III. (Kuhn+, 2015)

NASA Astrophysics Data System (ADS)

Kuhn, M. A.; Feigelson, E. D.; Getman, K. V.; Sills, A.; Bate, M. R.; Borissova, J.

2017-10-01

This paper uses the Paper I (Kuhn et al. 2014, J/ApJ/787/107) catalog of 142 subclusters of young stars in 17 MYStIX star-forming regions. These subclusters were found and characterized using the stars in the MYStIX Probable Complex Member (MPCM) catalogs from Broos et al. (2013, J/ApJS/209/32). The multiwavelength data analysis efforts that went into this catalog are described by Feigelson et al. (2013, J/ApJS/209/26), King et al. (2013, J/ApJS/209/28), Kuhn et al. (2013, J/ApJS/209/27; 2013, J/ApJS/209/29), Naylor et al. (2013, J/ApJS/209/30), Povich et al. (2013, J/ApJS/209/31), and Townsley et al. (2014, J/ApJS/213/1), which provided uniform data coverage across the 17 star-forming regions investigated here, including the most comprehensive and reliable lists of young stars in many of the nearest MSFRs. (3 data files).

On the mass of dense star clusters in starburst galaxies from spectrophotometry

NASA Astrophysics Data System (ADS)

Fleck, J.-J.; Boily, C. M.; Lançon, A.; Deiters, S.

2006-07-01

The mass of unresolved young star clusters derived from spectrophotometric data may well be off by a factor of 2 or more once the migration of massive stars driven by mass segregation is accounted for. We quantify this effect for a large set of cluster parameters, including variations in the stellar initial mass function (IMF), the intrinsic cluster mass, and mean mass density. Gas-dynamical models coupled with the Cambridge stellar evolution tracks allow us to derive a scheme to recover the real cluster mass given measured half-light radius, one-dimensional velocity dispersion and age. We monitor the evolution with time of the ratio of real to apparent mass through the parameter η. When we compute η for rich star clusters, we find non-monotonic evolution in time when the IMF stretches beyond a critical cut-off mass of 25.5Msolar. We also monitor the rise of colour gradients between the inner and outer volume of clusters: we find trends in time of the stellar IMF power indices overlapping well with those derived for the Large Magellanic Cloud cluster NGC 1818 at an age of 30Myr. We argue that the core region of massive Antennae clusters should have suffered from much segregation despite their low ages. We apply these results to a cluster mass function, and find that the peak of the mass distribution would appear to observers shifted to lower masses by as much as 0.2dex. The star formation rate derived for the cluster population is then underestimated by from 20 to 50 per cent.

On the missing second generation AGB stars in NGC 6752

NASA Astrophysics Data System (ADS)

Cassisi, Santi; Salaris, Maurizio; Pietrinferni, Adriano; Vink, Jorick S.; Monelli, Matteo

2014-11-01

In recent years the view of Galactic globular clusters as simple stellar populations has changed dramatically, it is now thought that basically all globular clusters host multiple stellar populations, each with its own chemical abundance pattern and colour-magnitude diagram sequence. Recent spectroscopic observations of asymptotic giant branch stars in the globular cluster NGC 6752 have disclosed a low [Na/Fe] abundance for the whole sample, suggesting that they are all first generation stars, and that all second generation stars fail to reach the AGB in this cluster. A scenario proposed to explain these observations invokes strong mass loss in second generation horizontal branch stars - all located at the hot side of the blue and extended horizontal branch of this cluster - possibly induced by the metal enhancement associated to radiative levitation. This enhanced mass loss would prevent second generation stars from reaching the asymptotic giant branch phase, thus explaining at the same time the low value of the ratio between horizontal branch and asymptotic giant branch stars (the R2 parameter) observed in NGC 6752. We have critically discussed this mass-loss scenario, finding that the required mass-loss rates are of the order of 10-9 M⊙ yr-1, significantly higher than current theoretical and empirical constraints. By making use of synthetic horizontal branch simulations, we demonstrate that our modelling correctly predicts the R2 parameter for NGC 6752, without the need to invoke very efficient mass loss during the core He-burning stage. As a test of our stellar models we show that we can reproduce the observed value of R2 for both M 3, a cluster of approximately the same metallicity and with a redder horizontal branch morphology, and M 13, a cluster with a horizontal branch very similar to NGC 6752. However, our simulations for the NGC 6752 horizontal branch predict however the presence of a significant fraction of second generation stars (about 50%) along the cluster asymptotic giant branch. We conclude that there is no simple explanation for the lack of second generation stars in the spectroscopically surveyed sample, although the interplay between mass loss (with low rates) and radiative levitation may play a role in explaining this puzzle.

CHEMICAL ABUNDANCES OF MEMBER STARS IN THE OPEN CLUSTER NGC 2632 (PRAESEPE)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, X. L.; Chen, Y. Q.; Zhao, G.

2015-11-15

Based on high-resolution, high signal-to-noise ratio spectra, we present abundances of 17 elements (Fe, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Co, Ni, Y, Zr, Ba, La) for six stars (one Am star, one F dwarf star, and four GK giant stars) and radial velocities for 18 proper-motion selected member stars in the open cluster NGC 2632. In the Am star, s-process elements Y and Ba are clearly overabundant, which may be considered as an indicator of a peculiar Am star. The average [Fe/H] is 0.16 ± 0.06 from four GK giant member stars, which is similarmore » to that of solar-type stars in the literature. As compared with dwarf stars, significant overabundances are found for Na, Mg, and Ba elements in our giant stars, which can be explained by the evolutionary effect. We also detect a star-to-star scatter of [Na/Fe] ratios among four giants which locate approximately at the same position in the CMD. Finally, we perform an analysis on the possible connection between the abundance and spatial structure of NGC 2632, but we find no inhomogeneous abundance among different clumps of stars in this cluster based on our limited sample.« less

The dynamical origin of multiple populations in intermediate-age clusters in the Magellanic Clouds

NASA Astrophysics Data System (ADS)

Hong, Jongsuk; de Grijs, Richard; Askar, Abbas; Berczik, Peter; Li, Chengyuan; Wang, Long; Deng, Licai; Kouwenhoven, M. B. N.; Giersz, Mirek; Spurzem, Rainer

2017-11-01

Numerical simulations were carried out to study the origin of multiple stellar populations in the intermediate-age clusters NGC 411 and NGC 1806 in the Magellanic Clouds. We performed NBODY6++ simulations based on two different formation scenarios, an ad hoc formation model where second-generation (SG) stars are formed inside a cluster of first-generation (FG) stars using the gas accumulated from the external intergalactic medium and a minor merger model of unequal mass (MSG/MFG ∼ 5-10 per cent) clusters with an age difference of a few hundred million years. We compared our results such as the radial profile of the SG-to-FG number ratio with observations on the assumption that the SG stars in the observations are composed of cluster members, and confirmed that both the ad hoc formation and merger scenarios reproduce the observed radial trend of the SG-to-FG number ratio, which shows less centrally concentrated SG than FG stars. It is difficult to constrain the formation scenario for the multiple populations by only using the spatial distribution of the SG stars. SG stars originating from the merger scenario show a significant velocity anisotropy and rotational features compared to those from the ad hoc formation scenario. Thus, observations aimed at kinematic properties like velocity anisotropy or rotational velocities for SG stars should be obtained to better understand the formation of the multiple populations in these clusters. This is, however, beyond current instrumentation capabilities.

A Proper Motion Search for Stars Escaping from Globular Clusters with High Velocities

NASA Astrophysics Data System (ADS)

Meusinger, H.; Scholz, R.-D.; Irwin, M.

The dynamical evolution of globular clusters, in particular during the late phases, may be strongly influenced by the energy transfer from binaries to passing stars. As a by-product of this process, stars with high velocities are expected, perhaps high enough to escape from the cluster. Accurate proper motions are the only suitable tool to identify candidates for such high-velocity cluster stars. In order to perform such a search, we use a catalogue of absolute proper motions and UBV magnitudes for about 104 stars with B < 20 in a field of 10 square degrees centered on the globular cluster M3. The data were derived from more than 80 photographic plates taken between 1965 and 1995 with the Tautenburg Schmidt telescope and measured by means of the APM facility, Cambridge. The stellar sample is complete to B = 18.5 and comprises nearly all post-main-sequence stars in the halo of M3 and its surrounding. The proper motions are of Hipparcos-like accuracy (median error 1 mas/yr) in this magnitude range. We find several dozens of candidates, distributed over the whole field, with proper motions and colours consistent with the assumption of their origin from the cluster. Further conclusions can be drawn only on the basis of radial velocity measurements for the candidates and of estimates for the field-star contamination by means of simulations of the Galactic structure and kinematics in this field.

COS Spectroscopy of White Dwarf Companions to Blue Stragglers

NASA Astrophysics Data System (ADS)

Gosnell, Natalie M.; Geller, Aaron M.; Knigge, Christian; Mathieu, Robert D.; Sills, Alison; Leiner, Emily; Leigh, Nathan

2017-01-01

Complete membership studies of open stellar clusters reveal that 25% of the evolved stars follow alternative pathways in stellar evolution, meaning something in the history of these stars changed their composition or mass (or both). In order to draw a complete picture of stellar evolution we must include these canonically "strange" stars in our definition of standard stellar populations. The formation mechanism of blue straggler stars, traditionally defined to be brighter and bluer than the main sequence turnoff in a star cluster, has been an outstanding question for almost six decades. Recent Hubble Space Telescope (HST) far-ultraviolet (far-UV) observations directly reveal that the blue straggler stars in the old (7 Gyr) open cluster NGC 188 are predominantly formed through mass transfer. We will present HST far-UV COS spectroscopy of white dwarf companions to blue stragglers. These white dwarfs are the remnants of the mass transfer formation process. The effective temperatures and surface gravities of the white dwarfs delineate the timeline of blue straggler formation in this cluster. The existence of these binaries in a well-studied cluster environment provides an unprecedented opportunity to observationally constrain mass transfer models and inform our understanding of many other alternative pathway stellar products.

Imprints of dynamical interactions on brown dwarf pairing statistics and kinematics

NASA Astrophysics Data System (ADS)

Sterzik, M. F.; Durisen, R. H.

2003-03-01

We present statistically robust predictions of brown dwarf properties arising from dynamical interactions during their early evolution in small clusters. Our conclusions are based on numerical calculations of the internal cluster dynamics as well as on Monte-Carlo models. Accounting for recent observational constraints on the sub-stellar mass function and initial properties in fragmenting star forming clumps, we derive multiplicity fractions, mass ratios, separation distributions, and velocity dispersions. We compare them with observations of brown dwarfs in the field and in young clusters. Observed brown dwarf companion fractions around 15 +/- 7% for very low-mass stars as reported recently by Close et al. (\\cite{CSFB03}) are consistent with certain dynamical decay models. A significantly smaller mean separation distribution for brown dwarf binaries than for binaries of late-type stars can be explained by similar specific energy at the time of cluster formation for all cluster masses. Due to their higher velocity dispersions, brown-dwarfs and low-mass single stars will undergo time-dependent spatial segregation from higher-mass stars and multiple systems. This will cause mass functions and binary statistics in star forming regions to vary with the age of the region and the volume sampled.

Effect of tidal fields on star clusters

NASA Technical Reports Server (NTRS)

Chernoff, David; Weinberg, Martin

1991-01-01

We follow the dynamical evolution of a star cluster in a galactic tidal field using a restricted N-body code. We find large asymmetric distortions in the outer profile of the cluster in the first 10 or so crossing times as material is lost. Prograde stars escape preferentially and establish a potentially observable retrograde rotation in the halo. We present the rate of particle loss and compare with the prescription proposed by Lee and Ostriker (1987).

Comparison of calculated and observed integral magnitudes for the globular cluster M13

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gerashchenko, A.N.; Kadla, Z.I.

On the basis of a study of the distribution of stars in the central region of the globular cluster M13 it is found that integral photoelectric observations cover stars down to about the point of turnoff from the main sequence. Here the distribution of giants and stars of the horizontal branch as a function of distance from the center of the cluster is the same within limits of 0

Unusual satellite data: A black hole?. [International Ultraviolet Explorer observations

NASA Technical Reports Server (NTRS)

1978-01-01

Data obtained by the NASA-launched European Space Agency's International Ultraviolet Explorer satellite suggests the possibility of a massive black hole at the center of some globular clusters (star groups) in our galaxy. Six of these clusters, three of them X-ray sources, were closely examined. Onboard short wavelength UV instrumentation penetrated the background denseness of the clusters 15,000 light years away where radiation, probably from a group of 10 to 20 bright blue stars orbiting the core, was observed. The stars may well be orbiting a massive black hole the size of 1,000 solar systems. The existence of the black hole is uncertain. The dynamics of the stars must be studied first to determine how they rotate in relation to the center of the million-star cluster. This may better indicate what provides the necessary gravitational pull that holds them in orbit.

