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Sample records for globular cluster b514

  1. THE VARIABLE STAR POPULATION OF THE GLOBULAR CLUSTER B514 IN THE ANDROMEDA GALAXY

    SciTech Connect

    Clementini, Gisella; Contreras, Rodrigo; Federici, Luciana; Cacciari, Carla; Merighi, Roberto; Fusi Pecci, Flavio; Smith, Horace A.; Catelan, Marcio; Marconi, Marcella; Kinemuchi, Karen; Pritzl, Barton J. E-mail: kuehncha@msu.ed E-mail: beers@pa.msu.ed E-mail: marcella@na.astro.i E-mail: kinemuchi@astro.ufl.ed

    2009-10-20

    A rich harvest of RR Lyrae stars has been identified for the first time in B514, a metal-poor ([Fe/H] approx- 1.95 +- 0.10 dex) globular cluster (GC) of the Andromeda galaxy (M31), based on Hubble Space Telescope Wide Field Planetary Camera 2 and Advanced Camera for Surveys time-series observations. We have detected and derived periods for 89 RR Lyrae stars (82 fundamental-mode, RRab, and 7 first-overtone, RRc, pulsators, respectively) among 161 candidate variables identified in the cluster. The average period of the RR Lyrae variables ((Pab) = 0.58 days and (Pc) = 0.35 days, for RRab and RRc pulsators, respectively) and the position in the period-amplitude diagram both suggest that B514 is likely an Oosterhoff type I cluster. This appears to be in disagreement with the general behavior of the metal-poor GCs in the Milky Way, which show instead Oosterhoff type II pulsation properties. The average apparent magnitude of the RR Lyrae stars sets the mean level of the cluster horizontal branch at (V(RR)) = 25.18 +- 0.02 (sigma = 0.16 mag, on 81 stars). By adopting a reddening E(B - V) = 0.07 +- 0.02 mag, the above metallicity and M {sub V} = 0.44 +- 0.05 mag for the RR Lyrae variables of this metallicity, we derive a distance modulus of mu{sub 0} = 24.52 +- 0.08 mag, corresponding to a distance of about 800 +- 30 kpc, based on a value of M {sub V} that sets mu{sub 0}(LMC)=18.52 mag.

  2. The youngest globular clusters

    NASA Astrophysics Data System (ADS)

    Beck, Sara

    2015-11-01

    It is likely that all stars are born in clusters, but most clusters are not bound and disperse. None of the many protoclusters in our Galaxy are likely to develop into long-lived bound clusters. The super star clusters (SSCs) seen in starburst galaxies are more massive and compact and have better chances of survival. The birth and early development of SSCs takes place deep in molecular clouds, and during this crucial stage the embedded clusters are invisible to optical or UV observations but are studied via the radio-infrared supernebulae (RISN) they excite. We review observations of embedded clusters and identify RISN within 10 Mpc whose exciting clusters have ≈ 106 M⊙ or more in volumes of a few pc3 and which are likely to not only survive as bound clusters, but to evolve into objects as massive and compact as Galactic globulars. These clusters are distinguished by very high star formation efficiency η, at least a factor of 10 higher than the few percent seen in the Galaxy, probably due to the violent disturbances their host galaxies have undergone. We review recent observations of the kinematics of the ionized gas in RISN showing outflows through low-density channels in the ambient molecular cloud; this may protect the cloud from feedback by the embedded H II region.

  3. ROTATING GLOBULAR CLUSTERS

    SciTech Connect

    Bianchini, P.; Varri, A. L.; Bertin, G.; Zocchi, A.

    2013-07-20

    Internal rotation is thought to play a major role in the dynamics of some globular clusters. However, in only a few cases has internal rotation been studied by the quantitative application of realistic and physically justified global models. Here, we present a dynamical analysis of the photometry and three-dimensional kinematics of {omega} Cen, 47 Tuc, and M15, by means of a recently introduced family of self-consistent axisymmetric rotating models. The three clusters, characterized by different relaxation conditions, show evidence of differential rotation and deviations from sphericity. The combination of line-of-sight velocities and proper motions allows us to determine their internal dynamics, predict their morphology, and estimate their dynamical distance. The well-relaxed cluster 47 Tuc is interpreted very well by our model; internal rotation is found to explain the observed morphology. For M15, we provide a global model in good agreement with the data, including the central behavior of the rotation profile and the shape of the ellipticity profile. For the partially relaxed cluster {omega} Cen, the selected model reproduces the complex three-dimensional kinematics; in particular, the observed anisotropy profile, characterized by a transition from isotropy to weakly radial anisotropy and then to tangential anisotropy in the outer parts. The discrepancy found for the steep central gradient in the observed line-of-sight velocity dispersion profile and for the ellipticity profile is ascribed to the condition of only partial relaxation of this cluster and the interplay between rotation and radial anisotropy.

  4. Novae in globular clusters

    SciTech Connect

    Kato, Mariko; Hachisu, Izumi; Henze, Martin

    2013-12-10

    We present the first light-curve analysis of Population II novae that appeared in M31 globular clusters. Our light-curve models, based on the optically thick wind theory, reproduce well both the X-ray turn-on and turnoff times with the white dwarf (WD) mass of about 1.2 M {sub ☉} for M31N 2007-06b in Bol 111 and about 1.37 M {sub ☉} for M31N 2010-10f in Bol 126. The transient supersoft X-ray source CXO J004345 in Bol 194 is highly likely a nova remnant of 1.2-1.3 M {sub ☉} WD. These WD masses are quite consistent with the temperatures deduced from X-ray spectra. We also present the dependence of nova light curves on the metallicity in the range from [Fe/H] = 0.4 to –2.7. Whereas strong optically thick winds are accelerated in Galactic disk novae owing to a large Fe opacity peak, only weak winds occur in Population II novae with low Fe abundance. Thus, nova light curves are systematically slow in low Fe environment. For an extremely low Fe abundance normal nova outbursts may not occur unless the WD is very massive. We encourage V or y filter observation rather than R as well as high cadence X-ray monitorings to open quantitative studies of extragalactic novae.

  5. The Ages of Globular Clusters

    NASA Astrophysics Data System (ADS)

    McNamara, D. H.

    2001-03-01

    We examine the luminosity levels of the main-sequence turnoffs, MTOv, and horizontal branches, Mv(HB), in 16 globular clusters. An entirely new approach of inferring the luminosity levels by utilizing high-amplitude δ Scuti variables (HADS) is introduced. When the MTOv values are compared with theoretical values inferred from models, we find all 16 clusters (metal-strong to metal-poor) are coeval with an average age of ~11.3 Gyr. A considerable scatter of Mv(HB) values of clusters at similar [Fe/H] values is found. A trend for clusters with blue horizontal branches to have brighter Mv(HB) than clusters with blue-red horizontal branches is suggested by the data. The Mv(HB) values appear to depend on another or other parameters in addition to the [Fe/H] values. In spite of this problem, we derive an equation relating Mv(HB) values of globular clusters to their [Fe/H] values. We also derive an equation relating the MTOv values of clusters to their [Fe/H] values. Both of these equations can be utilized to find cluster distances. The distance modulus of the LMC is found to be 18.66 from the VTO values of three LMC globular clusters; RR Lyrae stars in seven globular clusters yield 18.61, and RR Lyrae stars in the LMC bar yield 18.64.

  6. Field star interactions with globular clusters

    NASA Astrophysics Data System (ADS)

    Peng, Wei

    1992-09-01

    We investigate a new interaction of globular clusters with galactic field stars. By dynamical friction, high-velocity field stars passing through individual globular clusters are decelerated. This frictional interaction contributes to cluster heating, and, in conjunction with disk shocking and other mechanisms, it helps regulate the evolution of globular clusters. Moreover, penetrating field stars with low relative velocities can even be captured by globular clusters. Our calculated rate of captures suggest that there is a substantial population of stars having an origin external to the globulars in which they now reside. Intriguing candidates for this 'immigrant' population include some blue straggler stars and short-period pulsars.

  7. Field star diffusion in globular clusters

    NASA Astrophysics Data System (ADS)

    Peng, Wei; Weisheit, Jon C.

    1992-10-01

    We investigate a new interaction of globular clusters with galactic field stars: the deceleration (by dynamical friction) of high-velocity field stars diffusing through individual globular clusters. This frictional interaction contributes to cluster heating and, in conjunction with disk shocking and other mechanisms, helps to regulate the evolution of globular clusters. Moreover, penetrating field stars of low relative velocity can even be captured by globular clusters. Our calculated rate of capture suggests that there is a modest population of stars having an origin external to the clusters in which they now reside. Intriguing candidates for this 'immigrant' population include some blue stragglers and short-period pulsars.

  8. X-ray diagnostics of globular clusters

    NASA Technical Reports Server (NTRS)

    Grindlay, J. E.

    1982-01-01

    The presence of compact X-ray sources in globular clusters allows diagnostic studies of both the X-ray sources themselves and the globular clusters to be carried out. A review of much of this work, primarily based on Einstein X-ray observations and supporting studies of globular clusters at radio through UV wavelengths, is presented. The compact X-ray sources in globular clusters are found to be compact binaries containing neutron stars and - in a separate lower luminosity component of an apparently bimodal luminosity function - possibly white dwarfs. Implications for the formation and evolution of compact binary X-ray sources in globular clusters and in the galactic bulge are discussed. In particular, new evidence is presented that the galactic bulge sources may be compact binaries in the remnants of disrupted globular clusters.

  9. The globular cluster system of NGC 5128

    NASA Astrophysics Data System (ADS)

    Woodley, Kristin Anne

    2010-11-01

    The globular cluster system of a nearby giant elliptical galaxy, NGC 5128 is studied to place constraints on the formation history of the galaxy. In this thesis, we have identified 190 new globular clusters via radial velocity measurements, bringing the total known population of globular clusters to 605 within this galaxy. We have examined the colour and spatial distributions of the globular cluster system and find it is bimodal in colour, with both a red and blue globular cluster population. The blue population is more spatially extended than the red, and both populations fall off in number density with radius as a power-law. There is a clear lack of globular clusters along the isophotal minor axis of the galaxy beyond a galactocentric radius of 15' warranting further search. With this new dataset, we have measured the ages, metallicities, and formation timescales for 72 globular clusters. The spectroscopic metallicity distribution function is bimodal indicating there is a metal-rich and metal-poor globular cluster population that corresponds to the red and blue globular clusters, respectively. We find the majority of both metal-rich (56%) and metal-poor (92%) globular clusters are older than 8 Gyr, comparable to the Milky Way globular cluster system. We do find a smaller fraction, 18% of our sample, are metal-rich globular clusters with ages younger than 5 Gyr, while the remaining globular clusters have intermediate ages between 5--8 Gyr. The formation times of these globular clusters, estimated by their alpha-to-iron abundance ratios, indicate they formed quickly, on a timescale similar to globular clusters in most spiral galaxies, but on slower timescales than those in some other giant elliptical galaxies. The kinematics of the full globular cluster system is analyzed, as well as for the metal-rich and metal-poor globular clusters separately, as a function of galactocentric radius. We find the metal-poor globular cluster system has a small rotation signature of

  10. Rotation and flattening of globular clusters

    NASA Technical Reports Server (NTRS)

    Fall, S. M.; Frenk, C. S.

    1985-01-01

    Methods for measuring globular cluster ellipticities and the results of such measurements are reviewed, and the processes that determine the shapes of globular clusters and the ways in which they change with time are discussed. The use of the virial tensor theorem to study the connection between the global rotation, velocity anisotropy, and the shape of a self-gravitating system is addressed, and the employment of N-body models to simulate the evolution of globular clusters with initially anisotropic velocity distributions is examined. The application of a simple evaporation model and Fokker-Planck integrations to study the two-body diffusion in globular clusters is reviewed.

  11. Luminosity Functions for Globular Clusters

    NASA Astrophysics Data System (ADS)

    Silvestri, Fabio; Ventura, Paolo; D'Antona, Francesca; Mazzitelli, Italo

    1998-12-01

    We present theoretical mass-luminosity relations and luminosity functions (LFs) for globular cluster stars, from luminosities above the horizontal branch down to the minimum luminosity of hydrogen-burning stars. The LFs are available for metal mass fraction Z from Z = 10-4 to Z = 4 × 10-3, in the Johnson V band and in the Bessell-Cousins I band, and are based on tracks especially computed for this program, with the input physics of the models developed recently by D'Antona et al., Mazzitelli et al., and D'Antona & Mazzitelli. Two typical comparisons with observations are presented and discussed: (1) comparisons and statistical analysis with the LFs of the lower giant branch, turnoff region, and upper main sequence of several globular clusters from low to high metallicity, (2) derivation of the initial mass function (IMF) for the stars below the turnoff for several globular clusters for which Hubble Space Telescope data are available. In the first analysis we find that, for relatively large metallicities (Z >= 10-3) a good fit between theoretical and observed LFs can be found, although a simple χ2 statistical analysis shows that it is not possible to derive a strongly preferred age (or, equivalently, distance modulus) from the LF comparison. The fit with lower metallicity [Z ~ (1-2) × 10-4] LFs is less good but statistically acceptable. The main result is that the difference between observed and theoretical LFs of low-metallicity clusters reported by VandenBerg, Bolte, & Stetson appears to be much reduced in present models, and we give the possible reason why this happens and its consequences for the important parameter of the helium core mass at the flash. In the second application, we explore the effect of varying age and distance modulus on the mass function derived for a globular cluster. Distance moduli corresponding to the ``long'' distance scale (and relatively low ages) seem to be preferred based on these comparisons. The resulting index of the IMF is

  12. Extragalactic Globular Clusters: Tracers of Galaxy Evolution

    NASA Astrophysics Data System (ADS)

    Bassino, Lilia P.

    2008-09-01

    The study of globular cluster systems provides clues about different topics related to galaxy evolution. In the past years we have been investigating the globular cluster systems of galaxies in the Fornax and Antlia clusters, particularly those associated to the cluster-dominant galaxies. We present here the main results related to these systems. All of them have bimodal color distributions, even those around low-luminosity galaxies, that correspond to the metal-poor (``blue'') and metal-rich (``red'') globular cluster subpopulations. The radial and azimuthal projected areal distributions of the globular clusters are also analyzed. Total globular cluster populations are estimated through the luminosity functions. We stress on the properties of the globular cluster systems that allow us to trace possible interaction processes between the galaxies, like tidal stripping of globular clusters. The observational material consists of CCD images obtained with the wide-field MOSAIC Imager of the CTIO 4-m telescope (La Serena, Chile), and the FORS1 camera at the VLT ``Antu'' 8-m telescope (Cerro Paranal, Chile).

  13. Central Dynamics of Globular Clusters

    NASA Astrophysics Data System (ADS)

    Noyola, Eva; Baumgardt, H.

    2007-12-01

    Globular clusters have historically been classified into two groups due to their dynamical state. They are considered to be either pre-core-collapse or post-core-collapse systems. Clusters are considered as post-core-collapse when they show concentrated surface brightness profiles, with a steep central cusp; while pre-core collapse clusters are less concentrated and have flat central cores. Recent observational results show that some clusters have central surface brightness profiles with intermediate central slopes, showing shallow cusps. These observations could be explained by the presence of a single or a binary intermediate mass black hole in the center of the clusters. In this work, we create realistic synthetic images from the output of N-body models. The images attempt to mock the resolution and point spread function of the high resolution cameras on board the Hubble Space Telescope. The models are created with and without central black holes, where the no black hole models are allowed to reach core-collapse. We measure surface brightness profiles both from integrated light and from star counts. From the profiles, we obtain parameters such as central surface brightness slope, core radius, and half light radius. We also test how well a King model describes each profile. We find that the black hole models produce shallow cusps if the black hole is larger than a certain mass; while models without central black holes produce more concentrated profiles. This approach, allows to make a thorough comparison between observations and models.

  14. Where Are the Universe's Globular Clusters?

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-04-01

    Observations of globular clusters gravitationally-bound, spherical clusters of stars that orbit galaxies as satellites are critical to studies of galactic and stellar evolution. What type of galaxies host the largest total number of globular clusters in todays universe? A recent study answers this question.Total number of globular clusters vs. host galaxy luminosity for a catalog of ~400 galaxies of all types. [Harris 2016]Globular FavoritismGlobular clusters can be found in the halos of all galaxies above a critical brightness of about 107 solar luminosities (in practice, all but the smallest of dwarfs). The number of globulars a galaxy hosts is related to its luminosity: the Milky Way is host to ~150 globulars, the slightly brighterAndromeda galaxy may have several hundred globulars, and the extremelybright giant elliptical galaxy M87 likely has over ten thousand.But the number of galaxies is not evenly distributed in luminosity; tiny dwarf galaxies are extremely numerous in the universe, whereas giant ellipticals are far less common. So are most of the universes globulars found around dwarfs, simply because there are more dwarfs to host them? Or are the majority ofglobular clusters orbiting large galaxies? A scientist at McMaster University in Canada, William Harris, has done some calculations to find the answer.Finding the PeakHarris combines two components in his estimates:The Schechter function, a function that describes the relative number of galaxies per unit luminosity. This function drops off near a characteristic luminosity roughly that of our galaxy.Empirical data from ~400 galaxies that describe the average number of globulars per galaxy as a function of galaxy luminosity.Relative number of globular clusters in all galaxies at a given luminosity, for metal-poor globulars only (blue), metal-rich globulars only (red), and all globulars (black). The curves peak around the Schechter characteristic luminosity, and metal-poor globulars outnumber metal

  15. Close binary stars in globular clusters

    NASA Technical Reports Server (NTRS)

    Margon, Bruce

    1991-01-01

    Although close binary stars are thought theoretically to play a major role in globular cluster dynamics, virtually no non-degenerate close binaries are known in clusters. We review the status of observations in this area, and report on two new programs which are finally yielding candidate systems suitable for further study. One of the objects, a close eclipsing system in omega Cen, is also a big straggler, thus finally proving firm evidence that globular cluster blue stragglers really are binary stars.

  16. Gamma-ray Emission from Globular Clusters

    NASA Astrophysics Data System (ADS)

    Tam, Pak-Hin T.; Hui, Chung Y.; Kong, Albert K. H.

    2016-03-01

    Over the last few years, the data obtained using the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope has provided new insights on high-energy processes in globular clusters, particularly those involving compact objects such as MilliSecond Pulsars (MSPs). Gamma-ray emission in the 100 MeV to 10 GeV range has been detected from more than a dozen globular clusters in our galaxy, including 47 Tucanae and Terzan 5. Based on a sample of known gammaray globular clusters, the empirical relations between gamma-ray luminosity and properties of globular clusters such as their stellar encounter rate, metallicity, and possible optical and infrared photon energy densities, have been derived. The measured gamma-ray spectra are generally described by a power law with a cut-off at a few gigaelectronvolts. Together with the detection of pulsed γ-rays from two MSPs in two different globular clusters, such spectral signature lends support to the hypothesis that γ-rays from globular clusters represent collective curvature emission from magnetospheres of MSPs in the clusters. Alternative models, involving Inverse-Compton (IC) emission of relativistic electrons that are accelerated close to MSPs or pulsar wind nebula shocks, have also been suggested. Observations at >100 GeV by using Fermi/LAT and atmospheric Cherenkov telescopes such as H.E.S.S.-II, MAGIC-II, VERITAS, and CTA will help to settle some questions unanswered by current data.

  17. X-ray binaries in globular clusters

    NASA Technical Reports Server (NTRS)

    Grindlay, Jonathan E.

    1988-01-01

    X-ray and optical studies of compact binaries and globular clusters are reviewed. Topics covered include, the formation of compact binaries by three-body interactions and by tidal capture, studies of the 11 minute binary in NGC 6624 and the 8.5 hour binary in M 15 (AC211), and an evolutionary model for compact binary formation. Optical searches for X-ray binaries in globular clusters are examined including CCD surveys and studies of NGC 6712. In addition, globular clusters with central cusps in their surface brightness profiles, questions concerning the blue color of binaries, diffuse line emission from CVs, and the possibility that X-ray burst sources in the galactic bulge were formed by tidal capture in globular clusters which have since been disrupted are discussed.

  18. Stellar Populations Archive: The Globular Clusters

    NASA Astrophysics Data System (ADS)

    Zurek, D. R.; Ouellette, J. O.; Shara, M.; Hurley, J.; Ferguson, H.

    2001-12-01

    The Hubble Space Telescope cycle 10 panels have awarded us an archival grant to create a web based archive for the 101 Galactic globular clusters observed with WFPC2 on the Hubble Space Telescope. We will reduce all globular cluster WFPC2 data using ALLFrame in order to provide a photometric database which is precise and consistent from cluster to cluster. In addition the American Museum of Natural History has recently acquired three special purpose computers (GRAPE6) for dynamical simulations of stellar clusters. The simulations will be archived and a public database will be made available. The archive will go online early 2002 and as each cluster is reduced it will be made public. It is hoped that this "service to the community" will encourage comparitive studies of the Galactic globular cluster system. This database will also produce a library of template stellar populations with widespread applications.

  19. UV-bright stars in globular clusters

    NASA Technical Reports Server (NTRS)

    Landsman, Wayne B.

    1994-01-01

    This paper highlights globular cluster studies with Ultraviolet Imaging Telescope (UIT) in three areas: the discrepancy between observed ultraviolet HB magnitudes and predictions of theoretical HB models; the discovery of two hot subdwarfs in NGC 1851, a globular not previously known to contain such stars; and spectroscopic follow up of newly identified UV-bright stars in M79 and w Cen. I also present results of a recent observation of NGC 6397 with the Voyager ultraviolet spectrometer.

  20. Millisecond radio pulsars in globular clusters

    NASA Technical Reports Server (NTRS)

    Verbunt, Frank; Lewin, Walter H. G.; Van Paradijs, Jan

    1989-01-01

    It is shown that the number of millisecond radio pulsars, in globular clusters, should be larger than 100, applying the standard scenario that all the pulsars descend from low-mass X-ray binaries. Moreover, most of the pulsars are located in a small number of clusters. The prediction that Teran 5 and Liller 1 contain at least about a dozen millisecond radio pulsars each is made. The observations of millisecond radio pulsars in globular clusters to date, in particular the discovery of two millisecond radio pulsars in 47 Tuc, are in agreement with the standard scenario, in which the neutron star is spun up during the mass transfer phase.

  1. UNCLOAKING GLOBULAR CLUSTERS IN THE INNER GALAXY

    SciTech Connect

    Alonso-Garcia, Javier; Catelan, Marcio; Minniti, Dante; Mateo, Mario; Sen, Bodhisattva; Banerjee, Moulinath; Von Braun, Kaspar E-mail: mcatelan@astro.puc.cl E-mail: mmateo@umich.edu E-mail: moulib@umich.edu

    2012-03-15

    Extensive photometric studies of the globular clusters located toward the center of the Milky Way have been historically neglected. The presence of patchy differential reddening in front of these clusters has proven to be a significant obstacle to their detailed study. We present here a well defined and reasonably homogeneous photometric database for 25 of the brightest Galactic globular clusters located in the direction of the inner Galaxy. These data were obtained in the B, V, and I bands using the Magellan 6.5 m Telescope and the Hubble Space Telescope. A new technique is extensively used in this paper to map the differential reddening in the individual cluster fields, and to produce cleaner, dereddened color-magnitude diagrams for all the clusters in the database. Subsequent papers will detail the astrophysical analysis of the cluster populations, and the properties of the obscuring material along the clusters' lines of sight.

  2. Reconstructing galaxy histories from globular clusters

    NASA Astrophysics Data System (ADS)

    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.

  3. 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. PMID:14702077

  4. Globular Clusters in the Galactic Bulge

    NASA Astrophysics Data System (ADS)

    Bica, E.; Ortolani, S.; Barbuy, B.

    2016-06-01

    A view of the Galactic bulge by means of their globular clusters is fundamental for a deep understanding of its formation and evolution. Connections between the globular cluster and field star properties in terms of kinematics, orbits, chemical abundances, and ages should shed light on different stellar population components. Based on spatial distribution and metallicity, we define a probable best list of bulge clusters, containing 43 entries. Future work on newly discovered objects, mostly from the VVV survey, is suggested. These candidates might alleviate the issue of missing clusters on the far side of the bulge. We discuss the reddening law affecting the cluster distances towards the centre of the Galaxy, and conclude that the most suitable total-to-selective absorption value appears to be R V=3.2, in agreement with recent analyses. An update of elemental abundances for bulge clusters is provided.

  5. 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. PMID:17203062

  6. The fundamental plane correlations for globular clusters

    NASA Technical Reports Server (NTRS)

    Djorgovski, S.

    1995-01-01

    In the parameter space whose axes include a radius (core, or half-light), a surface brightness (central, or average within the half-light radius), and the central projected velocity dispersion, globular clusters lie on a two-dimensional surface (a plane, if the logarithmic quantities are used). This is analogous to the 'fundamental plane' of elliptical galaxies. The implied bivariate correlations are the best now known for globular clusters. The derived scaling laws for the core properties imply that cluster cores are fully virialized, homologous systems, with a constant (M/L) ratio. The corresponding scaling laws on the half-light scale are differrent, but are nearly identical to those derived from the 'fundamental plane' of ellipticals. This may be due to the range of cluster concentrations, which are correlated with other parameters. A similar explanation for elliptical galaxies may be viable. These correlations provide new empirical constraints for models of globular cluster formation and evolution, and may also be usable as rough distance-indicator relations for globular clusters.

  7. Massive star archeology in globular clusters

    NASA Astrophysics Data System (ADS)

    Chantereau, W.; Charbonnel, C.; Meynet, G.

    2015-01-01

    Globular clusters are among the oldest structures in the Universe and they host today low-mass stars and no gas. However, there has been a time when they formed as gaseous objects hosting a large number of short-lived, massive stars. Many details on this early epoch have been depicted recently through unprecedented dissection of low-mass globular cluster stars via spectroscopy and photometry. In particular, multiple populations have been identified, which bear the nucleosynthetic fingerprints of the massive hot stars disappeared a long time ago. Here we discuss how massive star archeology can be done through the lense of these multiple populations.

  8. Color Distributions of 29 Galactic Globular Clusters

    NASA Astrophysics Data System (ADS)

    Sohn, Young-Jong; Byun, Yong-Ik; Yim, Hong-Suh; Rhee, Myung-Hyun; Chun, Mun-Suk

    1998-06-01

    U, B, and V CCD images are used to investigate the radial color gradients of twenty nine Galactic globular clusters - twenty two King type clusters and seven Post Core Collapse (PCC) clusters classified on their surface brightness distributions. For King type clusters, eight clusters show radial color gradients with redder center and seven clusters with bluer centers in (B-V). Seven King type clusters have redder centers in (U-B), and five King type clusters show radial color gradients with bluer center in the same color. Among seven PCC clusters, one cluster show a redder center and five clusters show bluer centers in (B-V). Two PCC clusters have redder centers in (U-B), four PCC clusters show radial color gradients with bluer centers in the same color. These results bring an evidence that the color gradient is not unique to PCC clusters with bluer center. >From the Pearson's correlation coefficient tests, we found the horizontal branch morphologies have weak correlations to the radial color gradients within globular clusters.

  9. MODELING THE METALLICITY DISTRIBUTION OF GLOBULAR CLUSTERS

    SciTech Connect

    Muratov, Alexander L.; Gnedin, Oleg Y. E-mail: ognedin@umich.ed

    2010-08-01

    Observed metallicities of globular clusters reflect physical conditions in the interstellar medium of their high-redshift host galaxies. Globular cluster systems in most large galaxies display bimodal color and metallicity distributions, which are often interpreted as indicating two distinct modes of cluster formation. The metal-rich and metal-poor clusters have systematically different locations and kinematics in their host galaxies. However, the red and blue clusters have similar internal properties, such as their masses, sizes, and ages. It is therefore interesting to explore whether both metal-rich and metal-poor clusters could form by a common mechanism and still be consistent with the bimodal distribution. We present such a model, which prescribes the formation of globular clusters semi-analytically using galaxy assembly history from cosmological simulations coupled with observed scaling relations for the amount and metallicity of cold gas available for star formation. We assume that massive star clusters form only during mergers of massive gas-rich galaxies and tune the model parameters to reproduce the observed distribution in the Galaxy. A wide, but not the entire, range of model realizations produces metallicity distributions consistent with the data. We find that early mergers of smaller hosts create exclusively blue clusters, whereas subsequent mergers of more massive galaxies create both red and blue clusters. Thus, bimodality arises naturally as the result of a small number of late massive merger events. This conclusion is not significantly affected by the large uncertainties in our knowledge of the stellar mass and cold gas mass in high-redshift galaxies. The fraction of galactic stellar mass locked in globular clusters declines from over 10% at z > 3 to 0.1% at present.