Chemical Abundances of Seven Outer Halo M31 Globular Clusters from the Pan-Andromeda Archaeological Survey

NASA Astrophysics Data System (ADS)

Sakari, Charli M.

2017-03-01

Observations of stellar streams in M31's outer halo suggest that M31 is actively accreting several dwarf galaxies and their globular clusters (GCs). Detailed abundances can chemically link clusters to their birth environments, establishing whether or not a GC has been accreted from a satellite dwarf galaxy. This talk presents the detailed chemical abundances of seven M31 outer halo GCs (with projected distances from M31 greater than 30 kpc), as derived from high-resolution integrated-light spectra taken with the Hobby Eberly Telescope. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS)-this talk presents the first determinations of integrated Fe, Na, Mg, Ca, Ti, Ni, Ba, and Eu abundances for these clusters. Four of the target clusters (PA06, PA53, PA54, and PA56) are metal-poor ([Fe/H] < -1.5), α-enhanced (though they are possibly less alpha-enhanced than Milky Way stars at the 1 sigma level), and show signs of star-to-star Na and Mg variations. The other three GCs (H10, H23, and PA17) are more metal-rich, with metallicities ranging from [Fe/H] = -1.4 to -0.9. While H23 is chemically similar to Milky Way field stars, Milky Way GCs, and other M31 clusters, H10 and PA17 have moderately-low [Ca/Fe], compared to Milky Way field stars and clusters. Additionally, PA17's high [Mg/Ca] and [Ba/Eu] ratios are distinct from Milky Way stars, and are in better agreement with the stars and clusters in the Large Magellanic Cloud (LMC). None of the clusters studied here can be conclusively linked to any of the identified streams from PAndAS; however, based on their locations, kinematics, metallicities, and detailed abundances, the most metal-rich PAndAS clusters H23 and PA17 may be associated with the progenitor of the Giant Stellar Stream, H10 may be associated with the SW Cloud, and PA53 and PA56 may be associated with the Eastern Cloud.

Galaxy evolution in merging clusters: The passive core of the "Train Wreck" cluster of galaxies, A 520

NASA Astrophysics Data System (ADS)

Deshev, Boris; Finoguenov, Alexis; Verdugo, Miguel; Ziegler, Bodo; Park, Changbom; Hwang, Ho Seong; Haines, Christopher; Kamphuis, Peter; Tamm, Antti; Einasto, Maret; Hwang, Narae; Park, Byeong-Gon

2017-11-01

Aims: The mergers of galaxy clusters are the most energetic events in the Universe after the Big Bang. With the increased availability of multi-object spectroscopy and X-ray data, an ever increasing fraction of local clusters are recognised as exhibiting signs of recent or past merging events on various scales. Our goal is to probe how these mergers affect the evolution and content of their member galaxies. We specifically aim to answer the following questions: is the quenching of star formation in merging clusters enhanced when compared with relaxed clusters? Is the quenching preceded by a (short-lived) burst of star formation? Methods: We obtained optical spectroscopy of >400 galaxies in the field of the merging cluster Abell 520. We combine these observations with archival data to obtain a comprehensive picture of the state of star formation in the members of this merging cluster. Finally, we compare these observations with a control sample of ten non-merging clusters at the same redshift from The Arizona Cluster Redshift Survey (ACReS). We split the member galaxies into passive, star forming or recently quenched depending on their spectra. Results: The core of the merger shows a decreased fraction of star forming galaxies compared to clusters in the non-merging sample. This region, dominated by passive galaxies, is extended along the axis of the merger. We find evidence of rapid quenching of the galaxies during the core passage with no signs of a star burst on the time scales of the merger (≲0.4 Gyr). Additionally, we report the tentative discovery of an infalling group along the main filament feeding the merger, currently at 2.5 Mpc from the merger centre. This group contains a high fraction of star forming galaxies as well as approximately two thirds of all the recently quenched galaxies in our survey. The reduced spectra are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/607/A131

HUBBLE UNCOVERS MYSTERY OBJECTS IN THE DENSE CORE OF A NEARBY STAR CLUSTER

NASA Technical Reports Server (NTRS)

2002-01-01

Piercing the heart of a glittering swarm of stars, NASA's sharp-eyed Hubble Space Telescope unveils the central region of the globular cluster M22, a 12- to 14-billion-year-old grouping of stars in the constellation Sagittarius. The telescope's view of the cluster's core measures 3.3 light-years across. The stars near the cluster's core are 100,000 times more numerous than those in the Sun's neighborhood. Buried in the glow of starlight are about six 'mystery objects,' which astronomers estimate are no larger than one quarter the mass of the giant planet Jupiter, the solar system's heftiest planet. The mystery objects are too far and dim for Hubble to see directly. Instead, the orbiting observatory detected these unseen celestial bodies by looking for their gravitational effects on the light from far distant stars. In this case, the stars are far beyond the cluster in the galactic bulge, about 30,000 light-years from Earth at the center of the Milky Way Galaxy. M22 is 8,500 light-years away. The invisible objects betrayed their presence by bending the starlight gravitationally and amplifying it, a phenomenon known as microlensing. From February 22 to June 15, 1999, Hubble's Wide Field and Planetary Camera 2 looked through this central region and monitored 83,000 stars. During that time the orbiting observatory recorded six unexpectedly brief microlensing events. In each case a background star jumped in brightness for less than 20 hours before dropping back to normal. These transitory spikes in brightness mean that the object passing in front of the star must have been much smaller than a normal star. Hubble also detected one clear microlensing event. In that observation a star appeared about 10 times brighter over an 18-day span before returning to normal. Astronomers traced the leap in brightness to a dwarf star in the cluster floating in front of the background star. The inset photo shows the entire globular cluster of about 10 million stars. M22 is about 60 light-years wide. The image was taken in June 1995 by the Burrell Schmidt telescope at the Case Western Reserve University's Warner and Swasey Observatory on Kitt Peak in Arizona. This release is issued jointly by NASA and ESA. Credits for Hubble image: NASA, Kailash Sahu, Stefano Casertano, Mario Livio, Ron Gilliland (Space Telescope Science Institute), Nino Panagia (European Space Agency/Space Telescope Science Institute), Michael Albrow and Mike Potter (Space Telescope Science Institute) Credits for ground-based image: Nigel A.Sharp, REU program/AURA/NOAO/NSF

VizieR Online Data Catalog: OCCASO survey. HRV for 12 open clusters (Casamiquela+, 2016)

NASA Astrophysics Data System (ADS)

Casamiquela, L.; Carrera, R.; Jordi, C.; Balaguer-Nunez, L.; Pancino, E.; Hidalgo, S. L.; Martinez-Vazquez, C. E.; Murabito, S.; Del Pino, A.; Aparicio, A.; Blanco-Cuaresma, S.; Gallart, C.

2016-05-01

We present results of radial velocities for stars in 12 completed clusters (77 stars), and the reference stars Arcturus and μ Leo. This is a total of 79 stars. We include radial velocities from individual spectra, and final radial velocities from combined spectra which reach a minimum signal-to-noise ratio of 70. Comparison with the literature is included in the cases which the stars had previous measurements. (2 data files).

A Study of The Binary and Anomalous Stellar Populations in Two Intermediate-Aged Open Clusters

NASA Astrophysics Data System (ADS)

Mathieu, Robert D.; Milliman, Katelyn; Geller, Aaron M.; Gosnell, Natalie

2010-08-01

``Anomalous'' stars, such as blue stragglers and more recently sub- subgiants, have been an enduring challenge for stellar evolution theory. It is now clear that in star clusters these systems are closely linked to the binary star populations. Furthermore, sophisticated N-body models show that stellar dynamical processes play a central role in the formation of such anomalous stars. These stars trace the interface between the classical fields of stellar evolution and stellar dynamics. We propose to expand our highly successful radial-velocity survey to include two new rich open clusters NGC 7789 (1.8 Gyr, -0.1 dex) and NGC 2506 (2.1 Gyr, -0.4 dex) as part of the WIYN Open Cluster Study (WOCS). Though these two clusters are both of intermediate age and of similar richness, they have quite different blue straggler populations. NGC 2506 has only 10 known blue stragglers, while NGC 7789 has at least 27, among the largest known populations of blue stragglers in an open cluster. Defining the hard-binary populations in these two clusters is critical for understanding the factors that determine blue straggler production rates. Our proposed observations will establish the hard- binary fraction and frequency distributions of orbital parameters (periods, eccentricities, mass-ratios, etc.) for orbital periods approaching the hard-soft boundary, and will provide a comprehensive survey of the blue stragglers and other anomalous stars, including secure cluster memberships and binary properties. These data will then form direct constraints for detailed N-body open cluster simulations from which we will study the impact of the hard-binary population on the production rates and mechanisms of blue stragglers.

Open clusters in Auriga OB2

NASA Astrophysics Data System (ADS)

Marco, Amparo; Negueruela, Ignacio

2016-06-01

We study the area around the H II region Sh 2-234, including the young open cluster Stock 8, to investigate the extent and definition of the association Aur OB2 and the possible role of triggering in massive cluster formation. We obtained Strömgren and J, H, KS photometry for Stock 8 and Strömgren photometry for two other cluster candidates in the area, which we confirm as young open clusters and name Alicante 11 and Alicante 12. We took spectroscopy of ˜33 early-type stars in the area, including the brightest cluster members. We calculate a common distance of 2.80^{+0.27}_{-0.24} kpc for the three open clusters and surrounding association. We derive an age 4-6 Ma for Stock 8, and do not find a significantly different age for the other clusters or the association. The star LS V +34°23, with spectral type O8 II(f), is likely the main source of ionization of Sh 2-234. We observe an important population of pre-main-sequence stars, some of them with discs, associated with the B-type members lying on the main sequence. We interpret the region as an area of recent star formation with some residual and very localized ongoing star formation. We do not find evidence for sequential star formation on a large scale. The classical definition of Aur OB2 has to be reconsidered, because its two main open clusters, Stock 8 and NGC 1893, are not at the same distance. Stock 8 is probably located in the Perseus arm, but other nearby H II regions whose distances also place them in this arm show quite different distances and radial velocities and, therefore, are not connected.

The Effects of Rotation on the Main-sequence Turnoff of Intermediate-age Massive Star Clusters

NASA Astrophysics Data System (ADS)

Yang, Wuming; Bi, Shaolan; Meng, Xiangcun; Liu, Zhie

2013-10-01

The double or extended main-sequence turnoffs (MSTOs) in the color-magnitude diagram (CMD) of intermediate-age massive star clusters in the Large Magellanic Cloud are generally interpreted as age spreads of a few hundred Myr. However, such age spreads do not exist in younger clusters (i.e., 40-300 Myr), which challenges this interpretation. The effects of rotation on the MSTOs of star clusters have been studied in previous works, but the results obtained are conflicting. Compared with previous works, we consider the effects of rotation on the main-sequence lifetime of stars. Our calculations show that rotating models have a fainter and redder MSTO with respect to non-rotating counterparts with ages between about 0.8 and 2.2 Gyr, but have a brighter and bluer MSTO when age is larger than 2.4 Gyr. The spread of the MSTO caused by a typical rotation rate is equivalent to the effect of an age spread of about 200 Myr. Rotation could lead to the double or extended MSTOs in the CMD of the star clusters with ages between about 0.8 and 2.2 Gyr. However, the extension is not significant, and it does not even exist in younger clusters. If the efficiency of the mixing were high enough, the effects of the mixing would counteract the effect of the centrifugal support in the late stage of evolution, and the rotationally induced extension would disappear in the old intermediate-age star clusters, but younger clusters would have an extended MSTO. Moreover, the effects of rotation might aid in understanding the formation of some "multiple populations" in globular clusters.

Strangeon and Strangeon Star

NASA Astrophysics Data System (ADS)

Xiaoyu, Lai; Renxin, Xu

2017-06-01

The nature of pulsar-like compact stars is essentially a central question of the fundamental strong interaction (explained in quantum chromo-dynamics) at low energy scale, the solution of which still remains a challenge though tremendous efforts have been tried. This kind of compact objects could actually be strange quark stars if strange quark matter in bulk may constitute the true ground state of the strong-interaction matter rather than 56Fe (the so-called Witten’s conjecture). From astrophysical points of view, however, it is proposed that strange cluster matter could be absolutely stable and thus those compact stars could be strange cluster stars in fact. This proposal could be regarded as a general Witten’s conjecture: strange matter in bulk could be absolutely stable, in which quarks are either free (for strange quark matter) or localized (for strange cluster matter). Strange cluster with three-light-flavor symmetry is renamed strangeon, being coined by combining “strange nucleon” for the sake of simplicity. A strangeon star can then be thought as a 3-flavored gigantic nucleus, and strangeons are its constituent as an analogy of nucleons which are the constituent of a normal (micro) nucleus. The observational consequences of strangeon stars show that different manifestations of pulsarlike compact stars could be understood in the regime of strangeon stars, and we are expecting more evidence for strangeon star by advanced facilities (e.g., FAST, SKA, and eXTP).