  10. Globular cluster x-ray sources

    PubMed Central

    Pooley, David

    2010-01-01

    Globular clusters and x-ray astronomy have a long and fruitful history. Uhuru and OSO-7 revealed highly luminous (> 1036 ergs-1) x-ray sources in globular clusters, and Einstein and ROSAT revealed a larger population of low-luminosity (< 1033 ergs-1) x-ray sources. It was realized early on that the high-luminosity sources were low-mass x-ray binaries in outburst and that they were orders of magnitude more abundant per unit mass in globular clusters than in the rest of the galaxy. However, the low-luminosity sources proved difficult to classify. Many ideas were put forth—low-mass x-ray binaries in quiescence (qLMXBs), cataclysmic variables (CVs), active main-sequence binaries (ABs), and millisecond pulsars (MSPs)—but secure identifications were scarce. In ROSAT observations of 55 clusters, about 25 low-luminosity sources were found. Chandra has now observed over 80 Galactic globular clusters, and these observations have revealed over 1,500 x-ray sources. The superb angular resolution has allowed for many counterpart identifications, providing clues to the nature of this population. It is a heterogeneous mix of qLMXBs, CVs, ABs, and MSPs, and it has been shown that the qLMXBs and CVs are both, in part, overabundant like the luminous LMXBs. The number of x-ray sources in a cluster correlates very well with its encounter frequency. This points to dynamical formation scenarios for the x-ray sources and shows them to be excellent tracers of the complicated internal dynamics. The relation between the encounter frequency and the number of x-ray sources has been used to suggest that we have misunderstood the dynamical states of globular clusters. PMID:20404204

  11. Understanding the Milky Way Globular Clusters

    NASA Astrophysics Data System (ADS)

    Chatterjee, Sourav

    2014-05-01

    Studying the evolution of globular clusters is of great interest for a variety of branches in astrophysics and cosmology. The high masses and stellar densities make them important targets for Galactic and extragalactic astronomy, and hotbeds for strong dynamical encounters facilitating several exotic sources. Their old ages provide a direct window into the major star formation episodes in the early universe. Until recently our numerical understanding for these systems was either limited by the number of stars simulations can treat or by omission of some physical processes. Northwestern group's Hénon-type Monte Carlo code CMC can lift these problems and allows creation of star-by-star cluster models that can be directly compared with the observational data. I will present our latest understanding of how these clusters evolve as a whole, explain bulk properties of the Milky Way globular clusters, and identify formation channels for some resolved exotic stellar populations.

  12. REGION OF GLOBULAR CLUSTER NGC 6397

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Right A NASA Hubble Space Telescope image of a small region (1.4 light-years across) in the globular star cluster NGC 6397 shows far fewer stars than would be expected in faint red dwarf stars were abundant. 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. This observation shows the surprising cutoff point below which nature apparently doesn't make many stars smaller that 1/5 the mass of our Sun. If there were lower mass stars in the cluster, then the image would contain an estimated 500 stars. This observation provides new insights into star formation in our Galaxy. Left A ground-based sky survey photograph of the globular cluster NGC 6397, one of the nearest and densest agglomerations of stars to Earth. The cluster is located 7,200 light-years away in the southern constellation Ara, and is one of 150 such objects which orbit our Milky Way Galaxy. Globular clusters are ideal laboratories for studying the formation and evolution of stars. This visible light picture was taken on March 3, 1994 with the Wide Field Planetary Camera 2, as part of the HST parallel observing program. Credit: F. Paresce, ST ScI and ESA and NASA

  13. Globular Cluster Colors Versus Population Synthesis Models

    NASA Astrophysics Data System (ADS)

    Barmby, Pauline; Jalilian, F. F.

    2011-05-01

    Although the stellar populations of globular clusters are not as simple as we used to believe, they are still the simplest populations available in the nearby universe. As such, they are extremely useful for testing stellar population synthesis models. Using recent mass estimates for Local Group globular clusters, we have compiled a sample of clusters with masses large enough that stochastic effects on integrated photometry should be minimal. We have measured integrated colors in the Spitzer/IRAC bands for as many of these as possible, paying careful attention to systematics in order to get the most accurate colors. We present a comparison of the results with the predictions of the latest generation of population synthesis models, including GALEV and FSPS. Support for this work was provided by a Discovery Grant and an Undergraduate Summer Research Award from NSERC and by an Ontario Early Researcher Award.

  14. Metallicities and Reddenings For Young Globular Clusters

    NASA Astrophysics Data System (ADS)

    Sarajedini, Ata; Layden, Andrew

    1996-04-01

    We have obtained VI CCD photometry for the young globular clusters Ruprecht 106, Terzan 7, and Arp 2 using the 0.9m and 1.5m telescopes at Cerro Tololo Inter-American Observatory. The resulting V ~ (V-I) color-magnitude diagrams extend from the tip of the red giant branch to approximately 3 magnitudes below the horizontal branch. We have applied the SRM method of Sarajedini (1994) to the RGBs of these clusters in order to measure their reddenings and metallicities. Furthermore, we have formulated the SRM method in the V ~ (B-V) color-magnitude plane and applied it to published BV photometry for these young globular clusters. The implications of these derived parameters will be discussed.

  15. HUBBLE SPIES GLOBULAR CLUSTER IN NEIGHBORING GALAXY

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Hubble Space Telescope has captured a view of a globular cluster called G1, a large, bright ball of light in the center of the photograph consisting of at least 300,000 old stars. G1, also known as Mayall II, orbits the Andromeda galaxy (M31), the nearest major spiral galaxy to our Milky Way. Located 130,000 light-years from Andromeda's nucleus, G1 is the brightest globular cluster in the Local Group of galaxies. The Local Group consists of about 20 nearby galaxies, including the Milky Way. The crisp image is comparable to ground-based telescope views of similar clusters orbiting the Milky Way. The Andromeda cluster, however, is nearly 100 times farther away. A glimpse into the cluster's finer details allow astronomers to see its fainter helium-burning stars whose temperatures and brightnesses show that this cluster in Andromeda and the oldest Milky Way clusters have approximately the same age. These clusters probably were formed shortly after the beginning of the universe, providing astronomers with a record of the earliest era of galaxy formation. During the next two years, astronomers will use Hubble to study about 20 more globular clusters in Andromeda. The color picture was assembled from separate images taken in visible and near-infrared wavelengths taken in July of 1994. CREDIT: Michael Rich, Kenneth Mighell, and James D. Neill (Columbia University), and Wendy Freedman (Carnegie Observatories), and NASA Image files in GIF and JPEG format and captions may be accessed on Internet via anonymous ftp from oposite.stsci.edu in /pubinfo.

  16. Globular cluster formation - The fossil record

    NASA Technical Reports Server (NTRS)

    Murray, Stephen D.; Lin, Douglas N. C.

    1992-01-01

    Properties of globular clusters which have remained unchanged since their formation are used to infer the internal pressures, cooling times, and dynamical times of the protocluster clouds immediately prior to the onset of star formation. For all globular clusters examined, it is found that the cooling times are much less than the dynamical times, implying that the protoclusters must have been maintained in thermal equilibrium by external heat sources, with fluxes consistent with those found in previous work, and giving the observed rho-T relation. Self-gravitating clouds cannot be stably heated, so that the Jeans mass forms an upper limit to the cluster masses. The observed dependence of protocluster pressure upon galactocentric position implies that the protocluster clouds were in hydrostatic equilibrium after their formation. The pressure dependence is well fitted by that expected for a quasi-statically evolving background hot gas, shock heated to its virial temperature. The observations and inferences are combined with previous theoretical work to construct a picture of globular cluster formation.

  17. VARIABLES IN GLOBULAR CLUSTER NGC 5024

    SciTech Connect

    Safonova, M.; Stalin, C. S. E-mail: stalin@iiap.res.in

    2011-12-15

    We present the results of a commissioning campaign to observe Galactic globular clusters for the search of microlensing events. The central 10' Multiplication-Sign 10' region of the globular cluster NGC 5024 was monitored using the 2 m Himalayan Chandra Telescope in R-band for a period of about 8 hr on 2010 March 24. Light curves were obtained for nearly 10,000 stars using a modified Differential Image Analysis technique. We identified all known variables within our field of view and revised the periods and status of some previously reported short-period variables. We report about 70 new variable sources and present their equatorial coordinates, periods, light curves, and possible types. Out of these, 15 are SX Phe stars, 10 are W UMa-type stars, and 14 are probable RR Lyrae stars. Nine of the newly discovered SX Phe stars and one eclipsing binary belong to the blue straggler star population.

  18. Gravitational interactions between globular and open clusters: an introduction

    NASA Astrophysics Data System (ADS)

    de la Fuente Marcos, R.; de la Fuente Marcos, C.; Reilly, D.

    2014-01-01

    Historically, it has been assumed that globular and open clusters never interact. However, recent evidence suggests that: globular clusters passing through the disk may be able to perturb giant molecular clouds (GMCs) triggering formation of open clusters and some old open clusters may be linked to accreted globulars. Here, we further explore the existence of possible dynamical connections between globular and open clusters, and realize that the most obvious link must be in the form of gravitational interactions. If open clusters are born out of GMCs, they have to move in similar orbits. If we accept that globulars can interact with GMCs, triggering star formation, it follows that globular and open clusters must also interact. Consistently, theoretical arguments as well as observational evidence, show that globular and open clusters certainly are interacting populations and their interactions are far more common than usually thought, especially for objects part of the bulge/disk. Monte Carlo calculations confirm that conclusion. Globular clusters seem capable of not only inducing formation of open clusters but, more often, their demise. Relatively frequent high speed cluster encounters or cluster harassment may also cause, on the long-term, slow erosion and tidal truncation on the globulars involved. The disputed object FSR 1767 (2MASS-GC04) may be, statistically speaking, the best example of an ongoing interaction.

  19. 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

  20. Understanding the Current Dynamical States of Globular Clusters

    NASA Astrophysics Data System (ADS)

    Pooley, David

    2008-09-01

    We appear to be on the verge of a major paradigm shift in our understanding of the current dynamical states of Galactic globular clusters. Fregeau (2008) brought together two recent theoretical breakthroughs as well as an observational breakthrough made possible by Chandra -- that a globular cluster's X-ray source population scales with its dynamical encounter frequency -- to persuasively argue that we have misunderstood the dynamical states of Galactic globular clusters. The observational evidence hinges on Chandra results from clusters which are classified as "core collapsed," of which there are only a handful of observations. I propose a nearly complete census with Chandra of the rest of the "core collapsed" globular clusters.

  1. Isolated elliptical galaxies and their globular cluster systems. II. NGC 7796 - globular clusters, dynamics, companion

    NASA Astrophysics Data System (ADS)

    Richtler, T.; Salinas, R.; Lane, R. R.; Hilker, M.; Schirmer, M.

    2015-02-01

    Context. Rich globular cluster systems, particularly the metal-poor part of them, are thought to be the visible manifestations of long-term accretion processes. The invisible part is the dark matter halo, which may show some correspondence to the globular cluster system. It is therefore interesting to investigate the globular cluster systems of isolated elliptical galaxies, which supposedly have not experienced extended accretion. Aims: We investigate the globular cluster system of the isolated elliptical NGC 7796, present new photometry of the galaxy, and use published kinematical data to constrain the dark matter content. Methods: Deep images in B and R, obtained with the VIsible MultiObject Spectrograph (VIMOS) at the VLT, form the data base. We performed photometry with DAOPHOT and constructed a spherical photometric model. We present isotropic and anisotropic Jeans-models and give a morphological description of the companion dwarf galaxy. Results: The globular cluster system has about 2000 members, so it is not as rich as those of giant ellipticals in galaxy clusters with a comparable stellar mass, but richer than many cluster systems of other isolated ellipticals. The colour distribution of globular clusters is bimodal, which does not necessarily mean a metallicity bimodality. The kinematic literature data are somewhat inconclusive. The velocity dispersion in the inner parts can be reproduced without dark matter under isotropy. Radially anisotropic models need a low stellar mass-to-light ratio, which would contrast with the old age of the galaxy. A MONDian model is supported by X-ray analysis and previous dynamical modelling, but better data are necessary for a confirmation. The dwarf companion galaxy NGC 7796-1 exhibits tidal tails, multiple nuclei, and very boxy isophotes. Conclusions: NGC 7796 is an old, massive isolated elliptical galaxy with no indications of later major star formation events as seen frequently in other isolated ellipticals. Its

  2. No energy equipartition in globular clusters

    NASA Astrophysics Data System (ADS)

    Trenti, Michele; van der Marel, Roeland

    2013-11-01

    It is widely believed that globular clusters evolve over many two-body relaxation times towards a state of energy equipartition, so that velocity dispersion scales with stellar mass as σ ∝ m-η with η = 0.5. We show here that this is incorrect, using a suite of direct N-body simulations with a variety of realistic initial mass functions and initial conditions. No simulated system ever reaches a state close to equipartition. Near the centre, the luminous main-sequence stars reach a maximum ηmax ≈ 0.15 ± 0.03. At large times, all radial bins convergence on an asymptotic value η∞ ≈ 0.08 ± 0.02. The development of this `partial equipartition' is strikingly similar across our simulations, despite the range of different initial conditions employed. Compact remnants tend to have higher η than main-sequence stars (but still η < 0.5), due to their steeper (evolved) mass function. The presence of an intermediate-mass black hole (IMBH) decreases η, consistent with our previous findings of a quenching of mass segregation under these conditions. All these results can be understood as a consequence of the Spitzer instability for two-component systems, extended by Vishniac to a continuous mass spectrum. Mass segregation (the tendency of heavier stars to sink towards the core) has often been studied observationally, but energy equipartition has not. Due to the advent of high-quality proper motion data sets from the Hubble Space Telescope, it is now possible to measure η for real clusters. Detailed data-model comparisons open up a new observational window on globular cluster dynamics and evolution. A first comparison of our simulations to observations of Omega Cen yields good agreement, supporting the view that globular clusters are not generally in energy equipartition. Modelling techniques that assume equipartition by construction (e.g. multi-mass Michie-King models) are approximate at best.

  3. Microlensing: Globular Cluster M22 Video File

    NASA Astrophysics Data System (ADS)

    2001-01-01

    A computerized animation begins outside a globular cluster similar to M22, with the center of the Milky Way in the distance. The camera flies through the center of the cluster and rests with a dark object in the distance. This object, a suspected brown star, passes in front of a star in the galactic bulge, bending its light gravitationally. This bending, or lensing, causes a momentary brightening of the background star. Another sequence begins with a ground-based view of the center of our galaxy in the upper right. We zoom in to reveal a ground-based view of the region surrounding the cluster and continue zooming to reveal the Hubble Space Telescope view of M22. In an interview with Kailash Sahu, Astronomer, he describes the Hubble results, explains why the objects in M22 can't be planets, and explains Hubble's role in the observations of M22. The last image was taken with Hubble's Wide Field and Planetary Camera 2 and pierces the heart of a globular cluster with its needle-sharp vision and uncovers tantalizing clues to what could potentially be a strange and unexpected population of wandering, planet-sized objects.

  4. Rosat Observations of Nine Globular Clusters

    NASA Technical Reports Server (NTRS)

    Rappaport, S.; Dewey, D.; Levine, A.; Macri, L.

    1994-01-01

    The ROSAT HRI was used to image fields around nine Galactic globular clusters that have central densities in the range of 10(exp 4) - 10(exp 5) solar mass pc(exp -3) and that had not previously been observed with the Einstein Observatory. We detected X-ray sources associated with Pal 2 and NGC 6304 with luminosities of 1.1 x 10(exp 34) ergs/s and 1.2 x 10(exp 33) ergs/s, respectively. No X-ray emission was detected from the source in Ter 6, thus confirming its transient nature. In all, there were 23 serendipitous sources found in the nine fields; none was apparently associated with any of the other seven clusters. The results are discussed in the context of low-luminosity cluster X-ray sources, in general.

  5. CVs and millisecond pulsar progenitors in globular clusters

    NASA Technical Reports Server (NTRS)

    Grindlay, J. E.; Cool, A. M.; Bailyn, C. D.

    1991-01-01

    The recent discovery of a large population of millisecond pulsars in globular clusters, together with earlier studies of both low luminosity X-ray sources and LMXBs in globulars, suggest there should be significant numbers of CVs in globulars. Although they have been searched for without success in selected cluster X-ray source fields, systematic surveys are lacking and would constrain binary production and both stellar and dynamical evolution in globular clusters. We describe the beginnings of such a search, using narrow band H-alpha imaging, and the sensitivities it might achieve.

  6. Globular Clusters as Cradles of Life and Advanced Civilizations

    NASA Astrophysics Data System (ADS)

    Di Stefano, Rosanne; Ray, Alak

    2016-01-01

    Globular clusters are bound groups of about a million stars and stellar remnants. They are old, largely isolated, and very dense. We consider what each of these special features can mean for the development of life, the evolution of intelligent life, and the long-term survival of technological civilizations. We find that, if they house planets, globular clusters provide ideal environments for advanced civilizations that can survive over long times. We therefore propose methods to search for planets in globular clusters. If planets are found and if our arguments are correct, searches for intelligent life are most likely to succeed when directed toward globular clusters. Globular clusters may be the first places in which distant life is identified in our own or in external galaxies.

  7. IC 1257: A New Globular Cluster in the Galactic Halo

    NASA Technical Reports Server (NTRS)

    Harris, W. E.; Phelps, R. L.; Madore, B. F.; Pevunova, O.; Skiff, B. A.; Crute, C.; Wilson, B.

    1996-01-01

    New CCD photometry of the faint, compact star cluster IC 1257 (L = 17? = +/- 15?obtained with the Palomar 5m telescope, reveals that it is a highly reddened globular cluster well beyond the Galactic center.

  8. Lithium-rich Giants in Globular Clusters

    NASA Astrophysics Data System (ADS)

    Kirby, Evan N.; Guhathakurta, Puragra; Zhang, Andrew J.; Hong, Jerry; Guo, Michelle; Guo, Rachel; Cohen, Judith G.; Cunha, Katia

    2016-03-01

    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 consistent 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. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

  9. Lack of Energy Equipartition in Globular Clusters

    NASA Astrophysics Data System (ADS)

    Trenti, Michele

    2013-05-01

    Abstract (2,250 Maximum Characters): It is widely believed that globular clusters evolve over many two-body relaxation times toward a state of energy equipartition, so that velocity dispersion scales with stellar mass as σ∝m^{-η} with η=0.5. I will show instead that this is incorrect, using a suite of direct N-body simulations with a variety of realistic initial mass functions and initial conditions. No simulated system ever reaches a state close to equipartition. Near the center, the luminous main-sequence stars reach a maximum η_{max 0.15±0.03. At large times, all radial bins convergence on an asymptotic value η_{∞ 0.08±0.02. The development of this ``partial equipartition'' is strikingly similar across simulations, despite the range of different initial conditions employed. Compact remnants tend to have higher η than main-sequence stars (but still η< 0.5), due to their steeper (evolved) mass function. The presence of an intermediate-mass black hole (IMBH) decreases η, consistent with our previous findings of a quenching of mass segregation under these conditions. All these results can be understood as a consequence of the Spitzer instability for two-component systems, extended by Vishniac to a continuous mass spectrum. Mass segregation (the tendency of heavier stars to sink toward the core) has often been studied observationally, but energy equipartition has not. Due to the advent of high-quality proper motion datasets from the Hubble Space Telescope, it is now possible to measure η for real clusters. Detailed data-model comparisons open up a new observational window on globular cluster dynamics, structure, evolution, initial conditions, and possible IMBHs. A first comparison of my simulations to observations of Omega Cen yields good agreement, supporting the view that globular clusters are not generally in energy equipartition. Modeling techniques that assume equipartition by construction (e.g., multi-mass Michie-King models) are thus approximate

  10. A search for novae in M 31 globular clusters

    NASA Technical Reports Server (NTRS)

    Ciardullo, Robin; Tamblyn, Peter; Phillips, A. C.

    1990-01-01

    By combining a local sky-fitting algorithm with a Fourier point-spread-function matching technique, nova outbursts have been searched for inside 54 of the globular clusters contained on the Ciardullo et al. (1987 and 1990) H-alpha survey frames of M 31. Over a mean effective survey time of about 2.0 years, no cluster exhibited a magnitude increase indicative of a nova explosion. If the cataclysmic variables (CVs) contained within globular clusters are similar to those found in the field, then these data imply that the overdensity of CVs within globulars is at least several times less than that of the high-luminosity X-ray sources. If tidal capture is responsible for the high density of hard binaries within globulars, then the probability of capturing condensed objects inside globular clusters may depend strongly on the mass of the remnant.

  11. A search for novae in M 31 globular clusters

    NASA Astrophysics Data System (ADS)

    Ciardullo, Robin; Tamblyn, Peter; Phillips, A. C.

    1990-10-01

    By combining a local sky-fitting algorithm with a Fourier point-spread-function matching technique, nova outbursts have been searched for inside 54 of the globular clusters contained on the Ciardullo et al. (1987 and 1990) H-alpha survey frames of M 31. Over a mean effective survey time of about 2.0 years, no cluster exhibited a magnitude increase indicative of a nova explosion. If the cataclysmic variables (CVs) contained within globular clusters are similar to those found in the field, then these data imply that the overdensity of CVs within globulars is at least several times less than that of the high-luminosity X-ray sources. If tidal capture is responsible for the high density of hard binaries within globulars, then the probability of capturing condensed objects inside globular clusters may depend strongly on the mass of the remnant.

  12. Deep Mixing Statistics in the Globular Cluster NGC5466

    NASA Astrophysics Data System (ADS)

    Wilkerson, Rachel

    2010-01-01

    Using VIRUS-P in conjunction with the 2.7m telescope at McDonald Observatory, we obtained a sample of red giants from the metal-poor globular cluster NGC 5466. The flux of the CN and CH absorption features was calculated using the band indices S3839 and SCH respectively. The results indicate that none of the red giants in this sample exhibit deep mixing abundances. To date all similarly surveyed globular clusters show at least some stars with deep mixing abundances. We discuss the cluster properties of NGC 5466 and how this lack of deep mixing stars makes NGC 5466 different from other metal-poor globular clusters.

  13. A DYING STAR IN GLOBULAR CLUSTER

    NASA Technical Reports Server (NTRS)

    2002-01-01

    A DYING STAR IN GLOBULAR CLUSTER M15 The globular cluster Messier 15 is shown in this color image obtained with the NASA Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2). Lying some 40,000 light-years from Earth in the direction of the constellation Pegasus, M15 is one of nearly 150 known globular clusters that form a vast halo surrounding our Milky Way galaxy. Each of these clusters is a spherical association of hundreds of thousands of ancient stars. The image, prepared by the Hubble Heritage team, attempts to show the stars in M15 in their true colors. The brightest cluster stars are red giants, with an orange color due to surface temperatures lower than our Sun's. Most of the fainter stars are hotter, giving them a bluish-white color. If we lived in the core of M15, our sky would blaze with tens of thousands of brilliant stars both day and night! Nestled among the myriads of stars visible in the Hubble image is an astronomical oddity. The pinkish object to the upper left of the cluster's core is a gas cloud surrounding a dying star. Known as Kuestner 648, this was the first planetary nebula to be identified in a globular cluster. In 1928, F. G. Pease, working at the 100-inch telescope of California's Mount Wilson Observatory, photographed the spectrum of K 648 and discovered the telltale bright emission of a nebular gas cloud rather than a normal star. In the ensuing 70 years, only three more planetary nebulae have been discovered in globular clusters. The stars in M15 and other globular clusters are estimated to be about 12 billion years old. They were among the first generations of stars to form in the Milky Way. Our Sun, by comparison, is a youthful 4.6 billion years old. As a star like the Sun ages, it exhausts the hydrogen that fuels its nuclear fusion, and increases in size to become a red giant. Then it ejects its outer layers into space, producing a planetary nebula. The remnant star at the center of the nebula gradually dies away as a

  14. A DYING STAR IN GLOBULAR CLUSTER

    NASA Technical Reports Server (NTRS)

    2002-01-01

    A DYING STAR IN GLOBULAR CLUSTER M15 The globular cluster Messier 15 is shown in this color image obtained with the NASA Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2). Lying some 40,000 light-years from Earth in the direction of the constellation Pegasus, M15 is one of nearly 150 known globular clusters that form a vast halo surrounding our Milky Way galaxy. Each of these clusters is a spherical association of hundreds of thousands of ancient stars. The image, prepared by the Hubble Heritage team, attempts to show the stars in M15 in their true colors. The brightest cluster stars are red giants, with an orange color due to surface temperatures lower than our Sun's. Most of the fainter stars are hotter, giving them a bluish-white color. If we lived in the core of M15, our sky would blaze with tens of thousands of brilliant stars both day and night! Nestled among the myriads of stars visible in the Hubble image is an astronomical oddity. The pinkish object to the upper left of the cluster's core is a gas cloud surrounding a dying star. Known as Kuestner 648, this was the first planetary nebula to be identified in a globular cluster. In 1928, F. G. Pease, working at the 100-inch telescope of California's Mount Wilson Observatory, photographed the spectrum of K 648 and discovered the telltale bright emission of a nebular gas cloud rather than a normal star. In the ensuing 70 years, only three more planetary nebulae have been discovered in globular clusters. The stars in M15 and other globular clusters are estimated to be about 12 billion years old. They were among the first generations of stars to form in the Milky Way. Our Sun, by comparison, is a youthful 4.6 billion years old. As a star like the Sun ages, it exhausts the hydrogen that fuels its nuclear fusion, and increases in size to become a red giant. Then it ejects its outer layers into space, producing a planetary nebula. The remnant star at the center of the nebula gradually dies away as a

  15. Most Massive Globular Cluster in Our Galaxy

    NASA Astrophysics Data System (ADS)

    1994-05-01

    Far down in the southern sky, in the constellation of Centaurus, a diffuse spot of light can be perceived with the unaided eye. It may be unimpressive, but when seen through a telescope, it turns out to be a beautiful, dense cluster of innumerable stars [1]. Omega Centauri, as this object is called, is the brightest of its type in the sky. We refer to it as a "globular cluster", due to its symmetric form. It belongs to our Milky Way galaxy and astrophysical investigations have shown that it is located at a distance of about 16,500 light-years (1 light-year = 9,460,000,000,000 km). Nobody knows for sure how many individual stars it contains, but recent estimates run into the millions. Most of these stars are more than 10,000 million years old and it is generally agreed that Omega Centauri has a similar age. Measurements of its motion indicate that Omega Centauri plows through the Milky Way in an elongated orbit. It is not easy to understand how it has managed to keep its stars together during such an extended period. MEASURING STELLAR VELOCITIES IN OMEGA CENTAURI A group of astronomers [2] have recently carried through a major investigation of Omega Centauri. After many nights of observations at the ESO La Silla observatory, they now conclude that not only is this globular cluster the brightest, it is indeed by far the most massive known in the Milky Way. The very time-consuming observations were made during numerous observing sessions over a period of no less than 13 years (1981-1993), with the photoelectric spectrometer CORAVEL mounted on the 1.5-m Danish telescope at La Silla. The CORAVEL instrument (COrelation RAdial VELocities) was built in a joint effort between the Geneva (Switzerland) and Marseilles (France) observatories. It functions according to the cross-correlation technique, by means of which the spectrum of the observed star is compared with a "standard stellar spectrum" [3]. HOW HEAVY IS OMEGA CENTAURI? In the present study, a total of 1701

  16. Ultraviolet Spectra of Globular Clusters in Andromeda

    NASA Astrophysics Data System (ADS)

    Peterson, R. C.

    1999-05-01

    As part of a NASA-funded effort with Ben Dorman of Goddard Space Flight Center, I am engaged in calculating spectra from first principles of solar-type stars of a wide range of metallicity. This paper reports on an extension of this work funded by the Hubble Space Telescope archival program, the derivation of fundamental parameters for several globular clusters in Andromeda (M31). Properties of the underlying stellar population are derived by matching archival HST spectra with composite spectra constructed by weighted coaddition of the calculated spectra for stars of appropriate spectral types. Armed with these ab initio calculations, this work explores the degeneracy in age and metallicity in the ultraviolet, and the affect of unknowns such as the relative abundance of light elements versus iron and the possible presence of blue stragglers or blue horizontal branch stars.

  17. Globular Cluster Streams as Galactic High-Precision Scales

    NASA Astrophysics Data System (ADS)

    Küpper, Andreas H. W.; Balbinot, Eduardo; Bonaca, Ana; Johnston, Kathryn V.; Hogg, David W.; Kroupa, Pavel; Santiago, Basilio X.

    2016-08-01

    Tidal streams of globular clusters are ideal tracers of the Galactic gravitational potential. Compared to the few known, complex and diffuse dwarf-galaxy streams, they are kinematically cold, have thin morphologies and are abundant in the halo of the Milky Way. Their coldness and thinness in combination with potential epicyclic substructure in the vicinity of the stream progenitor turns them into high-precision scales. With the example of Palomar 5, we demonstrate how modeling of a globular cluster stream allows us to simultaneously measure the properties of the disrupting globular cluster, its orbital motion, and the gravitational potential of the Milky Way.

  18. Globular Clusters as Cradles of Life and Advanced Civilizations

    NASA Astrophysics Data System (ADS)

    Di Stefano, R.; Ray, A.