The Chemical Composition of NGC 5824, a Globular Cluster without Iron Spread but with an Extreme Mg–Al Anticorrelation

NASA Astrophysics Data System (ADS)

Mucciarelli, Alessio; Lapenna, Emilio; Ferraro, Francesco R.; Lanzoni, Barbara

2018-05-01

NGC 5824 is a massive Galactic globular cluster suspected to have an intrinsic spread in its iron content, according to the strength of the calcium triplet lines. We present chemical abundances of 117 cluster giant stars using high-resolution spectra acquired with the multi-object spectrograph FLAMES. The metallicity distribution of 87 red giant branch stars is peaked at [Fe/H] = ‑2.11 ± 0.01 dex, while that derived from 30 asymptotic giant branch stars is peaked at [Fe/H] = ‑2.20 ± 0.01 dex. Both the distributions are compatible with a null spread, indicating that this cluster did not retain the ejecta of supernovae. The small iron abundance offset between the two groups of stars is similar to the abundances already observed among red and asymptotic giant branch stars in other clusters. The lack of intrinsic iron spread rules out the possibility that NGC 5824 is the remnant of a disrupted dwarf galaxy, as previously suggested. We also find evidence of the chemical anomalies usually observed in globular clusters, namely the Na–O and the Mg–Al anticorrelations. In particular, NGC 5824 exhibits a huge range of [Mg/Fe] abundance, observed in only a few metal-poor and/or massive clusters. We conclude that NGC 5824 is a normal globular cluster, without spread in [Fe/H] but with an unusually large spread in [Mg/Fe], possibly due to an efficient self-enrichment driven by massive asymptotic giant branch stars. Based on observations collected at the ESO-VLT under the program 095.D-0290.

The II Zw 40 Supernebula: 30 Doradus on Steroids

NASA Astrophysics Data System (ADS)

Leitherer, Claus

2015-10-01

We propose COS G140L spectroscopy of the enigmatic nearby blue compact dwarf galaxy II Zw 40. The galaxy hosts a nuclear super star cluster with a luminosity 10 times that of 30 Doradus, the most powerful giant HII region in the Local Group. The super star cluster has been suggested to be the ionizing source of a supernebula detected via its free-free radiation in the radio. The physical conditions, however, are much more complex, as demonstrated by the detection of the nebular He II and the mid-infrared line of [O IV] 25.9. These lines are unlikely to be related to hot stars and require a different powering source. II Zw 40 shares many similarities with the related blue compact dwarfs NGC 5253 and Henize 2-10, both of which have been studied extensively with HST, yet no ultraviolet spectroscopy has ever been obtained for II Zw 40. This small 4-orbit proposal will provide the necessary UV data to study the massive-star content directly. We will determine reddening, age, and the stellar initial mass function and perform a comparison with the local benchmark 30 Doradus. In particular we will investigate whether the hot stars are able to power the supernebula and the nebular high-excitation lines. Our modeling will utilize the latest generation of stellar evolutionary tracks with and without stellar rotation. If the stars fall short in terms of spectral hardness and luminosity, II Zw 40 may become the second candidate for a central black hole in a young starburst after Henize 2-10.

The Massive Star Content of NGC 3603

NASA Astrophysics Data System (ADS)

Melena, Nicholas W.; Massey, Philip; Morrell, Nidia I.; Zangari, Amanda M.

2008-03-01

We investigate the massive star content of NGC 3603, the closest known giant H II region. We have obtained spectra of 26 stars in the central cluster using the Baade 6.5 m telescope (Magellan I). Of these 26 stars, 16 had no previous spectroscopy. We also obtained photometry of all of the stars with previous or new spectroscopy, primarily using archival HST Advanced Camera for Surveys/High-Resolution Camera images. The total number of stars that have been spectroscopically classified in NGC 3603 now stands at 38. The sample is dominated by very early O-type stars (O3); there are also several (previously identified) H-rich WN+abs stars. We derive E(B - V) = 1.39, and find that there is very little variation in reddening across the cluster core, in agreement with previous studies. Our spectroscopic parallax is consistent with the kinematic distance only if the ratio of total to selective extinction is anomalously high within the cluster, as argued by Pandey et al. Adopting their reddening, we derive a distance of 7.6 kpc. We discuss the various distance estimates to the cluster, and note that although there has been a wide range of values in the recent literature (6.3-10.1 kpc) there is actually good agreement with the apparent distance modulus of the cluster—the disagreement has been the result of the uncertain reddening correction. We construct our H-R diagram using the apparent distance modulus with a correction for the slight difference in differential reddening from star to star. The resulting H-R diagram reveals that the most massive stars are highly coeval, with an age of 1-2 Myr, and of very high masses (120 Msun). The three stars with Wolf-Rayet features are the most luminous and massive, and are coeval with the non-WRs, in accord with what was found in the R136 cluster. There may be a larger age spread (1-4 Myr) for the lower mass objects (20-40 Msun). Two supergiants (an OC9.7 I and the B1 I star Sher 25) both have an age of about 4 Myr. We compare the stellar content of this cluster to that of R136, finding that the number of very high luminosity (Mbol <= -10) stars is only about 1.1-2.4× smaller in NGC 3603. The most luminous members in both clusters are H-rich WN+abs stars, basically "Of stars on steroids," relatively unevolved stars whose high luminosities results in high-mass loss rates, and hence spectra that mimic that of evolved WNs. To derive an initial-mass function for the massive stars in NGC 3603 requires considerably more spectroscopy; we estimate from a color-magnitude diagram that less than a third of the stars with masses above 20 Msun have spectral types known. This paper is based on data gathered with the 6.5 m Magellan telescopes located at Las Campanas Observatory, Chile.

Lithium in lower-main-sequence stars of the Alpha Persei cluster

NASA Technical Reports Server (NTRS)

Balachandran, Suchitra; Lambert, David L.; Stauffer, John R.

1988-01-01

Lithium abundances are presented for main-sequence stars of spectral types F, G, and K in the young open cluster Alpha Per. For 46 cluster members, a correlation between Li abundance and projected rotational velocity v sin i is found: all of the Li-poor stars are slow rotators. Two explanations are proposed to account for the correlation: (1) that the Li depletion is introduced following a rapid spin-down phase experienced by young low-mass stars, and that this episode of Li depletion may be the dominant one determining the spread of Li abundances among young low-mass main-sequence stars, and (2) that star formation has occurred over a finite period such that the older stars have undergone a spin-down and depletion of Li by a means that may or may not depend on rotation. The Li abundance in the warm and rapidly rotating stars appears to be undepleted, as is predicted by recent models of pre-main-sequence stars. The depletion observed in the cool stars exceeds the level predicted by these models.

Formation of young massive clusters from turbulent molecular clouds

NASA Astrophysics Data System (ADS)

Fujii, Michiko; Portegies Zwart, Simon

2015-08-01

We simulate the formation and evolution of young star clusters using smoothed-particle hydrodynamics (SPH) and direct N-body methods. We start by performing SPH simulations of the giant molecular cloud with a turbulent velocity field, a mass of 10^4 to 10^6 M_sun, and a density between 17 and 1700 cm^-3. We continue the SPH simulations for a free-fall time scale, and analyze the resulting structure of the collapsed cloud. We subsequently replace a density-selected subset of SPH particles with stars. As a consequence, the local star formation efficiency exceeds 30 per cent, whereas globally only a few per cent of the gas is converted to stars. The stellar distribution is very clumpy with typically a dozen bound conglomerates that consist of 100 to 10000 stars. We continue to evolve the stars dynamically using the collisional N-body method, which accurately treats all pairwise interactions, stellar collisions and stellar evolution. We analyze the results of the N-body simulations at 2 Myr and 10 Myr. From dense massive molecular clouds, massive clusters grow via hierarchical merging of smaller clusters. The shape of the cluster mass function that originates from an individual molecular cloud is consistent with a Schechter function with a power-law slope of beta = -1.73 at 2 Myr and beta = -1.67 at 10 Myr, which fits to observed cluster mass function of the Carina region. The superposition of mass functions have a power-law slope of < -2, which fits the observed mass function of star clusters in the Milky Way, M31 and M83. We further find that the mass of the most massive cluster formed in a single molecular cloud with a mass of M_g scales with 6.1 M_g^0.51 which also agrees with recent observation in M51. The molecular clouds which can form massive clusters are much denser than those typical in the Milky Way. The velocity dispersion of such molecular clouds reaches 20 km/s and it is consistent with the relative velocity of the molecular clouds observed near NGC 3603 and Westerlund 2, for which a triggered star formation by cloud-cloud collisions is suggested.

The 100 brigthest Blue Straggler Stars.

NASA Astrophysics Data System (ADS)

Morales Durán, C.; Llorente de Andrés, F.; Ahumada, J. A.

2015-05-01

Blue straggler stars (BSS) are characterized by their appearance in the CMD of globular and open clusters, in the Main Sequence extension, above the turn-off and blueward of this. In accordance with the Standard Theory of stellar evolution, BSS should be out of the Main Sequence and over the Giant Branch if they really belong to the cluster and are formed at the same time than the rest of cluster stars. There are several theories that try to explain the existence of BSS but at present prevails the idea that they can be the product of mass transfer in binaries (McCrea, 1964), and the luminosity of the receiver star is incremented in such a way that now it is over the Main Sequence turn-off point of its cluster. Also it is believed that they are the result of stellar fussion of two or several stars, specially in dense systems as the globular cluster nucleus. This work is focalised in all the BSS brihgter the V = 10 mag. that we have been able to identify in open clusters. It is a sample unprecedented by its number and as well it is a sample with plentiful observational information, it is why we hope to be able to assure their membership to the parent cluster and obtain reliable information about their possible origin.

UPDATED MASS SCALING RELATIONS FOR NUCLEAR STAR CLUSTERS AND A COMPARISON TO SUPERMASSIVE BLACK HOLES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Scott, Nicholas; Graham, Alister W.

2013-02-15

We investigate whether or not nuclear star clusters and supermassive black holes (SMBHs) follow a common set of mass scaling relations with their host galaxy's properties, and hence can be considered to form a single class of central massive object (CMO). We have compiled a large sample of galaxies with measured nuclear star cluster masses and host galaxy properties from the literature and fit log-linear scaling relations. We find that nuclear star cluster mass, M {sub NC}, correlates most tightly with the host galaxy's velocity dispersion: log M {sub NC} = (2.11 {+-} 0.31)log ({sigma}/54) + (6.63 {+-} 0.09), butmore » has a slope dramatically shallower than the relation defined by SMBHs. We find that the nuclear star cluster mass relations involving host galaxy (and spheroid) luminosity and stellar and dynamical mass, intercept with but are in general shallower than the corresponding black hole scaling relations. In particular, M {sub NC}{proportional_to}M {sup 0.55{+-}0.15} {sub Gal,dyn}; the nuclear cluster mass is not a constant fraction of its host galaxy or spheroid mass. We conclude that nuclear stellar clusters and SMBHs do not form a single family of CMOs.« less

Detecting outliers and learning complex structures with large spectroscopic surveys - a case study with APOGEE stars

NASA Astrophysics Data System (ADS)

Reis, Itamar; Poznanski, Dovi; Baron, Dalya; Zasowski, Gail; Shahaf, Sahar

2018-05-01

In this work, we apply and expand on a recently introduced outlier detection algorithm that is based on an unsupervised random forest. We use the algorithm to calculate a similarity measure for stellar spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE). We show that the similarity measure traces non-trivial physical properties and contains information about complex structures in the data. We use it for visualization and clustering of the data set, and discuss its ability to find groups of highly similar objects, including spectroscopic twins. Using the similarity matrix to search the data set for objects allows us to find objects that are impossible to find using their best-fitting model parameters. This includes extreme objects for which the models fail, and rare objects that are outside the scope of the model. We use the similarity measure to detect outliers in the data set, and find a number of previously unknown Be-type stars, spectroscopic binaries, carbon rich stars, young stars, and a few that we cannot interpret. Our work further demonstrates the potential for scientific discovery when combining machine learning methods with modern survey data.