    2016-08-01

    Globular clusters are ancient stellar populations in compact dense ellipsoids. There is no star formation and there are no core-collapse supernovae, but several lines of evidence suggest that globular clusters are rich in planets. If so, and if advanced civilizations can develop there, then the distances between these civilizations and other stars would be far smaller than typical distances between stars in the Galactic disk, facilitating interstellar communication and travel. The potent combination of long-term stability and high stellar densities provides a globular cluster opportunity. Yet the very proximity that promotes interstellar travel also brings danger, as stellar interactions can destroy planetary systems. We find, however, that large portions of many globular clusters are “sweet spots,” where habitable-zone planetary orbits are stable for long times. Globular clusters in our own and other galaxies are, therefore, among the best targets for searches for extraterrestrial intelligence (SETI). We use the Drake equation to compare the likelihood of advanced civilizations in globular clusters to that in the Galactic disk. We also consider free-floating planets, since wide-orbit planets can be ejected to travel through the cluster. Civilizations spawned in globular clusters may be able to establish self-sustaining outposts, reducing the probability that a single catastrophic event will destroy the civilization. Although individual civilizations may follow different evolutionary paths, or even be destroyed, the cluster may continue to host advanced civilizations once a small number have jumped across interstellar space. Civilizations residing in globular clusters could therefore, in a sense, be immortal.

  19. CENTRAL ROTATIONS OF MILKY WAY GLOBULAR CLUSTERS

    SciTech Connect

    Fabricius, Maximilian H.; Rukdee, Surangkhana; Saglia, Roberto P.; Bender, Ralf; Hopp, Ulrich; Thomas, Jens; Williams, Michael J.; Noyola, Eva; Opitsch, Michael

    2014-06-01

    Most Milky Way globular clusters (GCs) exhibit measurable flattening, even if on a very low level. Both cluster rotation and tidal fields are thought to cause this flattening. Nevertheless, rotation has only been confirmed in a handful of GCs, based mostly on individual radial velocities at large radii. We are conducting a survey of the central kinematics of Galactic GCs using the new Integral Field Unit instrument VIRUS-W. We detect rotation in all 11 GCs that we have observed so far, rendering it likely that a large majority of the Milky Way GCs rotate. We use published catalogs of GCs to derive central ellipticities and position angles. We show that in all cases where the central ellipticity permits an accurate measurement of the position angle, those angles are in excellent agreement with the kinematic position angles that we derive from the VIRUS-W velocity fields. We find an unexpected tight correlation between central rotation and outer ellipticity, indicating that rotation drives flattening for the objects in our sample. We also find a tight correlation between central rotation and published values for the central velocity dispersion, most likely due to rotation impacting the old dispersion measurements.

  20. Dynamical evolution of globular-cluster systems in clusters of galaxies

    SciTech Connect

    Muzzio, J.C.

    1987-04-01

    The dynamical processes that affect globular-cluster systems in clusters of galaxies are analyzed. Two-body and impulsive approximations are utilized to study dynamical friction, drag force, tidal stripping, tidal radii, globular-cluster swapping, tidal accretion, and galactic cannibalism. The evolution of galaxies and the collision of galaxies are simulated numerically; the steps involved in the simulation are described. The simulated data are compared with observations. Consideration is given to the number of galaxies, halo extension, location of the galaxies, distribution of the missing mass, nonequilibrium initial conditions, mass dependence, massive central galaxies, globular-cluster distribution, and lost globular clusters. 116 references.

  1. Is the Globular Cluster Colour-Metallicity Relation Universal?

    NASA Astrophysics Data System (ADS)

    Usher, Christopher; Sluggs Survey Team

    2015-01-01

    Visible at much greater distances than resolved stars, globular clusters are important tools for studying galaxy formation and assembly. Studies of extragalactic globular clusters typically use optical colours to derive metallicites. We use Keck DEIMOS spectroscopy and Subaru Suprime-Cam photometry from the SLUGGS Survey to investigate how the globular cluster colour-metallicity relation varies galaxy to galaxy and with globular cluster luminosity. As in previous studies we see variations in the shape of the relationship between (g - i) colour and the strength of the calcium triplet spectral feature. To measure weaker spectral features in the DEIMOS spectra, we stack the spectra by colour and by magnitude. Comparing spectra with the same colours and luminosities but from different galaxies, we see significant differences in the strengths of several spectral features, including the calcium triplet and weak iron lines. We interpret this as strong evidence that the globular cluster colour-metallicity relation varies galaxy-to-galaxy. We suggest differences in globular cluster ages between galaxies and in the abundances of light elements (helium, carbon, nitrogen and oxygen) between galaxies as possible explanations for the observed variations in the colour-metallicity relation.

  2. Mapping the differential reddening in globular clusters

    NASA Astrophysics Data System (ADS)

    Bonatto, C.; Campos, Fabíola; Kepler, S. O.

    2013-10-01

    We build differential-reddening maps for 66 Galactic globular clusters (GCs) with archival Hubble Space Telescope WFC/ACS F606W and F814W photometry. Because of the different GC sizes (characterized by the half-light radius Rh) and distances to the Sun, the WFC/ACS field of view (200 arcsec × 200 arcsec) coverage (Robs) lies in the range 1 ≲ Robs/Rh ≲ 15 for about 85 per cent of the sample, with about 10 per cent covering only the inner (Robs ≲ Rh) parts. We divide the WFC/ACS field of view across each cluster in a regular cell grid and extract the stellar-density Hess diagram from each cell, shifting it in colour and magnitude along the reddening vector until matching the mean diagram. Thus, the maps correspond to the internal dispersion of the reddening around the mean. Depending on the number of available stars (i.e. probable members with adequate photometric errors), the angular resolution of the maps range from ≈ 7 arcsec × 7 arcsec to ≈ 20 arcsec × 20 arcsec. We detect spatially variable extinction in the 66 GCs studied, with mean values ranging from < δE(B-V)> ≡ 0.018 (NGC 6981) up to <δE(B-V)> ≡ 0.016 (Palomar 2). Differential-reddening correction decreases the observed foreground reddening and the apparent distance modulus but, since they are related to the same value of E(B - V), the distance to the Sun is conserved. Fits to the mean-ridge lines of the highly extincted and photometrically scattered GC Palomar 2 show that age and metallicity also remain unchanged after the differential-reddening correction, but measurement uncertainties decrease because of the reduced scatter. The lack of systematic variations of <δE(B-V)> with both the foreground reddening and the sampled cluster area indicates that the main source of differential reddening is interstellar.

  3. Chemical Abundance Patterns of Galactic Bulge Globular Clusters

    NASA Astrophysics Data System (ADS)

    Johnson, Christian I.; Rich, R. M.; Kunder, A.; Pilachowski, C. A.

    2014-01-01

    The Galactic bulge globular clusters are interesting but poorly understood stellar systems. The number of bulge globular cluster stars for which detailed chemical abundance information is available is considerably smaller than for halo cluster stars. However, there is growing evidence that many of the bulge globular clusters exhibit interesting characteristics, such as: double horizontal branches, populations separated by more than a factor of two in metallicity, high metallicity clusters with very blue horizontal branches, and large star-to-star variations of heavy element abundances. In order to investigate some of these problems, we have obtained high resolution spectra of several stars in multiple bulge globular clusters in order to measure detailed chemical abundance patterns. We make use of both new observations with the WIYN-Hydra and Magellan-MIKE spectrographs, and also archival data from VLT-FLAMES. We measure the abundances of several light odd-Z, alpha, Fe-peak, and neutron-capture elements, and compare the bulge globular cluster patterns with those in halo clusters and the bulge field. C.I.J. acknowledges support through the Clay Fellowship administered by the Smithsonian Astrophysical Observatory.

  4. Population Models for Massive Globular Clusters

    NASA Astrophysics Data System (ADS)

    Lee, Young-Wook; Joo, Seok-Joo; Han, Sang-Il; Na, Chongsam; Lim, Dongwook; Roh, Dong-Goo

    2015-03-01

    Increasing number of massive globular clusters (GCs) in the Milky Way are now turned out to host multiple stellar populations having different heavy element abundances enriched by supernovae. Recent observations have further shown that [CNO/Fe] is also enhanced in metal-rich subpopulations in most of these GCs, including ω Cen and M22 (Marino et al. 2011, 2012). In order to reflect this in our population modeling, we have expanded the parameter space of Y 2 isochrones and horizontal-branch (HB) evolutionary tracks to include the cases of normal and enhanced nitrogen abundances ([N/Fe] = 0.0, 0.8, and 1.6). The observed variations in the total CNO content were reproduced by interpolating these nitrogen enhanced stellar models. Our test simulations with varying N and O abundances show that, once the total CNO sum ([CNO/Fe]) is held constant, both N and O have almost identical effects on the HR diagram (see Fig. 1).

  5. Radio sources in the field of globular clusters

    NASA Technical Reports Server (NTRS)

    Johnson, H. M.

    1976-01-01

    Eleven globular clusters have been surveyed in the continuum at 2695 and 8085 MHz with the NRAO Green Bank interferometer. The nine clusters discussed probably show unresolved sources with flux densities of the order of 10 mJy. Most of them are not at cluster centers. The dual-frequency data are mostly consistent with nonthermal spectra or with partial resolution, at the higher frequency, of sources with flat spectra. The planetary nebula in M15 has been detected. The empirical statistics of cosmic field sources suggest that they may account for some, but not all, of the sources in the field of globular clusters.

  6. Enrichment by supernovae in globular clusters with multiple populations.

    PubMed

    Lee, Jae-Woo; Kang, Young-Woon; Lee, Jina; Lee, Young-Wook

    2009-11-26

    The most massive globular cluster in the Milky Way, omega Centauri, is thought to be the remaining core of a disrupted dwarf galaxy, as expected within the model of hierarchical merging. It contains several stellar populations having different heavy elemental abundances supplied by supernovae-a process known as metal enrichment. Although M 22 appears to be similar to omega Cen, other peculiar globular clusters do not. Therefore omega Cen and M 22 are viewed as exceptional, and the presence of chemical inhomogeneities in other clusters is seen as 'pollution' from the intermediate-mass asymptotic-giant-branch stars expected in normal globular clusters. Here we report Ca abundances for seven globular clusters and compare them to omega Cen. Calcium and other heavy elements can only be supplied through numerous supernovae explosions of massive stars in these stellar systems, but the gravitational potentials of the present-day clusters cannot preserve most of the ejecta from such explosions. We conclude that these globular clusters, like omega Cen, are most probably the relics of more massive primeval dwarf galaxies that merged and disrupted to form the proto-Galaxy. PMID:19940919

  7. CCD photometry of globular clusters in NGC 3377

    NASA Astrophysics Data System (ADS)

    Harris, William E.

    1990-08-01

    New CCD photometry to a limit B = 24.3 is presented for a sample of globular clusters around NGC 337, an intermediate-size elliptical in the Leo group. Assuming that the luminosity function of its globular clusters follows the same log-normal form as in other galaxies, a peak frequency level (turnover magnitude) of B(0) = 23.35 + or - 0.40 is derived for a combined sample of clusters in NGC 3377 and NGC 3379. Matching this to the globular clusters in the Milky Way then gives a distance modulus (m - M)B (Leo) = 30.2, or d = (10.7 + or - 2.2) Mpc, in good agreement with other recent distance-scale methods. The clusters around NGC 3377 follow a highly ellipsoidal space distribution; two different tests show that their distribution is at least as flattened as the E6 shape of the galaxy itself.

  8. CCD photometry of globular clusters in NGC 3377

    SciTech Connect

    Harris, W.E. )

    1990-09-01

    New CCD photometry to a limit B = 24.3 is presented for a sample of globular clusters around NGC 337, an intermediate-size elliptical in the Leo group. Assuming that the luminosity function of its globular clusters follows the same log-normal form as in other galaxies, a peak frequency level (turnover magnitude) of B(0) = 23.35 + or - 0.40 is derived for a combined sample of clusters in NGC 3377 and NGC 3379. Matching this to the globular clusters in the Milky Way then gives a distance modulus (m - M)B (Leo) = 30.2, or d = (10.7 + or - 2.2) Mpc, in good agreement with other recent distance-scale methods. The clusters around NGC 3377 follow a highly ellipsoidal space distribution; two different tests show that their distribution is at least as flattened as the E6 shape of the galaxy itself. 31 refs.

  9. Carbon and nitrogen abundance variations in globular cluster red giants

    NASA Astrophysics Data System (ADS)

    Martell, Sarah L.

    2008-06-01

    This dissertation describes investigations into two of the persistent questions of elemental abundances in Galactic globular clusters: the phenomenon of deep mixing, observed through the progressive depletion of surface carbon abundance as stars evolve along the red giant branch, and abundance bimodality, a phenomenon observed only in globular clusters, in which a subset of stars in a given globular cluster have a distinctive pattern of elemental enhancements and depletions relative to the Solar pattern. The first chapter gives an introduction to the history of globular cluster abundance studies, with particular focus on low-resolution spectroscopy. For both deep mixing and abundance bimodality, the leading theoretical models and the data which support and challenge them are laid out. Each section ends with a description of presently-unanswered questions; these are the motivation for the various projects contained in this dissertation. The second chapter describes the use of molecular handstrengths for determining elemental abundances from low-resolution spectra, and introduces a new CH bandstrength index that is designed to be sensitive to carbon abundance and insensitive to nitrogen abundance in Pop. II red giants over a wide range of metallicity. Various CH indices defined elsewhere in the literature are also discussed, and are shown to have comparable accuracy to the new index only over a limited range of stellar properties. Carbon abundances determined using the new CH index are compared to literature abundances for a few stars, and general concordance with published abundances is found. The third chapter contains a large-scale application of the new CH index: a survey of present-day carbon abundances and calculated carbon depletion rates in bright red giants belonging to eleven Galactic globular clusters spanning the full metallicity range of halo globular clusters. Targets were selected with similar evolutionary states, were observed with one instrument on

  10. The globular cluster system of NGC 6822

    NASA Astrophysics Data System (ADS)

    Veljanoski, J.; Ferguson, A. M. N.; Mackey, A. D.; Huxor, A. P.; Hurley, J. R.; Bernard, E. J.; Côté, P.; Irwin, M. J.; Martin, N. F.; Burgett, W. S.; Chambers, K. C.; Flewelling, H.; Kudritzki, R.; Waters, C.

    2015-09-01

    We present a comprehensive analysis of the globular cluster (GC) system of the Local Group dwarf irregular galaxy NGC 6822. Our study is based on homogeneous optical and near-IR photometry, as well as long-slit spectroscopic observations which are used to determine new radial velocities for six GCs, two of which had no previous spectroscopic information. We construct optical-near-IR colour-colour diagrams and through comparison to simple stellar population models infer that the GCs have old ages consistent with being 9 Gyr or older, while their metallicities are in the range between -1.6 ≲ [Fe/H] ≲ -0.4. We conduct a kinematic analysis of the GC population and find tentative evidence for weak net rotation of the GC system, in the same sense as that exhibited by the underlying spheroid. The most likely amplitude of rotation is ≈10 km s-1, approximately half the magnitude of the observed velocity dispersion. Finally, we use the GCs to estimate the dynamical mass of NGC 6822 within ˜11 kpc and we formally find it to be in the range between (3 and 4) × 109 M⊙. This implies an overall mass-to-light ratio in the range of ˜30-40 and indicates that NGC 6822 is highly dark-matter-dominated. The mass and the corresponding mass-to-light ratio estimates are affected by various additional systematic effects due to limitations of the data and the model that are not necessary reflected in the formal uncertainties.

  11. Shedding light on lithium evolution. the globular cluster perspective

    NASA Astrophysics Data System (ADS)

    Korn, Andreas J.

    I shall review what has been learnt during 20 years of lithium observations in stars belonging to metal-poor globular clusters. The focus will be on little evolved main- sequence, turnoff-point (TOP) and subgiant-branch (SGB) stars expected to display Spite-plateau lithium abundances like those found in the majority of field stars of similar metallicities. But is the Spite plateau of globular clusters the same as those of field stars? What effect does, e.g., cluster-internal pollution have on lithium abundances in the now dominant second generation of stars? It will be shown that it is primarily our incomplete knowledge of the temperature scale of Population II stars which currently limits the diagnostic power of globular clusters as regards the stellar-surface evolution of lithium.

  12. 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.

  13. Astronomers Ponder Lack of Planets in Globular Cluster

    NASA Technical Reports Server (NTRS)

    2000-01-01

    This videotape has seven segments, discussing and showing the evidence for the proposition that the galactic clusters do not have many planets. Specifically the segments show: (1) Dr. Ron Gilliland discussing the process of looking for "Hot Jupiters" (i.e., planets about the size of Jupiter, which are hotter than Jupiter) in the globular clusters, (2) a zoom into 47 Tucanae globular cluster, (3) an animation of a planet passing between the host star and the earth with a brightness graph, (4) the same animation as before without the graph, (5) Ron Gilliland of the Space Telescope Science Institute (STScI) discussing possible interpretations of his findings in the 47 Tucanae globular cluster, (6) Ron Gilliland examining the images of 47 Tucanae, and (7) images of 47 Tucanae watching for variations in brightness.

  14. RETENTION OF STELLAR-MASS BLACK HOLES IN GLOBULAR CLUSTERS

    SciTech Connect

    Morscher, Meagan; Umbreit, Stefan; Farr, Will M.; Rasio, Frederic A. E-mail: s-umbreit@northwestern.edu E-mail: rasio@northwestern.edu

    2013-01-20

    Globular clusters should be born with significant numbers of stellar-mass black holes (BHs). It has been thought for two decades that very few of these BHs could be retained through the cluster lifetime. With masses {approx}10 M{sub Sun }, BHs are {approx}20 times more massive than an average cluster star. They segregate into the cluster core, where they may eventually decouple from the remainder of the cluster. The small-N core then evaporates on a short timescale. This is the so-called Spitzer instability. Here we present the results of a full dynamical simulation of a globular cluster containing many stellar-mass BHs with a realistic mass spectrum. Our Monte Carlo simulation code includes detailed treatments of all relevant stellar evolution and dynamical processes. Our main finding is that old globular clusters could still contain many BHs at present. In our simulation, we find no evidence for the Spitzer instability. Instead, most of the BHs remain well mixed with the rest of the cluster, with only the innermost few tens of BHs segregating significantly. Over the 12 Gyr evolution, fewer than half of the BHs are dynamically ejected through strong binary interactions in the cluster core. The presence of BHs leads to long-term heating of the cluster, ultimately producing a core radius on the high end of the distribution for Milky Way globular clusters (and those of other galaxies). A crude extrapolation from our model suggests that the BH-BH merger rate from globular clusters could be comparable to the rate in the field.

  15. The Evolution of the Globular Cluster System in a Triaxial Galaxy: Can a Galactic Nucleus Form by Globular Cluster Capture?

    NASA Astrophysics Data System (ADS)

    Capuzzo-Dolcetta, Roberto

    1993-10-01

    Among the possible phenomena inducing evolution of the globular cluster system in an elliptical galaxy, dynamical friction due to field stars and tidal disruption caused by a central nucleus is of crucial importance. The aim of this paper is the study of the evolution of the globular cluster system in a triaxial galaxy in the presence of these phenomena. In particular, the possibility is examined that some galactic nuclei have been formed by frictionally decayed globular clusters moving in a triaxial potential. We find that the initial rapid growth of the nucleus, due mainly to massive clusters on box orbits falling in a short time scale into the galactic center, is later slowed by tidal disruption induced by the nucleus itself on less massive clusters in the way described by Ostriker, Binney, and Saha. The efficiency of dynamical friction is such to carry to the center of the galaxy enough globular cluster mass available to form a compact nucleus, but the actual modes and results of cluster-cluster encounters in the central potential well are complicated phenomena which remains to be investigated. The mass of the resulting nucleus is determined by the mutual feedback of the described processes, together with the initial spatial, velocity, and mass distributions of the globular cluster family. The effect on the system mass function is studied, showing the development of a low- and high-mass turnover even with an initially flat mass function. Moreover, in this paper is discussed the possibility that the globular cluster fall to the galactic center has been a cause of primordial violent galactic activity. An application of the model to M31 is presented.

  16. Integrated UV fluxes and the HB morphology of Globular Clusters

    NASA Astrophysics Data System (ADS)

    Landsman, W. B.; Catelan, M.; O'Connell, R. W.; Pereira, D.; Stecher, T. P.

    2001-12-01

    The UV ( ~ 1500 Å) flux of a globular cluster will be dominated by its blue horizontal branch (HB) population, provided that such a population is present. Thus, the integrated UV - V color of a globular cluster can provide an indication of its HB morphology, without the need to resolve the cluster into a color-magnitude diagram. To date, UV photometry of extragalactic clusters are available for only a few globulars in M31 (e.g. Bohlin et al. 1993, ApJ, 417, 127), but additional UV photometry of extragalactic globulars is soon expected from GALEX (Yi et al. 2001, AAS, 198, 5501), and from STIS FUV-MAMA observations of M87 (HST program 8643). Here we calibrate the relation between UV flux and HB morphology for Galactic globular clusters. The OAO-2 and ANS data tabulated by deBoer (1985, A&A, 142, 321) are supplemented with photometry of 14 globular clusters observed with the Ultraviolet Imaging Telescope (UIT), and a few cluster cores observed with the STIS FUV-MAMA. The UIT data is especially useful since its 40' diameter FOV was sufficient to completely encompass most of the observed clusters, while allowing isolation of hot field and UV-bright stars. We compare the observed Galactic UV color - HB morphology relation with synthetic HB models as a function of age and metallicity. We also estimate the effect of radiative levitation of heavy metals in hot HB stars (e.g. Moehler et al. 2000, , A&A, 360, 120) on the integrated UV flux. This work is funded by STScI grant GO-8358.01.

  17. Color maps of X-ray globular clusters

    NASA Technical Reports Server (NTRS)

    Bailyn, Charles D.; Grindlay, Jonathan E.; Cohn, Haldan; Lugger, Phyllis M.

    1988-01-01

    The results of a search for optical counterparts to X-ray sources in six globular clusters, 47 Tuc, NGC 1851, NGC 6441, NGC 6624, NGC 6712, and M15, are reported. Maps of the U-B color of the central regions of the clusters were prepared. A candidate for the optical counterpart of the source in NGC 6712 was found, along with a blue region near the X-ray source in 47 Tuc. Upper limits on the colors and magnitudes of possible optical counterparts are reported for the other three clusters. The use of color maps to determine color gradients in globular clusters is explored. It is found that, while such gradients do exist and vary from cluster to cluster, they can be explained by crowding effects. Crude limits are placed on the excess populations of blue objects such as CVs, which have been postulated to be concentrated in the centers of dense clusters.

  18. Chemical Compositions of Stars in Globular Cluster NGC 2419

    NASA Astrophysics Data System (ADS)

    Kadakia, Shimonee; Smecker-Hane, T.; Bosler, T.

    2007-05-01

    We determine the chemical abundances of 19 red giant branch stars in the Galactic globular cluster NGC 2419. Lying at a distance of 84.2 kpc and a galactocentric distance of 91.5 kpc, NGC 2419 is the fourth brightest globular cluster in the Milky Way with a total magnitude of M_V = -9.6 mag, which is significantly brighter than M_V = -7.5 mag, the typical peak of the globular cluster luminosity functions in external galaxies. Our results will give an insight of whether NGC 2419 is in fact a globular cluster or a core of a disrupted galaxy that merged with the Milky Way. We have used IRAF to reduce spectra we have taken with the DEIMOS spectrograph on the the Keck I 10-meter telescope. Using the strengths of the Ca II triplet absorption lines at approximately 8600 Angstrom, we will determine the chemical abundance of each star. If the chemical abundances differ by significantly more than the observational errors would predict then we can conclude the cluster is a remnant of the core of a galaxy that merged with the Milky Way and not a normal globular cluster, because most globular clusters formed quickly from a well mixed gas cloud, and thus their stars have nearly identical ages and chemical compositions. We gratefully acknowledge financial support from a UROP grant to SK and NSF grant AST-0307863 to TSH. These data were obtained at the Keck Observatory, operated by the California Inst. of Technology, Univ. of California and NASA and made possible by generous financial support from the W.M. Keck Foundation.

  19. Detection of a large-scale structure of intracluster globular clusters in the Virgo cluster.

    PubMed

    Lee, Myung Gyoon; Park, Hong Soo; Hwang, Ho Seong

    2010-04-16

    Globular clusters are usually found in galaxies, and they are excellent tracers of dark matter. Long ago it was suggested that intracluster globular clusters (IGCs) may exist that are bound to a galaxy cluster rather than to any single galaxy. Here we present a map showing the large-scale distribution of globular clusters over the entire Virgo cluster. It shows that IGCs are found out to 5 million light years from the Virgo center and that they are concentrated in several substructures that are much larger than galaxies. These objects might have been mostly stripped off from low-mass dwarf galaxies. PMID:20223950

  20. Metallicity and star formation history of globular clusters

    NASA Astrophysics Data System (ADS)

    Zhang, Mei; Ma, Er

    1993-01-01

    Using population synthesis method, the star formation history in globular clusters has been studied. No single star formation mode with a constant star formation rate (SER) and an invariable initial mass function (IMF) can fit the observations of globular clusters. There are at least two stages of star formation: a pollution stage and a starburst stage. In the pollution stage, either the IMF is very peculiar (only form massive stars), or its SFR is so small that the low-mass stars form only a little. A starburst then follows to form most stars in the globular cluster. Within the framework of Fall and Rees'model, the collisions between warm clouds in the two phase medium may provide a suitable external cause to stimulate the starburst.

  1. Metallicity and star formation history of globular clusters

    NASA Astrophysics Data System (ADS)

    Zhang, Mei; Ma, Er

    1993-03-01

    Using population synthesis method, the star formation history in globular clusters has been studied. No single star formation mode with a constant star formation rate (SER) and an invariable initial mass function (IMF) can fit the observations of globular clusters. There are at least two stages of star formation: a pollution stage and a starburst stage. In the pollution stage, either the IMF is very peculiar (only form massive stars), or its SFR is so small that the low-mass stars form only a little. A starburst then follows to form most stars in the globular cluster. Within the framework of Fall and Rees' model, the collisions between warm clouds in the two phase medium may provide a suitable external cause to stimulate the starburst.

  2. HST Imaging of the Globular Clusters in the Formax Cluster: Color and Luminosity Distributions

    NASA Technical Reports Server (NTRS)

    Grillmair, C. J.; Forbes, D. A.; Brodie, J.; Elson, R.

    1998-01-01

    We examine the luminosity and B - I color distribution of globular clusters for three early-type galaxies in the Fornax cluster using imaging data from the Wide Field/Planetary Camera 2 on the Hubble Space Telescope.

  3. Nearby Spiral Galaxy Globular Cluster Systems. II. Globular Cluster Metallicities in NGC 300

    NASA Astrophysics Data System (ADS)

    Nantais, Julie B.; Huchra, John P.; Barmby, Pauline; Olsen, Knut A. G.

    2010-03-01

    We present new metallicity estimates for globular cluster (GC) candidates in the Sd spiral NGC 300, one of the nearest spiral galaxies outside the Local Group. We have obtained optical spectroscopy for 44 Sculptor Group GC candidates with the Boller and Chivens (B&C) spectrograph on the Baade Telescope at Las Campanas Observatory. There are two GCs in NGC 253 and 12 objects in NGC 300 with globular-cluster-like spectral features, nine of which have radial velocities above 0 km s-1. The remaining three, due to their radial velocities being below the expected 95% confidence limit for velocities of NGC 300 halo objects, are flagged as possible foreground stars. The non-cluster-like candidates included 13 stars, 15 galaxies, and an H II region. One GC, four galaxies, two stars, and the H II region from our sample were identified in archival Hubble Space Telescope images. For the GCs, we measure spectral indices and estimate metallicities using an empirical calibration based on Milky Way GCs. The GCs of NGC 300 appear similar to those of the Milky Way. Excluding possible stars and including clusters from the literature, the GC system (GCS) has a velocity dispersion of 68 km s-1 and has no clear evidence of rotation. The mean metallicity for our full cluster sample plus one literature object is [Fe/H] = -0.94, lying above the relationship between mean GC metallicity and overall galaxy luminosity. Excluding the three low-velocity candidates, we obtain a mean [Fe/H] = -0.98, still higher than expected, raising the possibility of significant foreground star contamination even in this sample. Visual confirmation of genuine GCs using high-resolution space-based imagery could greatly reduce the potential problem of interlopers in small samples of GCSs in low-radial-velocity galaxies. Data for this project were obtained at the Baade 6.5 m telescope, Las Campanas Observatory, Chile. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint

  4. Co-evolution of galactic nuclei and globular cluster systems

    SciTech Connect

    Gnedin, Oleg Y.; Ostriker, Jeremiah P.; Tremaine, Scott

    2014-04-10

    We revisit the hypothesis that dense galactic nuclei are formed from inspiraling globular clusters. Recent advances in the understanding of the continuous formation of globular clusters over cosmic time and the concurrent evolution of the galaxy stellar distribution allow us to construct a simple model that matches the observed spatial and mass distributions of clusters in the Galaxy and the giant elliptical galaxy M87. In order to compare with observations, we model the effects of dynamical friction and dynamical evolution, including stellar mass loss, tidal stripping of stars, and tidal disruption of clusters by the growing galactic nucleus. We find that inspiraling globular clusters form a dense central structure, with mass and radius comparable to the typical values in observed nuclear star clusters (NSCs) in late-type and low-mass early-type galaxies. The density contrast associated with the NSC is less pronounced in giant elliptical galaxies. Our results indicate that the NSC mass as a fraction of mass of the galaxy stellar spheroid scales as M{sub NSC}/M{sub ∗}≈0.0025 M{sub ∗,11}{sup −0.5}. Thus disrupted globular clusters could contribute most of the mass of NSCs in galaxies with stellar mass below 10{sup 11} M {sub ☉}. The inner part of the accumulated cluster may seed the growth of a central black hole via stellar dynamical core collapse, thereby relieving the problem of how to form luminous quasars at high redshift. The seed black hole may reach ∼10{sup 5} M {sub ☉} within ≲ 1 Gyr of the beginning of globular cluster formation.