Cosmic Spider is Good Mother

NASA Astrophysics Data System (ADS)

2006-04-01

Hanging above the Large Magellanic Cloud (LMC) - one of our closest galaxies - in what some describe as a frightening sight, the Tarantula nebula is worth looking at in detail. Also designated 30 Doradus or NGC 2070, the nebula owes its name to the arrangement of its brightest patches of nebulosity that somewhat resemble the legs of a spider. This name, of the biggest spiders on Earth, is also very fitting in view of the gigantic proportions of the celestial nebula - it measures nearly 1,000 light years across! ESO PR Photo 11/06 ESO PR Photo 13b/06 Tarantula's Central Cluster, R136 The Tarantula nebula is the largest emission nebula in the sky and also one of the largest known star-forming regions in all the Milky Way's neighbouring galaxies. Located about 170,000 light-years away, in the southern constellation Dorado (The Swordfish), it can be seen with the unaided eye. As shown in this image obtained with the FORS1 multi-mode instrument on ESO's Very Large Telescope, its structure is fascinatingly complex, with a large number of bright arcs and apparently dark areas in between. Inside the giant emission nebula lies a cluster of young, massive and hot stars, denoted R 136, whose intense radiation and strong winds make the nebula glow, shaping it into the form of a giant arachnid. The cluster is about 2 to 3 million years old, that is, almost from 'yesterday' in the 13.7 billion year history of the Universe. Several of the brighter members in the immediate surroundings of the dense cluster are among the most massive stars known, with masses well above 50 times the mass of our Sun. The cluster itself contains more than 200 massive stars. ESO PR Photo 11/06 ESO PR Photo 13c/06 The Stellar Cluster Hodge 301 In the upper right of the image, another cluster of bright, massive stars is seen. Known to astronomers as Hodge 301, it is about 20 million years old, or about 10 times older than R136. The more massive stars of Hodge 301 have therefore already exploded as supernovae, blasting material away at tremendous speed and creating a web of entangled filaments. More explosions will come soon - in astronomical terms - as three red supergiants are indeed present in Hodge 301 that will end their life in the gigantic firework of a supernova within the next million years. ESO PR Photo 13d/06 ESO PR Photo 13d/06 Gas Pillars in Tarantula Nebula While some stars are dying in this spidery cosmic inferno, others are yet to be born. Some structures, seen in the lower part of the image, have the appearance of elephant trunks, not unlike the famous and fertile "Pillars of Creation" at the top of which stars are forming. In fact, it seems that stars form all over the place in this gigantic stellar nursery and in all possible masses, at least down to the mass of our Sun. In some places, in a marvellous recycling process, it is the extreme radiation from the hot and massive stars and the shocks created by the supernova explosions that has compressed the gas to such extent to allow stars to form. To the right and slightly below the central cluster, a red bubble is visible. The star that blows the material making this bubble is thought to be 20 times more massive, 130 000 times more luminous, 10 times larger and 6 times hotter than our Sun. A possible fainter example of such a bubble is also visible just above the large red bubble in the image. ESO PR Photo 13e/06 ESO PR Photo 13e/06 Red Bubbles in Tarantula Nebula Earlier colour composite images of the Tarantula nebula have been made with other instruments and/or filters at ESO's telescopes, e.g. PR Photo 05a/00 in visual light with FORS2 at the VLT at Paranal, and PR Photos 14a-g/02 and 34a-h/04 with the Wide-Field Imager at the ESO/MPG 2.2-m telescope at La Silla.

The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters - XIV. Multiple stellar populations within M 15 and their radial distribution

NASA Astrophysics Data System (ADS)

Nardiello, D.; Milone, A. P.; Piotto, G.; Anderson, J.; Bedin, L. R.; Bellini, A.; Cassisi, S.; Libralato, M.; Marino, A. F.

2018-06-01

In the context of the Hubble Space Telescope UV Survey of Galactic globular clusters (GCs), we derived high-precision, multi-band photometry to investigate the multiple stellar populations in the massive and metal-poor GC M 15. By creating for red-giant branch (RGB) stars of the cluster a `chromosome map', which is a pseudo two-colour diagram made with appropriate combination of F275W, F336W, F438W, and F814W magnitudes, we revealed colour spreads around two of the three already known stellar populations. These spreads cannot be produced by photometric errors alone and could hide the existence of (two) additional populations. This discovery increases the complexity of the multiple-population phenomenon in M 15. Our analysis shows that M 15 exhibits a faint sub-giant branch (SGB), which is also detected in colour-magnitude diagrams (CMDs) made with optical magnitudes only. This poorly populated SGB includes about 5 per cent of the total number of SGB stars and evolves into a red RGB in the mF336W versus mF336W - mF814W CMD, suggesting that M 15 belongs to the class of Type II GCs. We measured the relative number of stars in each population at various radial distances from the cluster centre, showing that all of these populations share the same radial distribution within statistic uncertainties. These new findings are discussed in the context of the formation and evolution scenarios of the multiple populations.

Collisions in primordial star clusters. Formation pathway for intermediate mass black holes

NASA Astrophysics Data System (ADS)

Reinoso, B.; Schleicher, D. R. G.; Fellhauer, M.; Klessen, R. S.; Boekholt, T. C. N.

2018-06-01

Collisions were suggested to potentially play a role in the formation of massive stars in present day clusters, and have likely been relevant during the formation of massive stars and intermediate mass black holes within the first star clusters. In the early Universe, the first stellar clusters were particularly dense, as fragmentation typically only occurred at densities above 109 cm-3, and the radii of the protostars were enhanced as a result of larger accretion rates, suggesting a potentially more relevant role of stellar collisions. We present here a detailed parameter study to assess how the number of collisions and the mass growth of the most massive object depend on the properties of the cluster. We also characterize the time evolution with three effective parameters: the time when most collisions occur, the duration of the collisions period, and the normalization required to obtain the total number of collisions. We apply our results to typical Population III (Pop. III) clusters of about 1000 M⊙, finding that a moderate enhancement of the mass of the most massive star by a factor of a few can be expected. For more massive Pop. III clusters as expected in the first atomic cooling halos, we expect a more significant enhancement by a factor of 15-32. We therefore conclude that collisions in massive Pop. III clusters were likely relevant to form the first intermediate mass black holes.

B, V Photometry for ~19,000 Stars in and around the Magellanic Cloud Globular Clusters NGC 1466, NGC 1841, NGC 2210, NGC 2257, and Reticulum

NASA Astrophysics Data System (ADS)

Jeon, Young-Beom; Nemec, James M.; Walker, Alistair R.; Kunder, Andrea M.

2014-06-01

Homogeneous B, V photometry is presented for 19,324 stars in and around 5 Magellanic Cloud globular clusters: NGC 1466, NGC 1841, NGC 2210, NGC 2257, and Reticulum. The photometry is derived from eight nights of CCD imaging with the Cerro Tololo Inter-American Observatory 0.9 m SMARTS telescope. Instrumental magnitudes were transformed to the Johnson B, V system using accurate calibration relations based on a large sample of Landolt-Stetson equatorial standard stars, which were observed on the same nights as the cluster stars. Residual analysis of the equatorial standards used for the calibration, and validation of the new photometry using Stetson's sample of secondary standards in the vicinities of the five Large Magellanic Cloud clusters, shows excellent agreement with our values in both magnitudes and colors. Color-magnitude diagrams reaching to the main-sequence turnoffs at V ~ 22 mag, sigma-magnitude diagrams, and various other summaries are presented for each cluster to illustrate the range and quality of the new photometry. The photometry should prove useful for future studies of the Magellanic Cloud globular clusters, particularly studies of their variable stars.

A Look into the Hellish Cradles of Suns and Solar Systems

NASA Astrophysics Data System (ADS)

2009-09-01

New images released today by ESO delve into the heart of a cosmic cloud, called RCW 38, crowded with budding stars and planetary systems. There, young stars bombard fledgling suns and planets with powerful winds and blazing light, helped in their task by short-lived, massive stars that explode as supernovae. In some cases, this onslaught cooks away the matter that may eventually form new solar systems. Scientists think that our own Solar System emerged from such an environment. The dense star cluster RCW 38 glistens about 5500 light years away in the direction of the constellation Vela (the Sails). Like the Orion Nebula Cluster, RCW 38 is an "embedded cluster", in that the nascent cloud of dust and gas still envelops its stars. Astronomers have determined that most stars, including the low mass, reddish ones that outnumber all others in the Universe, originate in these matter-rich locations. Accordingly, embedded clusters provide scientists with a living laboratory in which to explore the mechanisms of star and planetary formation. "By looking at star clusters like RCW 38, we can learn a great deal about the origins of our Solar System and others, as well as those stars and planets that have yet to come", says Kim DeRose, first author of the new study that appears in the Astronomical Journal. DeRose did her work on RCW 38 as an undergraduate student at the Harvard-Smithsonian Center for Astrophysics, USA. Using the NACO adaptive optics instrument on ESO's Very Large Telescope [1], astronomers have obtained the sharpest image yet of RCW 38. They focused on a small area in the centre of the cluster that surrounds the massive star IRS2, which glows in the searing, white-blue range, the hottest surface colour and temperatures possible for stars. These dramatic observations revealed that IRS2 is actually not one, but two stars - a binary system consisting of twin scorching stars, separated by about 500 times the Earth-Sun distance. In the NACO image, the astronomers found a handful of protostars - the faintly luminous precursors to fully realised stars - and dozens of other candidate stars that have eked out an existence here despite the powerful ultraviolet light radiated by IRS2. Some of these gestating stars may, however, not get past the protostar stage. IRS2's strong radiation energises and disperses the material that might otherwise collapse into new stars, or that has settled into so-called protoplanetary discs around developing stars. In the course of several million years, the surviving discs may give rise to the planets, moons and comets that make up planetary systems like our own. As if intense ultraviolet rays were not enough, crowded stellar nurseries like RCW 38 also subject their brood to frequent supernovae when giant stars explode at the ends of their lives. These explosions scatter material throughout nearby space, including rare isotopes - exotic forms of chemical elements that are created in these dying stars. This ejected material ends up in the next generation of stars that form nearby. Because these isotopes have been detected in our Sun, scientists have concluded that the Sun formed in a cluster like RCW 38, rather than in a more rural portion of the Milky Way. "Overall, the details of astronomical objects that adaptive optics reveals are critical in understanding how new stars and planets form in complex, chaotic regions like RCW 38", says co-author Dieter Nürnberger. Notes [1] The name "NACO" is a combination of the Nasmyth Adaptive Optics System (NAOS) and the Near-Infrared Imager and Spectrograph (CONICA). Adaptive optics cancels out most of the image-distorting turbulence in Earth's atmosphere caused by temperature variations and wind. More information This research was presented in a paper that appeared in the Astronomical Journal: A Very Large Telescope / NACO study of star formation in the massive embedded cluster RCW 38, by DeRose et al. (2009, AJ, 138, 33-45). The team is composed of K.L. DeRose, T.L. Bourke, R.A. Gutermuth and S.J. Wolk (Harvard-Smithsonian Center for Astrophysics, Cambridge, USA), S.T. Megeath (Department of Physics and Astronomy, The University of Toledo, USA), J. Alves (Centro Astronómico Hispano Alemán, Almeria, Spain), and D. Nürnberger (ESO). ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world's most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become "the world's biggest eye on the sky".

On the assessment of the nature of open star clusters and the determination of their basic parameters with limited data

NASA Astrophysics Data System (ADS)

Carraro, Giovanni; Baume, Gustavo; Seleznev, Anton F.; Costa, Edgardo

2017-07-01

Our knowledge of stellar evolution and of the structure and chemical evolution of the Galactic disk largely builds on the study of open star clusters. Because of their crucial role in these relevant topics, large homogeneous catalogues of open cluster parameters are highly desirable. Although efforts have been made to develop automatic tools to analyse large numbers of clusters, the results obtained so far vary from study to study, and sometimes are very contradictory when compared to dedicated studies of individual clusters. In this work we highlight the common causes of these discrepancies for some open clusters, and show that at present dedicated studies yield a much better assessment of the nature of star clusters, even in the absence of ideal data-sets. We make use of deep, wide-field, multi-colour photometry to discuss the nature of six strategically selected open star clusters: Trumpler 22, Lynga 6, Hogg 19, Hogg 21, Pismis 10 and Pismis 14. We have precisely derived their basic parameters by means of a combination of star counts and photometric diagrams. Trumpler 22 and Lynga 6 are included in our study because they are widely known, and thus provided a check of our data and methodology. The remaining four clusters are very poorly known, and their available parameters have been obtained using automatic tools only. Our results are in some cases in severe disagreement with those from automatic surveys.

Pre-main-sequence stars in the young cluster IC 2391

NASA Technical Reports Server (NTRS)

Stauffer, John; Hartmann, Lee W.; Jones, Burton F.; Mcnamara, Brian R.

1989-01-01

Seven or eight new, late-type members of the poor open cluster IC 2391 are identified, and membership is confirmed for two other stars. The new members fall approximately along a 3 x 10 to the 7th yr isochrone, which is the age estimated for the cluster on the basis of it super main-seqence turnoff. Echelle spectra were obtained for the most probable cluster members. Most show H-alpha in emission and a strong Li 6707 A absorption line, and a few are rapid rotators. The Li abundances for cluster stars cooler than the sun are considerably less than the primordial Li abundance, providing the first direct evidence for substantial premain-sequence Li burning. The rotational velocities show a range from about 15 to 150 km/s, with a distribution of rotational velocities not significantly different from that observed for low-mass stars in the Pleiades.