  5. MOCK OBSERVATIONS OF BLUE STRAGGLERS IN GLOBULAR CLUSTER MODELS

    SciTech Connect

    Sills, Alison; Glebbeek, Evert; Chatterjee, Sourav; Rasio, Frederic A. E-mail: e.glebbeek@astro.ru.nl E-mail: rasio@northwestern.edu

    2013-11-10

    We created artificial color-magnitude diagrams of Monte Carlo dynamical models of globular clusters and then used observational methods to determine the number of blue stragglers in those clusters. We compared these blue stragglers to various cluster properties, mimicking work that has been done for blue stragglers in Milky Way globular clusters to determine the dominant formation mechanism(s) of this unusual stellar population. We find that a mass-based prescription for selecting blue stragglers will select approximately twice as many blue stragglers than a selection criterion that was developed for observations of real clusters. However, the two numbers of blue stragglers are well-correlated, so either selection criterion can be used to characterize the blue straggler population of a cluster. We confirm previous results that the simplified prescription for the evolution of a collision or merger product in the BSE code overestimates their lifetimes. We show that our model blue stragglers follow similar trends with cluster properties (core mass, binary fraction, total mass, collision rate) as the true Milky Way blue stragglers as long as we restrict ourselves to model clusters with an initial binary fraction higher than 5%. We also show that, in contrast to earlier work, the number of blue stragglers in the cluster core does have a weak dependence on the collisional parameter Γ in both our models and in Milky Way globular clusters.

  6. Spectroscopy of the globular clusters in M87

    NASA Technical Reports Server (NTRS)

    Mould, J. R.; Oke, J. B.; Nemec, J. M.

    1987-01-01

    With a velocity dispersion of 370 + or - 50 km/sec the globular cluster system of M87 is kinematically hotter than the stars in the giant elliptical itself. This is consistent with the clusters' shallower density distribution for isotropic orbits. The mean metallicity of the 27 clusters in the sample analyzed here is no more than a factor of 2 more metal rich than the cluster system of the Milky Way, but considerably more metal poor than the integrated starlight in the field at a radius of 1' from the center of M87. There is no evidence for the existence of young clusters in the system. The mass-radius relation between 1' and 5' required to contain the globular clusters joins on to that required to contain the hot gas around M87.

  7. Spectroscopy of the globular clusters in M87

    NASA Technical Reports Server (NTRS)

    Mould, J. R.; Oke, J. B.; Nemec, J. M.

    1986-01-01

    With a velocity dispersion of 370 + or - 50 km/sec the globular cluster system of M87 is kinematically hotter than the stars in the giant elliptical itself. This is consistent with the clusters' shallower density distribution for isotropic orbits. The mean metallicity of the 27 clusters in the sample analyzed here is no more than a factor of 2 more metal rich than the cluster system of the Milky Way, but considerably more metal poowr than the integrated starlight in the field at a radius of 1' from the center of M87. There is no evidence for the existence of young clusters in the system. The mass-radius relation between 1' and 5' required to contain the globular clusters joins on to that required to contain the hot gas around M87.

  8. Slicing and dicing globular clusters: dynamically evolved single stellar populations

    NASA Astrophysics Data System (ADS)

    Sippel, Anna C.; Hurley, Jarrod R.

    2016-04-01

    We utilize direct N-body models of globular clusters including stellar evolution to calculate magnitudes for each star in the Hubble Space Telescope Advanced Camera for Surveys 555, 606 and 814 filters. This enables us to analyse the colour of dynamically evolved single stellar populations over time in colour-magnitude diagrams of both, resolved and integrated globular clusters. We find that the change of integrated cluster colour is driven predominantly by the colour of the brightest stars available and hence by stellar evolution, but not by the removal of low-mass stars. We show that even in mass-segregated clusters, different stellar populations are distributed over the entire cluster. This implies that evolved stars also exist within and outside the half-mass radius.

  9. Far-ultraviolet radiation from disk globular clusters

    NASA Technical Reports Server (NTRS)

    Rich, R. M.; Minniti, Dante; Liebert, James

    1993-01-01

    IUE spectra obtained in a survey of the metal-rich disk system of globular clusters are presented. Significant FUV fluxes were detected in the 1200-2000-A short-wavelength (SWP) range of the IUE Observatory in several disk globular clusters. These clusters are the most metal-rich known to have an FUV flux. Three clusters show spectral energy distrbutions (SEDs) clearly rising at shorter wavelengths, not unlike the upturns observed in the bulges of metal-rich elliptical galaxies. Several others with weak SWP detections appear to have flat or uncertain spectral energy distributions. Blue stragglers provide a possible explanation for flux redder than 2000 A in clusters showing weaker flux in the SWP region, and with flat or declining SEDs.

  10. The effect of galaxy triaxiality on globular clusters

    NASA Technical Reports Server (NTRS)

    Ostriker, J. P.; Binney, J.; Saha, P.

    1989-01-01

    Evidence of triaxiality and central mass concentration in the Galaxy and in M31 and M87 is examined. It is proposed that globular clusters on box orbits in the inner parts of these systems pass close enough to the center to be destroyed by tidal shocks. Remaining clusters will preferentially populate tube orbits with relatively high angular momentum. The process is used to explain the cluster distribution in M87 reported by Lauer and Kormendy (1986). Models are presented for cluster destruction by massive black holes in M87's dark halo. Consideration is given to techniques for testing the suggestion that clusters form only on box orbits.

  11. DARK MATTER HALOS IN GALAXIES AND GLOBULAR CLUSTER POPULATIONS

    SciTech Connect

    Hudson, Michael J.; Harris, Gretchen L.; Harris, William E.

    2014-05-20

    We combine a new, comprehensive database for globular cluster populations in all types of galaxies with a new calibration of galaxy halo masses based entirely on weak lensing. Correlating these two sets of data, we find that the mass ratio η ≡ M {sub GCS}/M {sub h} (total mass in globular clusters, divided by halo mass) is essentially constant at (η) ∼ 4 × 10{sup –5}, strongly confirming earlier suggestions in the literature. Globular clusters are the only known stellar population that formed in essentially direct proportion to host galaxy halo mass. The intrinsic scatter in η appears to be at most 0.2 dex; we argue that some of this scatter is due to differing degrees of tidal stripping of the globular cluster systems between central and satellite galaxies. We suggest that this correlation can be understood if most globular clusters form at very early stages in galaxy evolution, largely avoiding the feedback processes that inhibited the bulk of field-star formation in their host galaxies. The actual mean value of η also suggests that about one-fourth of the initial gas mass present in protogalaxies collected into giant molecular clouds large enough to form massive, dense star clusters. Finally, our calibration of (η) indicates that the halo masses of the Milky Way and M31 are (1.2 ± 0.5) × 10{sup 12} M {sub ☉} and (3.9 ± 1.8) × 10{sup 12} M {sub ☉}, respectively.

  12. BVRI CCD photometry of the globular cluster NGC 2808

    SciTech Connect

    Alcaino, G.; Liller, W.; Alvarado, F.; Wenderoth, E. )

    1990-03-01

    As a part of a continuing program, CCD color-magnitude diagrams are presented for the bright globular cluster NGC 2808 in the four colors comprising BVRI. From a comparison of four different CMDs with theoretical isochrones, an age of 16 + or - 2 Gyr is obtained, assuming a value for Fe/H near -1.3. 28 refs.

  13. Possible systematic decreases in the age of globular clusters

    SciTech Connect

    Shi, X.; Schramm, D. N.; Dearborn, D. S.P.; Truran, J. W.

    1994-03-01

    The ages of globular clusters inferred from observations depends sensitively on assumptions like the initial helium abundance and the mass loss rate. A high helium abundance (e.g., Y\\approx0.28) or a mass loss rate of \\sim10^{-11}M_\\odot yr^{-1} near the main sequence turn-off region lowers the current age estimate from 14 Gyr to about 10--12 Gyr, significantly relaxing the constraints on the Hubble constant, allowing values as high as 60km/sec/Mpc for a universe with the critical density and 90km/sec/Mpc for a baryon-only universe. Possible mechanisms for the helium enhancement in globular clusters are discussed, as are arguments for an instability strip induced mass loss near the turn-off. Ages lower than 10 Gyr are not possible even with the operation of both of these mechanisms unless the initial helium abundance in globular clusters is >0.30, which would conflict with indirect measurements of helium abundances in globular clusters.

  14. Recent Advances in High Dispersion Spectroscopy of Globular Cluster Stars

    NASA Astrophysics Data System (ADS)

    Gratton, R. G.

    The author discusses some problems concerning the derivation of accurate metal abundances for globular clusters using high dispersion spectra from both the old photographic and the most recent CCD data. The discrepant low abundances found by Cohen (1980), from photographic material for M71 giants, are found to be due to the use of too high microturbulences.

  15. Spectroscopy of candidate young globular clusters in NGC 1275

    NASA Technical Reports Server (NTRS)

    Zepf, Stephen E.; Carter, Dave; Sharples, Ray M.; Ashman, Keith

    1995-01-01

    We present spectra of the brightest member of the population of compact blue objects discovered in the peculiar galaxy NGC 1275 by Holtzman et al. (1992) using Hubble Space Telescope images. These spectra show strong Balmer absorption lines like those observed in A-type stars, as expected if the object is a young globular cluster. The age estimated from the strength of the Balmer lines is about 0.5 Gyr, although ages ranging from 0.1 Gyr to 0.9 Gyr cannot be confidently excluded given current models of stellar populations. If these estimated ages are adopted for the young cluster population of NGC 1275 as a whole, the fading predicted by stellar populations models gives a luminosity function which is consistent with that of the Galactic globular cluster system convolved with the observational selection function for the NGC 1275 system. We also use the equivalent widths of the Mg b and Fe 5270 features to constrain the metallicity of the young cluster. Combining these absorption-line widths with the age estimates from the Balmer lines and stellar population models, we find a metallicity of roughly solar, based on the Mg b index, and somewhat higher for the Fe 5270 index. The radial velocity of the absorption lines of the cluster spectrum is offset from the emission lines of the galaxy spectrum at the same position by - 130 km/s, providing further evidence for the identification of the object as a global cluster and opening up the future possibility of studying the kinematics of young cluster systems. The discovery of objects with the characteristics of young globular clusters in NGC 1275, which shows evidence of a recent interaction or merger, supports the hypothesis that galaxy interactions and mergers are favorable sites for the formation of globular clusters.

  16. Blue Stragglers in Globular Clusters: Observations, Statistics and Physics

    NASA Astrophysics Data System (ADS)

    Knigge, Christian

    This chapter explores how we might use the observed statistics of blue stragglers in globular clusters to shed light on their formation. This means we will touch on topics also discussed elsewhere in this book, such as the discovery and implications of bimodal radial distributions and the "double sequences" of blue stragglers that have recently been found in some clusters. However, we will focus particularly on the search for a "smoking gun" correlation between the number of blue stragglers in a given globular cluster and a physical cluster parameter that would point towards a particular formation channel. As we shall see, there is little evidence for an intrinsic correlation between blue straggler numbers and stellar collision rates, even in dense cluster cores. On the other hand, there is a clear correlation between blue straggler numbers and the total (core) mass of the cluster. This would seem to point towards a formation channel involving binaries, rather than dynamical encounters. However, the correlation between blue straggler numbers and actual binary numbers—which relies on recently determined empirical binary fractions—is actually weaker than that with core mass. We explain how this surprising result may be reconciled with a binary formation channel if binary fractions depend almost uniquely on core mass. If this is actually the case, it would have significant implications for globular cluster dynamics more generally.

  17. Predictions of a population of cataclysmic variables in globular clusters

    NASA Technical Reports Server (NTRS)

    Di Stefano, R.; Rappaport, S.

    1994-01-01

    We have studied the number of cataclysmic variables (CVs) that should be active in globular clusters during the present epoch as a result of binary formation via two-body tidal capture. We predict the orbital period and luminosity distributions of CVs in globular clusters. The results arebased on Monte Carlo simulations combined with evolution calculations appropriate to each system formed during the lifetime of two specific globular clusters, omega Cen and 47 Tuc. From our study of these two clusters, which represent the range of core densities and states of mass segregation that are likely to be interesting, we extrapolate our results to the Galactic globlular cluster system. Although there is at present little direct observational evidence of CVs in globular clusters, we find that there should be a large number of active systems. We predict that there should be more than approximately 100 CVs in both 47 Tuc and omega Cen and several thousand in the Galactic globular cluster system. These numbers are based on two-body processes alone and represent a lower bound on the number of systems that may have been formed as a result of stellar interaction within globular clusters. The relation between these calculations and the paucity of optically detected CVs in globular clusters is discussed. Should future observations fail to find convincing evidence of a substantial population of cluster CVs, then the two-body tidal capture scenario is likely to be seriously constrained. Of the CVs we espect in 47 Tuc and omega Cen, approximately 45 and 20, respectively, should have accretion luminosities above 10(exp 33) ergs/s. If one utilizes a relation for converting accretion luminosity to hard X-ray luminosity that is based on observations of Galactic plane CVs, even these sources will not exhibit X-ray luminosities above 10(exp 33) ergs/s. While we cannot account directly for the most luminous subset of the low-luminosity globular cluster X-ray sources without assuming an

  18. Forming Centaurus A's High M/L Globular Clusters

    NASA Astrophysics Data System (ADS)

    Sauda Bovill, Mia; Hyazinth Puzia, Thomas; Ricotti, Massimo; Taylor, Matthew Alan

    2015-08-01

    Using a set of high resolution N-body simulations, we develop a formation model for the high M/L globular clusters recently found around Centaurus A. In our model, the high M/L clusters are not true globular clusters, but rather the stripped cores of dwarf galaxies which underwent an epoch of efficient, concentrated star formation, possibly before or during the epoch of reionization. Given the known stellar masses of these objects, their halos virialized with relatively high masses and high redshift, falling into the Centaurus A with > 109 Msolar. These criteria give us a distribution of surviving subhalos which is consistent with the number and three dimensional distribution of the high M/L clusters within 100 pc of Centaurus A.