BLUE STRAGGLERS IN GLOBULAR CLUSTER 47 TUCANAE

NASA Technical Reports Server (NTRS)

2002-01-01

The core of globular cluster 47 Tucanae is home to many blue stragglers, rejuvenated stars that glow with the blue light of young stars. A ground-based telescope image (on the left) shows the entire crowded core of 47 Tucanae, located 15,000 light-years away in the constellation Tucana. Peering into the heart of the globular cluster's bright core, the Hubble Space Telescope's Wide Field and Planetary Camera 2 separated the dense clump of stars into many individual stars (image on right). Some of these stars shine with the light of old stars; others with the blue light of blue stragglers. The yellow circles in the Hubble telescope image highlight several of the cluster's blue stragglers. Analysis for this observation centered on one massive blue straggler. Astronomers theorize that blue stragglers are formed either by the slow merger of stars in a double-star system or by the collision of two unrelated stars. For the blue straggler in 47 Tucanae, astronomers favor the slow merger scenario. This image is a 3-color composite of archival Hubble Wide Field and Planetary Camera 2 images in the ultraviolet (blue), blue (green), and violet (red) filters. Color tables were assigned and scaled so that the red giant stars appear orange, main-sequence stars are white/green, and blue stragglers are appropriately blue. The ultraviolet images were taken on Oct. 25, 1995, and the blue and violet images were taken on Sept. 1, 1995. Credit: Rex Saffer (Villanova University) and Dave Zurek (STScI), and NASA

Map-based trigonometric parallaxes of open clusters: Coma

NASA Technical Reports Server (NTRS)

Gatewood, George

1995-01-01

This is the fourth study in a series to determine the direct trigonometric parallaxes of four of the nearest open star clusters, the Hyades, the Pleiades, the Praesepe, and the nearby cluster in Coma (Gatewood et al. 1990; Gatewood et al. 1992); Gatewood & Kiewiet de Jonge 1994). The results for the open star cluster in Coma are compared with those of the other three clusters, and the members are found to be significantly subluminous. The trigonometric parallax of the cluster is estimated from that of three members studied with the Multichannel Astrometric Photometer (MAP) at the Thaw Refractor of the University of Pittsburgh's Allegheny Observatory. The weighted mean parallax of the cluster is +13.53 +/- 0.54 mass (0.00054 min), corresponding to a distance modulus of 4.34 +/- 0.09 mag. The U-B excess of the Coma cluster members may be used to adjust the observed absolute magnitudes and the B-V measurements as suggested by Sandage & Eggen (1959). The agreement obtained in this manner suggests that, like subdwarf stars, the stars of the Coma cluster appear subluminous because of line blanketing. One of the three members observed in this study was recognized as a member by its parallax and is the faintest known member of the cluster.

Map-based trigonometric parallaxes of open clusters: Coma

NASA Astrophysics Data System (ADS)

Gatewood, George

1995-06-01

This is the fourth study in a series to determine the direct trigonometric parallaxes of four of the nearest open star clusters, the Hyades, the Pleiades, the Praesepe, and the nearby cluster in Coma (Gatewood et al. 1990; Gatewood et al. 1992); Gatewood & Kiewiet de Jonge 1994). The results for the open star cluster in Coma are compared with those of the other three clusters, and the members are found to be significantly subluminous. The trigonometric parallax of the cluster is estimated from that of three members studied with the Multichannel Astrometric Photometer (MAP) at the Thaw Refractor of the University of Pittsburgh's Allegheny Observatory. The weighted mean parallax of the cluster is +13.53 +/- 0.54 mass (0.00054 min), corresponding to a distance modulus of 4.34 +/- 0.09 mag. The U-B excess of the Coma cluster members may be used to adjust the observed absolute magnitudes and the B-V measurements as suggested by Sandage & Eggen (1959). The agreement obtained in this manner suggests that, like subdwarf stars, the stars of the Coma cluster appear subluminous because of line blanketing. One of the three members observed in this study was recognized as a member by its parallax and is the faintest known member of the cluster.

The impact of radiation feedback on the assembly of star clusters in a galactic context

NASA Astrophysics Data System (ADS)

Guillard, Nicolas; Emsellem, Eric; Renaud, Florent

2018-07-01

Massive star clusters are observed in galaxies spanning a broad range of luminosities and types, and are assumed to form in dense gas-rich environments. Using a parsec-resolution hydrodynamical simulation of an isolated gas-rich low-mass galaxy, we discuss here the non-linear effects of stellar feedback on the properties of star clusters with a focus on the progenitors of nuclear clusters. Our simulation shows two categories of star clusters: those for which feedback expels gas leftovers associated with their formation sites, and those, in a denser environment, around which feedback fails to totally clear the gas. We confirm that radiation feedback (photoionization and radiative pressure) plays a more important role than Type II supernovae in destroying dense gas structures, and in altering or quenching the subsequent cluster formation. Radiation feedback also disturbs the cluster mass growth, by increasing the internal energy of the gas component to the point at which radiation pressure overcomes the cluster gravity. We discuss how these effects may depend on the local properties of the interstellar medium, and also on the details of the subgrid recipes, which can affect the available cluster gas reservoirs, the evolution of potential nuclear cluster progenitors, and the overall galaxy morphology.

Star Formation History In Merging Galaxies

NASA Astrophysics Data System (ADS)

Chien, Li-Hsin

2009-01-01

Interacting and merging galaxies are believed to play an important role in many aspects of galactic evolution. Their violent interactions can trigger starbursts, which lead to formation of young globular clusters. Therefore the ages of these young globular clusters can be interpreted to yield the timing of interaction-triggered events, and thus provide a key to reconstruct the star formation history in merging galaxies. The link between galaxy interaction and star formation is well established, but the triggers of star formation in interacting galaxies are still not understood. To date there are two competing formulas that describe the star formation mechanism--density-dependent and shock-induced rules. Numerical models implementing the two rules predict significantly different star formation histories in merging galaxies. My dissertation combines these two distinct areas of astrophysics, stellar evolution and galactic dynamics, to investigate the star formation history in galaxies at various merging stages. Begin with NGC 4676 as an example, I will briefly describe its model and illustrate the idea of using the ages of clusters to constrain the modeling. The ages of the clusters are derived from spectra that were taken with multi-object spectroscopy on Keck. Using NGC 7252 as a second example, I will present a state of the art dynamical model which predicts NGC7252's star formation history and other properties. I will then show a detailed comparison and analysis between the clusters and the modeling. In the end, I will address this important link as the key to answer the fundamental question of my thesis: what is the trigger of star formation in merging galaxies?

LITHIUM-RICH GIANTS IN GLOBULAR CLUSTERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kirby, Evan N.; Cohen, Judith G.; Guhathakurta, Puragra

Although red giants deplete lithium on their surfaces, some giants are Li-rich. Intermediate-mass asymptotic giant branch (AGB) stars can generate Li through the Cameron–Fowler conveyor, but the existence of Li-rich, low-mass red giant branch (RGB) stars is puzzling. Globular clusters are the best sites to examine this phenomenon because it is straightforward to determine membership in the cluster and to identify the evolutionary state of each star. In 72 hours of Keck/DEIMOS exposures in 25 clusters, we found four Li-rich RGB and two Li-rich AGB stars. There were 1696 RGB and 125 AGB stars with measurements or upper limits consistentmore » with normal abundances of Li. Hence, the frequency of Li-richness in globular clusters is (0.2 ± 0.1)% for the RGB, (1.6 ± 1.1)% for the AGB, and (0.3 ± 0.1)% for all giants. Because the Li-rich RGB stars are on the lower RGB, Li self-generation mechanisms proposed to occur at the luminosity function bump or He core flash cannot explain these four lower RGB stars. We propose the following origin for Li enrichment: (1) All luminous giants experience a brief phase of Li enrichment at the He core flash. (2) All post-RGB stars with binary companions on the lower RGB will engage in mass transfer. This scenario predicts that 0.1% of lower RGB stars will appear Li-rich due to mass transfer from a recently Li-enhanced companion. This frequency is at the lower end of our confidence interval.« less

Neutron Star Discovered Where a Black Hole Was Expected

NASA Astrophysics Data System (ADS)

2005-11-01

A very massive star collapsed to form a neutron star and not a black hole as expected, according to new results from NASA's Chandra X-ray Observatory. This discovery shows that nature has a harder time making black holes than previously thought. Scientists found this neutron star -- a dense whirling ball of neutrons about 12 miles in diameter -- in an extremely young star cluster. Astronomers were able to use well-determined properties of other stars in the cluster to deduce that the progenitor of this neutron star was at least 40 times the mass of the Sun. ESO Optical Image of Westerlund 1 ESO Optical Image of Westerlund 1 "Our discovery shows that some of the most massive stars do not collapse to form black holes as predicted, but instead form neutron stars," said Michael Muno, a UCLA postdoctoral Hubble Fellow and lead author of a paper to be published in The Astrophysical Journal Letters. When very massive stars make neutron stars and not black holes, they will have a greater influence on the composition of future generations of stars. When the star collapses to form the neutron star, more than 95% of its mass, much of which is metal-rich material from its core, is returned to the space around it. "This means that enormous amounts of heavy elements are put back into circulation and can form other stars and planets," said J. Simon Clark of the Open University in the United Kingdom. Animation: Dissolve from Optical to X-ray Image of Westerlund 1 Animation: Dissolve from Optical to X-ray Image of Westerlund 1 Astronomers do not completely understand how massive a star must be to form a black hole rather than a neutron star. The most reliable method for estimating the mass of the progenitor star is to show that the neutron star or black hole is a member of a cluster of stars, all of which are close to the same age. Because more massive stars evolve faster than less massive ones, the mass of a star can be estimated from if its evolutionary stage is known. Neutron stars and black holes are the end stages in the evolution of a star, so their progenitors must have been among the most massive stars in the cluster. Muno and colleagues discovered a pulsing neutron star in a cluster of stars known as Westerlund 1. This cluster contains a hundred thousand or more stars in a region only 30 light years across, which suggests that all the stars were born in a single episode of star formation. Based on optical properties such as brightness and color some of the normal stars in the cluster are known to have masses of about 40 suns. Since the progenitor of the neutron star has already exploded as a supernova, its mass must have been more than 40 solar masses. 2MASS Infrared Image of Westerlund 1 2MASS Infrared Image of Westerlund 1 Introductory astronomy courses sometimes teach that stars with more than 25 solar masses become black holes -- a concept that until recently had no observational evidence to test it. However, some theories allow such massive stars to avoid becoming black holes. For example, theoretical calculations by Alexander Heger of the University of Chicago and colleagues indicate that extremely massive stars blow off mass so effectively during their lives that they leave neutron stars when they go supernovae. Assuming that the neutron star in Westerlund 1 is one of these, it raises the question of where the black holes observed in the Milky Way and other galaxies come from. Other factors, such as the chemical composition of the star, how rapidly it is rotating, or the strength of its magnetic field might dictate whether a massive star leaves behind a neutron star or a black hole. The theory for stars of normal chemical composition leaves a small window of initial masses - between about 25 and somewhat less than 40 solar masses - for the formation of black holes from the evolution of single massive stars. The identification of additional neutron stars or the discovery of black holes in young star clusters should further constrain the masses and properties of neutron star and black hole progenitors. The work described by Muno was based on two Chandra observations on May 22 and June 18, 2005. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the agency's Science Mission Directorate. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass. Additional information and images are available at: http://chandra.harvard.edu and http://chandra.nasa.gov

Dynamical age differences among coeval star clusters as revealed by blue stragglers.

PubMed

Ferraro, F R; Lanzoni, B; Dalessandro, E; Beccari, G; Pasquato, M; Miocchi, P; Rood, R T; Sigurdsson, S; Sills, A; Vesperini, E; Mapelli, M; Contreras, R; Sanna, N; Mucciarelli, A

2012-12-20

Globular star clusters that formed at the same cosmic time may have evolved rather differently from the dynamical point of view (because that evolution depends on the internal environment) through a variety of processes that tend progressively to segregate stars more massive than the average towards the cluster centre. Therefore clusters with the same chronological age may have reached quite different stages of their dynamical history (that is, they may have different 'dynamical ages'). Blue straggler stars have masses greater than those at the turn-off point on the main sequence and therefore must be the result of either a collision or a mass-transfer event. Because they are among the most massive and luminous objects in old clusters, they can be used as test particles with which to probe dynamical evolution. Here we report that globular clusters can be grouped into a few distinct families on the basis of the radial distribution of blue stragglers. This grouping corresponds well to an effective ranking of the dynamical stage reached by stellar systems, thereby permitting a direct measure of the cluster dynamical age purely from observed properties.

Globular Cluster Contributions to the Galactic Halo

NASA Astrophysics Data System (ADS)

Martell, Sarah; Grebel, Eva; Lai, David

2010-08-01

The goal of this project is to confirm chemically that globular clusters are the source of as much as half the population of the Galactic halo. Using moderate-resolution spectroscopy from the SEGUE survey, we have identified a previously unknown population of halo field giants with distinctly strong CN features. CN variations are typically only observed in globular clusters, so these stars are interpreted as immigrants to the halo that originally formed in globular clusters. In one night of Keck/HIRES time, we will obtain high-quality, high- resolution spectra for five such stars, and determine abundances of O, Na, Mg, Al, alpha, iron-peak and neutron-capture elements. With this information we can state clearly whether these unusual CN-strong halo stars carry the full abundance pattern seen in CN-strong globular cluster stars, with depleted C, O, and Mg and enhanced N, Na, and Al. This type of coarse ``chemical tagging'' will allow a clearer division of the Galactic halo into contributions from globular clusters and from dwarf galaxies, and will place constraints on theoretical models of globular cluster formation and evolution.

A two-step initial mass function:. Consequences of clustered star formation for binary properties

NASA Astrophysics Data System (ADS)

Durisen, R. H.; Sterzik, M. F.; Pickett, B. K.

2001-06-01

If stars originate in transient bound clusters of moderate size, these clusters will decay due to dynamic interactions in which a hard binary forms and ejects most or all the other stars. When the cluster members are chosen at random from a reasonable initial mass function (IMF), the resulting binary characteristics do not match current observations. We find a significant improvement in the trends of binary properties from this scenario when an additional constraint is taken into account, namely that there is a distribution of total cluster masses set by the masses of the cloud cores from which the clusters form. Two distinct steps then determine final stellar masses - the choice of a cluster mass and the formation of the individual stars. We refer to this as a ``two-step'' IMF. Simple statistical arguments are used in this paper to show that a two-step IMF, combined with typical results from dynamic few-body system decay, tends to give better agreement between computed binary characteristics and observations than a one-step mass selection process.

MASS OUTFLOW AND CHROMOSPHERIC ACTIVITY OF RED GIANT STARS IN GLOBULAR CLUSTERS. II. M13 AND M92

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meszaros, Sz.; Dupree, A. K.; Szalai, T.

High-resolution spectra of 123 red giant stars in the globular cluster M13 and 64 red giant stars in M92 were obtained with Hectochelle at the MMT telescope. Emission and line asymmetries in H{alpha} and Ca II K are identified, characterizing motions in the extended atmospheres and seeking differences attributable to metallicity in these clusters and M15. On the red giant branch, emission in H{alpha} generally appears in stars with T {sub eff} {approx}< 4500 K and log L/L {sub sun}{approx}> 2.75. Fainter stars showing emission are asymptotic giant branch (AGB) stars or perhaps binary stars. The line-bisector for H{alpha} revealsmore » the onset of chromospheric expansion in stars more luminous than log (L/L {sub sun}) {approx} 2.5 in all clusters, and this outflow velocity increases with stellar luminosity. However, the coolest giants in the metal-rich M13 show greatly reduced outflow in H{alpha} most probably due to decreased T {sub eff} and changing atmospheric structure. The Ca II K{sub 3} outflow velocities are larger than shown by H{alpha} at the same luminosity and signal accelerating outflows in the chromospheres. Stars clearly on the AGB show faster chromospheric outflows in H{alpha} than RGB objects. While the H{alpha} velocities on the RGB are similar for all metallicities, the AGB stars in the metal-poor M15 and M92 have higher outflow velocities than in the metal-rich M13. Comparison of these chromospheric line profiles in the paired metal-poor clusters, M15 and M92, shows remarkable similarities in the presence of emission and dynamical signatures, and does not reveal a source of the 'second-parameter' effect.« less

Mapping the spatial distribution of star formation in cluster galaxies at z ~0.5 with the Grism Lens-Amplified Survey from Space (GLASS)

NASA Astrophysics Data System (ADS)

Vulcani, Benedetta

2015-08-01

What physical processes regulate star formation in dense environments? Understanding why galaxy evolution is environment dependent is one of the key questions of current astrophysics. I will present the first characterization of the spatial distribution of star formation in cluster galaxies at z~0.5, in order to quantify the role of different physical processes that are believed to be responsible for shutting down star formation. The analysis makes use of data from the Grism Lens-Amplified Survey from Space (GLASS), a large HST cycle-21 program targeting 10 massive galaxy clusters with extensive HST imaging from CLASH and the Frontier Field Initiative. The program consists of 140 primary and 140 parallel orbits of near-infrared WCF3 and optical ACS slitless grism observations, which result in 3D spectroscopy of hundreds of galaxies. The grism data are used to produce spatially resolved maps of the star formation density, while the stellar mass density and optical surface brightness are obtained from multiband imaging. I will describe quantitative measures of the spatial location and extend of the star formation rate, showing that about half of the cluster members with significant Halpha detection have diffused star formation, larger than the optical counterpart. This suggests that star formation occurs out to larger radii than the rest frame continuum. For some systems, nuclear star forming regions are found. I will also present a comparison between the Halpha distribution observed in cluster and field galaxies. The characterization of the spatial distribution of Halpha provides a new window, yet poorly exploited, on the mechanisms that regulate star formation and morphological transformation in dense environments.

Extinction in the Star Cluster SAI 113 and Galactic Structure in Carina

NASA Astrophysics Data System (ADS)

Carraro, Giovanni; Turner, David G.; Majaess, Daniel J.; Baume, Gustavo L.; Gamen, Roberto; Molina Lera, José A.

2017-04-01

Photometric CCD UB VI C photometry obtained for 4860 stars surrounding the embedded southern cluster SAI 113 (Skiff 8) is used to examine the reddening in the field and derive the distance to the cluster and nearby van Genderen 1. Spectroscopic color excesses for bright cluster stars, photometric reddenings for A3 dwarfs, and dereddening of cluster stars imply that the reddening and extinction laws match results derived for other young clusters in Carina: {E}U-B/{E}B-V≃ 0.64 and {R}V≃ 4. SAI 113 displays features that may be linked to a history of dynamical interactions among member stars: possible circumstellar reddening and rapid rotation of late B-type members, ringlike features in star density, and a compact core, with most stars distributed randomly across the field. The group van Genderen 1 resembles a stellar asterism, with potential members distributed randomly across the field. Distances of 3.90 ± 0.19 kpc and 2.49 ± 0.09 kpc are derived for SAI 113 and van Genderen 1, respectively, with variable reddenings {E}B-V ranging from 0.84 to 1.29 and 0.23 to 1.28. The SRC variables CK Car and EV Car may be outlying members of van Genderen 1, thereby of use for calibrating the period-luminosity relation for pulsating M supergiants. More importantly, the anomalous reddening and extinction evident in Carina and nearby regions of the Galactic plane in the fourth quadrant impact the mapping of spiral structure from young open clusters. The distribution of spiral arms in the fourth quadrant may be significantly different from how it is often portrayed.

A New Look at the Molecular Gas in M42 and M43: Possible Evidence for Cloud–Cloud Collision that Triggered Formation of the OB Stars in the Orion Nebula Cluster

NASA Astrophysics Data System (ADS)

Fukui, Yasuo; Torii, Kazufumi; Hattori, Yusuke; Nishimura, Atsushi; Ohama, Akio; Shimajiri, Yoshito; Shima, Kazuhiro; Habe, Asao; Sano, Hidetoshi; Kohno, Mikito; Yamamoto, Hiroaki; Tachihara, Kengo; Onishi, Toshikazu

2018-06-01

The Orion Nebula Cluster toward the H II region M42 is the most outstanding young cluster at the smallest distance (410 pc) among the rich high-mass stellar clusters. By newly analyzing the archival molecular data of the 12CO(J = 1–0) emission at 21″ resolution, we identified at least three pairs of complementary distributions between two velocity components at 8 and 13 km s‑1. We present a hypothesis that the two clouds collided with each other and triggered formation of the high-mass stars, mainly toward two regions including the nearly 10 O stars in M42 and the B star, NU Ori, in M43. The timescale of the collision is estimated to be ∼0.1 Myr by a ratio of the cloud size and velocity corrected for projection, which is consistent with the age of the youngest cluster members less than 0.1 Myr. The majority of the low-mass cluster members were formed prior to the collision in the last Myr. We discuss the implications of the present hypothesis and the scenario of high-mass star formation by comparing with the other eight cases of triggered O-star formation via cloud–cloud collision.

Quantifying the coexistence of massive black holes and dense nuclear star clusters

NASA Astrophysics Data System (ADS)

Graham, Alister W.; Spitler, Lee R.

2009-08-01

In large spheroidal stellar systems, such as elliptical galaxies, one invariably finds a 106-109Msolar supermassive black hole at their centre. In contrast, within dwarf elliptical galaxies one predominantly observes a 105-107Msolar nuclear star cluster. To date, few galaxies have been found with both types of nuclei coexisting and even less have had the masses determined for both central components. Here, we identify one dozen galaxies housing nuclear star clusters and supermassive black holes whose masses have been measured. This doubles the known number of such hermaphrodite nuclei - which are expected to be fruitful sources of gravitational radiation. Over the host spheroid (stellar) mass range 108-1011Msolar, we find that a galaxy's nucleus-to-spheroid (baryon) mass ratio is not a constant value but decreases from a few per cent to ~0.3 per cent such that log[(MBH + MNC)/Msph] = -(0.39 +/- 0.07) log[Msph/1010Msolar] - (2.18 +/- 0.07). Once dry merging commences and the nuclear star clusters disappear, this ratio is expected to become a constant value. As a byproduct of our investigation, we have found that the projected flux from resolved nuclear star clusters is well approximated with Sérsic functions having a range of indices from ~0.5 to ~3, the latter index describing the Milky Way's nuclear star cluster.

Building black holes: supercomputer cinema.

PubMed

Shapiro, S L; Teukolsky, S A

1988-07-22

A new computer code can solve Einstein's equations of general relativity for the dynamical evolution of a relativistic star cluster. The cluster may contain a large number of stars that move in a strong gravitational field at speeds approaching the speed of light. Unstable star clusters undergo catastrophic collapse to black holes. The collapse of an unstable cluster to a supermassive black hole at the center of a galaxy may explain the origin of quasars and active galactic nuclei. By means of a supercomputer simulation and color graphics, the whole process can be viewed in real time on a movie screen.

Hyperfast pulsars as the remnants of massive stars ejected from young star clusters

NASA Astrophysics Data System (ADS)

Gvaramadze, Vasilii V.; Gualandris, Alessia; Portegies Zwart, Simon

2008-04-01

Recent proper motion and parallax measurements for the pulsar PSR B1508+55 indicate a transverse velocity of ~1100kms-1, which exceeds earlier measurements for any neutron star. The spin-down characteristics of PSR B1508+55 are typical for a non-recycled pulsar, which implies that the velocity of the pulsar cannot have originated from the second supernova disruption of a massive binary system. The high velocity of PSR B1508+55 can be accounted for by assuming that it received a kick at birth or that the neutron star was accelerated after its formation in the supernova explosion. We propose an explanation for the origin of hyperfast neutron stars based on the hypothesis that they could be the remnants of a symmetric supernova explosion of a high-velocity massive star which attained its peculiar velocity (similar to that of the pulsar) in the course of a strong dynamical three- or four-body encounter in the core of dense young star cluster. To check this hypothesis, we investigated three dynamical processes involving close encounters between: (i) two hard massive binaries, (ii) a hard binary and an intermediate-mass black hole (IMBH) and (iii) a single stars and a hard binary IMBH. We find that main-sequence O-type stars cannot be ejected from young massive star clusters with peculiar velocities high enough to explain the origin of hyperfast neutron stars, but lower mass main-sequence stars or the stripped helium cores of massive stars could be accelerated to hypervelocities. Our explanation for the origin of hyperfast pulsars requires a very dense stellar environment of the order of 106- 107starspc-3. Although such high densities may exist during the core collapse of young massive star clusters, we caution that they have never been observed.

Hubble Admires a Youthful Globular Star Cluster

NASA Image and Video Library

2017-12-08

Hubble sees an unusal global cluster that is enriching the interstellar medium with metals Globular clusters offer some of the most spectacular sights in the night sky. These ornate spheres contain hundreds of thousands of stars, and reside in the outskirts of galaxies. The Milky Way contains over 150 such clusters — and the one shown in this NASA/ESA Hubble Space Telescope image, named NGC 362, is one of the more unusual ones. As stars make their way through life they fuse elements together in their cores, creating heavier and heavier elements — known in astronomy as metals — in the process. When these stars die, they flood their surroundings with the material they have formed during their lifetimes, enriching the interstellar medium with metals. Stars that form later therefore contain higher proportions of metals than their older relatives. By studying the different elements present within individual stars in NGC 362, astronomers discovered that the cluster boasts a surprisingly high metal content, indicating that it is younger than expected. Although most globular clusters are much older than the majority of stars in their host galaxy, NGC 362 bucks the trend, with an age lying between 10 and 11 billion years old. For reference, the age of the Milky Way is estimated to be above 13 billion years. This image, in which you can view NGC 362’s individual stars, was taken by Hubble’s Advanced Camera for Surveys (ACS). Credit: ESA/Hubble& NASA NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

A common origin for globular clusters and ultra-faint dwarfs in simulations of the first galaxies

DOE PAGES

Ricotti, Massimo; Parry, Owen H.; Gnedin, Nickolay Y.

2016-11-09

In this study, the first in a series on galaxy formation before reionization, we focus on understanding what determines the size and morphology of stellar objects in the first low-mass galaxies, using parsec-scale cosmological simulations performed with an adaptive mesh hydrodynamics code. Although the dense gas in which stars are formed tends to have a disk structure, stars are found in spheroids with little rotation. Halos with masses betweenmore » $${10}^{6}\\,{M}_{\\odot }$$ and $$5\\times {10}^{8}\\,{M}_{\\odot }$$ form stars stochastically, with stellar masses in the range $${10}^{4}\\,{M}_{\\odot }$$ to $$2\\times {10}^{6}\\,{M}_{\\odot }$$. We observe, nearly independent of stellar mass, a large range of half-light radii for the stars, from a few parsecs to a few hundred parsecs and surface brightnesses and mass-to-light ratios ranging from those typical of globular clusters to ultra-faint dwarfs. In our simulations, stars form in dense stellar clusters with high gas-to-star conversion efficiencies and rather uniform metallicities. A fraction of these clusters remain bound after the gas is removed by feedback, but others are destroyed, and their stars, which typically have velocity dispersions of 20–40 km s –1, expand until they become bound by the dark matter halo. We thus speculate that the stars in ultra-faint dwarf galaxies may show kinematic and chemical signatures consistent with their origin in a few distinct stellar clusters. On the other hand, some globular clusters may form at the center of primordial dwarf galaxies and may contain dark matter, perhaps detectable in the outer parts.« less

A common origin for globular clusters and ultra-faint dwarfs in simulations of the first galaxies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ricotti, Massimo; Parry, Owen H.; Gnedin, Nickolay Y.

In this study, the first in a series on galaxy formation before reionization, we focus on understanding what determines the size and morphology of stellar objects in the first low-mass galaxies, using parsec-scale cosmological simulations performed with an adaptive mesh hydrodynamics code. Although the dense gas in which stars are formed tends to have a disk structure, stars are found in spheroids with little rotation. Halos with masses betweenmore » $${10}^{6}\\,{M}_{\\odot }$$ and $$5\\times {10}^{8}\\,{M}_{\\odot }$$ form stars stochastically, with stellar masses in the range $${10}^{4}\\,{M}_{\\odot }$$ to $$2\\times {10}^{6}\\,{M}_{\\odot }$$. We observe, nearly independent of stellar mass, a large range of half-light radii for the stars, from a few parsecs to a few hundred parsecs and surface brightnesses and mass-to-light ratios ranging from those typical of globular clusters to ultra-faint dwarfs. In our simulations, stars form in dense stellar clusters with high gas-to-star conversion efficiencies and rather uniform metallicities. A fraction of these clusters remain bound after the gas is removed by feedback, but others are destroyed, and their stars, which typically have velocity dispersions of 20–40 km s –1, expand until they become bound by the dark matter halo. We thus speculate that the stars in ultra-faint dwarf galaxies may show kinematic and chemical signatures consistent with their origin in a few distinct stellar clusters. On the other hand, some globular clusters may form at the center of primordial dwarf galaxies and may contain dark matter, perhaps detectable in the outer parts.« less

Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II

NASA Astrophysics Data System (ADS)

Contenta, Filippo; Balbinot, Eduardo; Petts, James A.; Read, Justin I.; Gieles, Mark; Collins, Michelle L. M.; Peñarrubia, Jorge; Delorme, Maxime; Gualandris, Alessia

2018-05-01

We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the `ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ˜45 pc from its centre. Using a grid of collisional N-body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results, therefore, favour a DM core. This implies that either a cold DM cusp was `heated up' at the centre of Eri II by bursty star formation or we are seeing an evidence for physics beyond cold DM.

Star formation activity in the southern Galactic H II region G351.63-1.25

NASA Astrophysics Data System (ADS)

Vig, S.; Ghosh, S. K.; Ojha, D. K.; Verma, R. P.; Tamura, M.

2014-06-01

The southern Galactic high-mass star-forming region, G351.63-1.25, is an H II region-molecular cloud complex with a luminosity of ˜2.0 × 105 L⊙, located at a distance of 2.4 kpc from the Sun. In this paper, we focus on the investigation of the associated H II region, embedded cluster and the interstellar medium in the vicinity of G351.63-1.25. We address the identification of exciting source(s) as well as the census of the stellar populations, in an attempt to unfold star formation activity in this region. The ionized gas distribution has been mapped using the Giant Metrewave Radio Telescope, India, at three frequencies: 1280, 610 and 325 MHz. The H II region shows an elongated morphology and the 1280 MHz map comprises six resolved high-density regions encompassed by diffuse emission spanning 1.4 × 1.0 pc2. Based on the measurements of flux densities at multiple radio frequencies, the brightest ultracompact core has electron temperature Te˜7647 {±} 153 K and emission measure, EM˜2.0 {±} 0.8×107 cm-6 pc. The zero-age main-sequence spectral type of the brightest radio core is O7.5. We have carried out near-infrared observations in the JHKs bands using the SIRIUS camera on the 1.4 m Infrared Survey Facility telescope. The near-infrared images reveal the presence of a cluster embedded in nebulous fan-shaped emission. The log-normal slope of the K-band luminosity function of the embedded cluster is found to be ˜0.27 ± 0.03, and the fraction of the near-infrared excess stars is estimated to be 43 per cent. These indicate that the age of the cluster is consistent with ˜1 Myr. Other available data of this region show that the warm (mid-infrared) and cold (millimetre) dust emission peak at different locations indicating progressive stages of star formation process. The champagne flow model from a flat, thin molecular cloud is used to explain the morphology of radio emission with respect to the millimetre cloud and infrared brightness.

GLOBULAR CLUSTERS AND SPUR CLUSTERS IN NGC 4921, THE BRIGHTEST SPIRAL GALAXY IN THE COMA CLUSTER

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Myung Gyoon; Jang, In Sung, E-mail: mglee@astro.snu.ac.kr, E-mail: isjang@astro.snu.ac.kr

2016-03-01

We resolve a significant fraction of globular clusters (GCs) in NGC 4921, the brightest spiral galaxy in the Coma cluster. We also find a number of extended bright star clusters (star complexes) in the spur region of the arms. The latter are much brighter and bluer than those in the normal star-forming region, being as massive as 3 × 10{sup 5} M{sub ⊙}. The color distribution of the GCs in this galaxy is found to be bimodal. The turnover magnitudes of the luminosity functions of the blue (metal-poor) GCs (0.70 < (V − I) ≤ 1.05) in the halo are estimated V(max) = 27.11 ± 0.09 mag and I(max) = 26.21 ± 0.11 mag.more » We obtain similar values for NGC 4923, a companion S0 galaxy, and two Coma cD galaxies (NGC 4874 and NGC 4889). The mean value for the turnover magnitudes of these four galaxies is I(max) = 26.25 ± 0.03 mag. Adopting M{sub I} (max) = −8.56 ± 0.09 mag for the metal-poor GCs, we determine the mean distance to the four Coma galaxies to be 91 ± 4 Mpc. Combining this with the Coma radial velocity, we derive a value of the Hubble constant, H{sub 0} = 77.9 ± 3.6 km s{sup −1} Mpc{sup −1}. We estimate the GC specific frequency of NGC 4921 to be S{sub N} = 1.29 ± 0.25, close to the values for early-type galaxies. This indicates that NGC 4921 is in the transition phase to S0s.« less

Interacting star clusters in the Large Magellanic Cloud. Overmerging problem solved by cluster group formation

NASA Astrophysics Data System (ADS)

Leon, Stéphane; Bergond, Gilles; Vallenari, Antonella

1999-04-01

We present the tidal tail distributions of a sample of candidate binary clusters located in the bar of the Large Magellanic Cloud (LMC). One isolated cluster, SL 268, is presented in order to study the effect of the LMC tidal field. All the candidate binary clusters show tidal tails, confirming that the pairs are formed by physically linked objects. The stellar mass in the tails covers a large range, from 1.8x 10(3) to 3x 10(4) \\msun. We derive a total mass estimate for SL 268 and SL 356. At large radii, the projected density profiles of SL 268 and SL 356 fall off as r(-gamma ) , with gamma = 2.27 and gamma =3.44, respectively. Out of 4 pairs or multiple systems, 2 are older than the theoretical survival time of binary clusters (going from a few 10(6) years to 10(8) years). A pair shows too large age difference between the components to be consistent with classical theoretical models of binary cluster formation (Fujimoto & Kumai \\cite{fujimoto97}). We refer to this as the ``overmerging'' problem. A different scenario is proposed: the formation proceeds in large molecular complexes giving birth to groups of clusters over a few 10(7) years. In these groups the expected cluster encounter rate is larger, and tidal capture has higher probability. Cluster pairs are not born together through the splitting of the parent cloud, but formed later by tidal capture. For 3 pairs, we tentatively identify the star cluster group (SCG) memberships. The SCG formation, through the recent cluster starburst triggered by the LMC-SMC encounter, in contrast with the quiescent open cluster formation in the Milky Way can be an explanation to the paucity of binary clusters observed in our Galaxy. Based on observations collected at the European Southern Observatory, La Silla, Chile}

The VMC Survey. XI. Radial Stellar Population Gradients in the Galactic Globular Cluster 47 Tucanae

NASA Astrophysics Data System (ADS)

Li, Chengyuan; de Grijs, Richard; Deng, Licai; Rubele, Stefano; Wang, Chuchu; Bekki, Kenji; Cioni, Maria-Rosa L.; Clementini, Gisella; Emerson, Jim; For, Bi-Qing; Girardi, Leo; Groenewegen, Martin A. T.; Guandalini, Roald; Gullieuszik, Marco; Marconi, Marcella; Piatti, Andrés E.; Ripepi, Vincenzo; van Loon, Jacco Th.

2014-07-01

We present a deep near-infrared color-magnitude diagram of the Galactic globular cluster 47 Tucanae, obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA) as part of the VISTA near-infrared Y, J, K s survey of the Magellanic System (VMC). The cluster stars comprising both the subgiant and red giant branches exhibit apparent, continuous variations in color-magnitude space as a function of radius. Subgiant branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core; similarly, red-giant-branch stars in the cluster's periphery are bluer than their more centrally located cousins. The observations can very well be described by adopting an age spread of ~0.5 Gyr as well as radial gradients in both the cluster's helium abundance (Y) and metallicity (Z), which change gradually from (Y = 0.28, Z = 0.005) in the cluster core to (Y = 0.25, Z = 0.003) in its periphery. We conclude that the cluster's inner regions host a significant fraction of second-generation stars, which decreases with increasing radius; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population.

Radial Stellar Population Gradients in the Galactic Globular Cluster 47 Tucanae

NASA Astrophysics Data System (ADS)

de Grijs, Richard; Li, Chengyuan; Deng, Licai

2015-01-01

We present a deep near-infrared color-magnitude diagram of the Galactic globular cluster 47 Tucanae, obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA) as part of the VISTA near-infrared Y, J, Ks survey of the Magellanic System (VMC). The cluster stars comprising both the subgiant and red-giant branches exhibit apparent, continuous variations in color-magnitude space as a function of radius. Subgiant-branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core; similarly, red-giant-branch stars in the cluster's periphery are bluer than their more centrally located cousins. The observations can very well be described by adopting an age spread of ~0.5 Gyr as well as radial gradients in both the cluster's helium abundance (Y) and metallicity (Z), which change gradually from Y = 0.28, Z = 0.005 in the cluster core to Y = 0.25, Z = 0.003 in its periphery. We conclude that the cluster's inner regions host a significant fraction of second-generation stars, which decreases with increasing radius; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population.

LoCuSS: pre-processing in galaxy groups falling into massive galaxy clusters at z = 0.2

NASA Astrophysics Data System (ADS)

Bianconi, M.; Smith, G. P.; Haines, C. P.; McGee, S. L.; Finoguenov, A.; Egami, E.

2018-01-01

We report direct evidence of pre-processing of the galaxies residing in galaxy groups falling into galaxy clusters drawn from the Local Cluster Substructure Survey (LoCuSS). 34 groups have been identified via their X-ray emission in the infall regions of 23 massive ( = 1015 M⊙) clusters at 0.15 < z < 0.3. Highly complete spectroscopic coverage combined with 24 μm imaging from Spitzer allows us to make a consistent and robust selection of cluster and group members including star-forming galaxies down to a stellar mass limit of M⋆ = 2 × 1010 M⊙. The fraction fSF of star-forming galaxies in infalling groups is lower and with a flatter trend with respect to clustercentric radius when compared to the rest of the cluster galaxy population. At R ≈ 1.3 r200, the fraction of star-forming galaxies in infalling groups is half that in the cluster galaxy population. This is direct evidence that star-formation quenching is effective in galaxies already prior to them settling in the cluster potential, and that groups are favourable locations for this process.

Herschel And Alma Observations Of The Ism In Massive High-Redshift Galaxy Clusters

NASA Astrophysics Data System (ADS)

Wu, John F.; Aguirre, Paula; Baker, Andrew J.; Devlin, Mark J.; Hilton, Matt; Hughes, John P.; Infante, Leopoldo; Lindner, Robert R.; Sifón, Cristóbal

2017-06-01

The Sunyaev-Zel'dovich effect (SZE) can be used to select samples of galaxy clusters that are essentially mass-limited out to arbitrarily high redshifts. I will present results from an investigation of the star formation properties of galaxies in four massive clusters, extending to z 1, which were selected on the basis of their SZE decrements in the Atacama Cosmology Telescope (ACT) survey. All four clusters have been imaged with Herschel/PACS (tracing star formation rate) and two with ALMA (tracing dust and cold gas mass); newly discovered ALMA CO(4-3) and [CI] line detections expand an already large sample of spectroscopically confirmed cluster members. Star formation rate appears to anti-correlate with environmental density, but this trend vanishes after controlling for stellar mass. Elevated star formation and higher CO excitation are seen in "El Gordo," a violent cluster merger, relative to a virialized cluster at a similar high (z 1) redshift. Also exploiting ATCA 2.1 GHz observations to identify radio-loud active galactic nuclei (AGN) in our sample, I will use these data to develop a coherent picture of how environment influences galaxies' ISM properties and evolution in the most massive clusters at early cosmic times.

The VMC survey. XI. Radial stellar population gradients in the galactic globular cluster 47 Tucanae

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Chengyuan; De Grijs, Richard; Deng, Licai

2014-07-20

We present a deep near-infrared color-magnitude diagram of the Galactic globular cluster 47 Tucanae, obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA) as part of the VISTA near-infrared Y, J, K{sub s} survey of the Magellanic System (VMC). The cluster stars comprising both the subgiant and red giant branches exhibit apparent, continuous variations in color-magnitude space as a function of radius. Subgiant branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core; similarly, red-giant-branch stars in the cluster's periphery are bluer than their more centrally located cousins. The observations can verymore » well be described by adopting an age spread of ∼0.5 Gyr as well as radial gradients in both the cluster's helium abundance (Y) and metallicity (Z), which change gradually from (Y = 0.28, Z = 0.005) in the cluster core to (Y = 0.25, Z = 0.003) in its periphery. We conclude that the cluster's inner regions host a significant fraction of second-generation stars, which decreases with increasing radius; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population.« less

A CN Band Survey of Red Giants in the Globular Cluster M53

NASA Astrophysics Data System (ADS)

Martell, S. L.; Smith, G. H.

2004-12-01

We investigate the star-to-star variations in λ 3883 CN bandstrength among red giant stars in the low-metallicity globular cluster M53 ([Fe/H] = --2.0). Our data were taken with the Kast spectrograph on the 3-meter Shane telescope at Lick Observatory in April 2001. Star-to-star variations in CN bandstrength are common in intermediate- and high-metallicity globular clusters ([Fe/H] ≥ --1.6). Our data were obtained to test whether that variation will also be present in a low-metallicity globular cluster, or whether it will be suppressed by the overall lack of metals in the stars. Our preliminary result is that the λ 3883 CN band is weak in our program stars, which span the brightest magnitude of the red giant branch. On visual inspection, the M53 giants appear to be similar in their CN bandstrength to the four CN-weak giants in NGC 6752 whose average spectrum is plotted in Fig. 4 of Norris et al. (1981, ApJ, 244, 205). This work is planned to form part of a larger study of the metallicity dependence of CN bandstrength and carbon abundance behavior on the upper giant branch of globular clusters. This work is supported by NSF grant AST 00-98453 and by an award from the ARCS foundation, Northern California Chapter.

HUBBLE PINPOINTS WHITE DWARFS IN GLOBULAR CLUSTER

NASA Technical Reports Server (NTRS)

2002-01-01

Peering deep inside a cluster of several hundred thousand stars, NASA's Hubble Space Telescope uncovered the oldest burned-out stars in our Milky Way Galaxy. Located in the globular cluster M4, these small, dying stars - called white dwarfs - are giving astronomers a fresh reading on one of the biggest questions in astronomy: How old is the universe? The ancient white dwarfs in M4 are about 12 to 13 billion years old. After accounting for the time it took the cluster to form after the big bang, astronomers found that the age of the white dwarfs agrees with previous estimates for the universe's age. In the top panel, a ground-based observatory snapped a panoramic view of the entire cluster, which contains several hundred thousand stars within a volume of 10 to 30 light-years across. The Kitt Peak National Observatory's 0.9-meter telescope took this picture in March 1995. The box at left indicates the region observed by the Hubble telescope. The Hubble telescope studied a small region of the cluster. A section of that region is seen in the picture at bottom left. A sampling of an even smaller region is shown at bottom right. This region is only about one light-year across. In this smaller region, Hubble pinpointed a number of faint white dwarfs. The blue circles pinpoint the dwarfs. It took nearly eight days of exposure time over a 67-day period to find these extremely faint stars. Globular clusters are among the oldest clusters of stars in the universe. The faintest and coolest white dwarfs within globular clusters can yield a globular cluster's age. Earlier Hubble observations showed that the first stars formed less than 1 billion years after the universe's birth in the big bang. So, finding the oldest stars puts astronomers within arm's reach of the universe's age. M4 is 7,000 light-years away in the constellation Scorpius. Hubble's Wide Field and Planetary Camera 2 made the observations from January through April 2001. These optical observations were combined to create the above images. Spectral data were also taken. Credit for Hubble telescope photos: NASA and H. Richer (University of British Columbia) Credit for ground-based photo: NOAO/AURA/NSF

A self-contamination model for the formation of globular star clusters

NASA Astrophysics Data System (ADS)

Brown, James Howard

Described here is a model of globular cluster formation which allows the self contamination of the cluster by an earlier generation of massive stars. It is first shown that such self-contamination naturally produces an Fe/H in the range from -2.5 to -1.0, precisely the same range observed in the metal poor (halo) globular clusters; this also seems to require that the disk clusters started with a substantial initial metallicity. To minimize the problem of creating homogeneous globular clusters, the second (currently observed) generation of stars is assumed to form in the expanding supershell around the first generation stars. Both numerical and analytic models are used to address this problem. The most important result of this investigation was that the late evolution of the supershell is the most important, and that this phase of the evolution is dominated by the external medium in which the cloud is embedded. This result and the requirement that only the most tightly bound systems may become globular clusters lead to the conclusion that a globular cluster with the mass and binding energy typically observed can be formed at star formation efficiences as low as 10-20 percent. Furthermore, self contamination requires that the typical Fe/H of a bound system be about -1.6, independent of the free parameters of the model, allowing the clusters and field stars to form with different metallicity distributions in spite of their forming at the same time. Since the formation of globular clusters in this model is tied to the external pressure, the halo globular cluster masses and distribution can be used as probes of the early galactic structure. In particular, this model requires an increase in the typical globular cluster mass as one moves out from the galactic center; the masses of the halo clusters are examined, and they show considerable evidence for such a gradient. Based on a pressure distribution derived from this data, the effect of the galactic tidal field on the model is also investigated using an N-body simulation.

Disentangling the gamma-ray emission towards Cygnus X: Sh2-104

NASA Astrophysics Data System (ADS)

Gotthelf, Eric

2015-09-01

We have just discovered distinct X-ray emission coincident with VER J2018+363, a TeV source recently resolved from the giant gamma-ray complex MGRO J2019+37 in the Cygnus region. NuSTAR reveals a hard point source and a diffuse nebula adjacent to and possibly part of Sh2-104, a compact HII region containing several young massive stellar clusters. There is reasonable evidence that these X-rays probe the origin of the gamma-ray flux, however, unrelated extragalactic sources need to be excluded. We propose a short Chandra observation to localize the X-ray emission to identify a putative pulsar or stellar counterpart(s). This is an important step to fully understand the energetics of the MGRO J2019+37 complex and the production of gamma-rays in star formation regions, in general.

The Gould’s Belt Distances Survey (GOBELINS) II. Distances and Structure toward the Orion Molecular Clouds

NASA Astrophysics Data System (ADS)

Kounkel, Marina; Hartmann, Lee; Loinard, Laurent; Ortiz-León, Gisela N.; Mioduszewski, Amy J.; Rodríguez, Luis F.; Dzib, Sergio A.; Torres, Rosa M.; Pech, Gerardo; Galli, Phillip A. B.; Rivera, Juana L.; Boden, Andrew F.; Evans, Neal J., II; Briceño, Cesar; Tobin, John J.

2017-01-01

We present the results of the Gould’s Belt Distances Survey of young star-forming regions toward the Orion Molecular Cloud Complex. We detected 36 young stellar objects (YSOs) with the Very Large Baseline Array, 27 of which have been observed in at least three epochs over the course of two years. At least half of these YSOs belong to multiple systems. We obtained parallax and proper motions toward these stars to study the structure and kinematics of the Complex. We measured a distance of 388 ± 5 pc toward the Orion Nebula Cluster, 428 ± 10 pc toward the southern portion L1641, 388 ± 10 pc toward NGC 2068, and roughly ˜420 pc toward NGC 2024. Finally, we observed a strong degree of plasma radio scattering toward λ Ori.

The old open cluster NGC 2112: updated estimates of fundamental parameters based on a membership analysis†

NASA Astrophysics Data System (ADS)

Carraro, G.; Villanova, S.; Demarque, P.; Moni Bidin, C.; McSwain, M. V.

2008-05-01

We report on a new, wide-field (20 × 20 arcmin2), multicolour (UBVI), photometric campaign in the area of the nearby old open cluster NGC 2112. At the same time, we provide medium-resolution spectroscopy of 35 (and high-resolution of additional 5) red giant and turn-off stars. This material is analysed with the aim to update the fundamental parameters of this traditionally difficult cluster, which is very sparse and suffers from heavy field star contamination. Among the 40 stars with spectra, we identified 21 bona fide radial velocity members which allow us to put more solid constraints on the cluster's metal abundance, long suggested to be as low as the metallicity of globulars. As indicated earlier by us on a purely photometric basis, the cluster [Fe/H] abundance is slightly supersolar ([Fe/H] = 0.16 +/- 0.03) and close to the Hyades value, as inferred from a detailed abundance analysis of three of the five stars with higher resolution spectra. Abundance ratios are also marginally supersolar. Based on this result, we revise the properties of NGC 2112 using stellar models from the Padova and Yale-Yonsei groups. For this metal abundance, we find that the cluster's age, reddening and distance values are 1.8 Gyr, 0.60 mag and 940 pc, respectively. Both the Yale-Yonsei and Padova models predict the same values for the fundamental parameters within the errors. Overall, NGC 2112 is a typical solar neighbourhood, thin-disc star cluster, sharing the same chemical properties of F-G stars and open clusters close to the Sun. This investigation outlines the importance of a detailed membership analysis in the study of disc star clusters. This paper includes data gathered with the 6.5 Magellan Telescopes, located at Las Campanas Observatory, Chile. The data discussed in this paper will be made available at the WEBDA open cluster data base http://www.univie.ac.at/webda, which is maintained by E. Paunzen and J.-C. Mermilliod. ‡ E-mail: gcarraro@eso.org (GC); sandro.villanova@unipd.it (SV); demarque@astro.yale.edu (PD); mbidin@das.uchile.cl (CMB); mcswain@lehigh.edu(MVM)

The Evolution of Rotation and Activity in Young Open Clusters: the Zero-Age Main Sequence.

NASA Astrophysics Data System (ADS)

Patten, Brian Michael

1995-01-01

I have undertaken a program of ground- and space -based observations to measure photometric rotation periods and X-ray luminosities for late-type stars in the young open clusters IC 2391 and IC 2602. With cluster ages of ~30 Myr, IC 2391 and IC 2602 are ideal sites in which to observe conditions at the ZAMS since the solar-type stars in these clusters have not been on the main sequence long enough to undergo significant magnetic braking. The ROSAT survey of IC 2391 revealed 80 X-ray sources, 44 of which were found to be associated with stars which are now classified as new cluster members. Among the solar-type stars in both IC 2391 and IC 2602, I find a factor of ~25 spread in the distribution of rotation periods, which range from 0.21 to 4.86 day. I also find a factor of ~10-20 spread in the range of LX about a median LX value of ~10^{30 } erg s^{-1} for both clusters. These results show conclusively that stars arrive on the ZAMS with a wide range of rotation rates and coronal activity levels. When compared to data from older clusters, such as the Pleiades and the Hyades, there is an overall decline observed in both the rotation rates and median X-ray luminosity of cluster members with increasing age, however, while the spread in the range of rotation rates decreases to a small value, the spread in the range of LX values as a fraction of the median is observed to increase with age. This behavior is best explained through a dependence of LX on P rot which is weak in the young clusters and strong in the older clusters. The Rossby diagram shows there is a tight correlation between L X/Lbol and the Rossby number, Prot divided by the convective turnover time. Young, rapidly rotating, main sequence stars lie along a plateau of magnetic saturation, where LX has a weak dependence on rotation period, while older, more slowly rotating stars lie in a region on the Rossby diagram where LX has a strong dependence on rotation period.