  19. Bright X-Ray Sources in M31 Globular Clusters

    NASA Astrophysics Data System (ADS)

    Di Stefano, R.; Kong, A. K. H.; Garcia, M. R.; Barmby, P.; Greiner, J.; Murray, S. S.; Primini, F. A.

    2002-05-01

    We have conducted Chandra observations of ~2560 arcmin2 (~131 kpc2) of M31 and find that the most luminous X-ray sources in most of our fields are in globular clusters. Of the 28 globular cluster X-ray sources in our fields, 15 are newly discovered. Approximately one-third of all the sources have LX([0.5-7] keV)>1037 ergs s-1, and approximately one-tenth of all the sources have LX([0.5-7] keV) close to or above 1038 ergs s-1. The most luminous source, in the globular cluster Bo 375, is consistently observed to have LX greater than 2×1038 ergs s-1. (1) We present data on the spectra and/or light curves of the five most luminous M31 globular cluster sources. (2) We explore possible explanations for the high X-ray luminosities of the brightest sources. These include that the X-ray sources may be composites, the radiation we receive may be beamed, metallicity effects could be at work, or the sources may be accreting black holes. We weigh each of these possibilities against the data. In addition, we introduce a neutron star model in which mass transfer proceeds on the thermal timescale of the donor star. Our model can produce luminosities of several times 1038 ergs s-1 and leads to a set of well-defined predictions. (3) We compute the X-ray luminosity function and the distribution of counts in wavebands that span the range of energies to which Chandra is sensitive. We find the peak X-ray luminosity is higher and that systems with LX>1037 ergs s-1 constitute a larger fraction of all GC sources than in our Galaxy. (4) We study the possible reasons for this difference between M31 and Galactic globular cluster X-ray sources and identify three promising explanations.

  20. Catalogue of Galactic globular-cluster surface-brightness profiles

    NASA Technical Reports Server (NTRS)

    Trager, S. C.; King, Ivan R.; Djorgovski, S.

    1995-01-01

    We present a catalogue of surface-brightness profiles (SBPs) of 125 Galactic globular clusters, the largest such collection ever gathered. The SPBs are constructed from generally inhomogeneous data, but are based heavily on the Berkeley Global Cluster Survey of Djorgovski & King. All but four of the SBPs have photometric zero points. We derive central surface brightness, King-model concentrations, core radii, half-light, and other fraction-of-light radii where data permit, and we briefly discuss their use.

  1. A SURVEY FOR PLANETARY NEBULAE IN M31 GLOBULAR CLUSTERS

    SciTech Connect

    Jacoby, George H.; De Marco, Orsola; Lee, Myung Gyoon; Herrmann, Kimberly A.; Hwang, Ho Seong; Davies, James E.; Kaplan, Evan E-mail: rbc@astro.psu.edu E-mail: mglee@astrog.snu.ac.kr E-mail: hhwang@cfa.harvard.edu E-mail: evanskaplan@gmail.com

    2013-05-20

    We report the results of an [O III] {lambda}5007 spectroscopic survey for planetary nebulae (PNe) located within the star clusters of M31. By examining R {approx} 5000 spectra taken with the WIYN+Hydra spectrograph, we identify 3 PN candidates in a sample of 274 likely globular clusters, 2 candidates in objects which may be globular clusters, and 5 candidates in a set of 85 younger systems. The possible PNe are all faint, between {approx}2.5 and {approx}6.8 mag down the PN luminosity function, and, partly as a consequence of our selection criteria, have high excitation, with [O III] {lambda}5007 to H{beta} ratios ranging from 2 to {approx}> 12. We discuss the individual candidates, their likelihood of cluster membership, and the possibility that they were formed via binary interactions within the clusters. Our data are consistent with the suggestion that PN formation within globular clusters correlates with binary encounter frequency, though, due to the small numbers and large uncertainties in the candidate list, this study does not provide sufficient evidence to confirm the hypothesis.

  2. The ACS Survey of Galactic Globular Clusters. VIII. Effects of Environment on Globular Cluster Global Mass Functions

    NASA Astrophysics Data System (ADS)

    Paust, Nathaniel E. Q.; Reid, I. Neill; Piotto, Giampaolo; Aparicio, Antonio; Anderson, Jay; Sarajedini, Ata; Bedin, Luigi R.; Chaboyer, Brian; Dotter, Aaron; Hempel, Maren; Majewski, Steven; Marín-Franch, A.; Milone, Antonino; Rosenberg, Alfred; Siegel, Michael

    2010-02-01

    We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic Globular Clusters to construct global present-day mass functions for 17 globular clusters utilizing multi-mass King models to extrapolate from our observations to the global cluster behavior. The global present-day mass functions for these clusters are well matched by power laws from the turnoff, ≈0.8 M sun, to 0.2-0.3 M sun on the lower main sequence. The slopes of those power-law fits, α, have been correlated with an extensive set of intrinsic and extrinsic cluster properties to investigate which parameters may influence the form of the present-day mass function. We do not confirm previous suggestions of correlations between α and either metallicity or Galactic location. However, we do find a strong statistical correlation with the related parameters central surface brightness, μ V , and inferred central density, ρ0. The correlation is such that clusters with denser cores (stronger binding energy) tend to have steeper mass functions (a higher proportion of low-mass stars), suggesting that dynamical evolution due to external interactions may have played a key role in determining α. Thus, the present-day mass function may owe more to nurture than to nature. Detailed modeling of external dynamical effects is therefore a requisite for determining the initial mass function for Galactic globular clusters.

  3. THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. VIII. EFFECTS OF ENVIRONMENT ON GLOBULAR CLUSTER GLOBAL MASS FUNCTIONS

    SciTech Connect

    Paust, Nathaniel E. Q.; Reid, I. Neill; Anderson, Jay E-mail: inr@stsci.edu

    2010-02-15

    We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic Globular Clusters to construct global present-day mass functions for 17 globular clusters utilizing multi-mass King models to extrapolate from our observations to the global cluster behavior. The global present-day mass functions for these clusters are well matched by power laws from the turnoff, {approx}0.8 M {sub sun}, to 0.2-0.3 M {sub sun} on the lower main sequence. The slopes of those power-law fits, {alpha}, have been correlated with an extensive set of intrinsic and extrinsic cluster properties to investigate which parameters may influence the form of the present-day mass function. We do not confirm previous suggestions of correlations between {alpha} and either metallicity or Galactic location. However, we do find a strong statistical correlation with the related parameters central surface brightness, {mu} {sub V}, and inferred central density, {rho}{sub 0}. The correlation is such that clusters with denser cores (stronger binding energy) tend to have steeper mass functions (a higher proportion of low-mass stars), suggesting that dynamical evolution due to external interactions may have played a key role in determining {alpha}. Thus, the present-day mass function may owe more to nurture than to nature. Detailed modeling of external dynamical effects is therefore a requisite for determining the initial mass function for Galactic globular clusters.

  4. Globular Cluster Systems in Brightest Cluster Galaxies. II. NGC 6166

    NASA Astrophysics Data System (ADS)

    Harris, William E.; Blakeslee, John P.; Whitmore, Bradley C.; Gnedin, Oleg Y.; Geisler, Douglas; Rothberg, Barry

    2016-01-01

    We present new deep photometry of the globular cluster system (GCS) around NGC 6166, the central supergiant galaxy in Abell 2199. Hubble Space Telescope data from the Advanced Camera for Surveys and WFC3 cameras in F475W and F814W are used to determine the spatial distribution of the GCS, its metallicity distribution function (MDF), and the dependence of the MDF on galactocentric radius and on GC luminosity. The MDF is extremely broad, with the classic red and blue subpopulations heavily overlapped, but a double-Gaussian model can still formally match the MDF closely. The spatial distribution follows a Sérsic-like profile detectably to a projected radius of at least Rgc = 250 kpc. To that radius, the total number of clusters in the system is NGC = 39000 ± 2000, the global specific frequency is SN = 11.2 ± 0.6, and 57% of the total are blue, metal-poor clusters. The GCS may fade smoothly into the intracluster medium (ICM) of A2199; we see no clear transition from the core of the galaxy to the cD halo or the ICM. The radial distribution, projected ellipticity, and mean metallicity of the red (metal-richer) clusters match the halo light extremely well for {R}{gc}≳ 15 {{kpc}}, both of them varying as {σ }{MRGC}∼ {σ }{light}∼ {R}-1.8. By comparison, the blue (metal-poor) GC component has a much shallower falloff {σ }{MPGC}∼ {R}-1.0 and a more nearly spherical distribution. This strong difference in their density distributions produces a net metallicity gradient in the GCS as a whole that is primarily generated by the population gradient. With NGC 6166 we appear to be penetrating into a regime of high enough galaxy mass and rich enough environment that the bimodal two-phase description of GC formation is no longer as clear or effective as it has been in smaller galaxies.

  5. Variable stars in large Magellanic cloud globular clusters. III. Reticulum

    SciTech Connect

    Kuehn, Charles A.; Dame, Kyra; Smith, Horace A.; De Lee, Nathan E-mail: damekyra@msu.edu E-mail: nathan.delee@vanderbilt.edu; and others

    2013-06-01

    This is the third in a series of papers studying the variable stars in old globular clusters in the Large Magellanic Cloud. The primary goal of this series is to look at how the characteristics and behavior of RR Lyrae stars in Oosterhoff-intermediate systems compare to those of their counterparts in Oosterhoff-I/II systems. In this paper we present the results of our new time-series BVI photometric study of the globular cluster Reticulum. We found a total of 32 variables stars (22 RRab, 4 RRc, and 6 RRd stars) in our field of view. We present photometric parameters and light curves for these stars. We also present physical properties, derived from Fourier analysis of light curves, for some of the RR Lyrae stars. We discuss the Oosterhoff classification of Reticulum and use our results to re-derive the distance modulus and age of the cluster.

  6. Ellipticals with Kinematically Distinct Cores: WFPC2 Imaging of Globular Clusters

    NASA Astrophysics Data System (ADS)

    Forbes, Duncan A.; Franx, Marijn; Illingworth, Garth D.; Carollo, C. M.

    1996-08-01

    New globular clusters may form in the merger of two galaxies. Perhaps the best examples of merger remnants are the set of ellipticals with kinematically distinct cores. Here we present Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC2) imaging of 14 kinematically distinct core ellipticals to examine their globular cluster systems. In particular, we probe the galaxy central regions, for which we might expect to see the strongest signatures of some formation and destruction processes. These data increase substantially the number of extragalactic globular cluster systems studied to date. We have developed a method for galaxy subtraction and selection of globular clusters which results in about 200 globulars per galaxy to a limiting magnitude of V ~ 25. Simulations of artificial globulars are described also. We find that the globular cluster luminosity, and color, vary only weakly, if at all, with galactocentric distance. The mean colors of globular clusters are constant with globular cluster magnitude. Several clear trends are also present. First, globular cluster colors are bluer (more metal poor by ~0.5 dex) than the underlying galaxy starlight at any given galactocentric distance. Second, we find a good correlation over roughly 10 magnitudes between the mean globular cluster metallicity and parent galaxy luminosity of the form Z is proportional to L^0.4^. This relationship includes dwarf ellipticals, spiral galaxy bulges, and giant ellipticals. Third, we find that globular cluster surface density distribution can be described by a core model, for which the core radius correlates with galaxy luminosity. Last, for the sample as a whole, the globular cluster systems are closely aligned with the galaxy major axis and are slightly rounder than the galaxy itself, although their are some notable exceptions. Our results favor scenarios in which ellipticals form from massive, gas rich progenitors at early epochs. Detailed simulations of the formation of

  7. Globular clusters, Hipparcos, and the age of the galaxy

    PubMed Central

    Reid, Neill

    1998-01-01

    We discuss the impact of the results from the recent Hipparcos astrometric satellite on distance estimates of galactic globular clusters. Recalibrating the clusters not only implies a relatively small change in the distance to the Large Magellanic Cloud, and hence a rescaling of several estimates of the Hubble constant, but also leads to significantly younger cluster ages. Although the data are not yet conclusive, the results so far point to a likely resolution of the apparent paradox of a universe younger than its constituents, without requiring significant modifications to simple cosmological models. PMID:9419316

  8. Young globular clusters in the Milky Way Galaxy

    NASA Technical Reports Server (NTRS)

    Lin, D. N. C.; Richer, Harvey B.

    1992-01-01

    Ruprecht 106 and Pal 12 are two known globular clusters in the Milky Way that are unequivocally younger than other clusters of similar metallicity. The Galactic coordinates of Ruprecht 106 place it near to the Magellanic Stream in projection, suggesting a tidal capture from the Magellanic Clouds. It is demonstrated that a family of orbits for both clusters can be constructed that are consistent with this capture hypothesis and that these then lead to a prediction of 3 milliarcseconds per year for the proper motions of both Ruprecht 106 and Pal 12.

  9. The globular cluster system of NGC 1316. III. Kinematic complexity

    NASA Astrophysics Data System (ADS)

    Richtler, T.; Hilker, M.; Kumar, B.; Bassino, L. P.; Gómez, M.; Dirsch, B.

    2014-09-01

    Context. The merger remnant NGC 1316 (Fornax A) is one of the most important objects regarding the investigation of and thus an important object to study merger-related processes. A recent photometric study used globular clusters in NGC 1316 to constrain its star formation history, but without the knowledge of individual radial velocities. The kinematical properties of the globular cluster system in comparison with the diffuse stellar light might give more insight into the formation of NGC 1316. Of particular interest is the dark matter content. Planetary nebulae in NGC 1316 indicate a massive dark halo, and globular cluster velocities provide independent evidence. Aims: We aim at measuring radial velocities of globular clusters in NGC 1316. We use these kinematical data to investigate the global structure of NGC 1316 and to constrain the dark matter content. Methods: We perform multiobject spectroscopy with VLT/FORS2 and MXU. Out of 562 slits, we extract radial velocities for 177 globular clusters. Moreover, we measure radial velocities of the integrated galaxy light, using slits with a sufficiently bright sky. To these data, we add 20 cluster velocities from the literature. In an appendix, we identify new morphological features of NGC 1316 and its companion galaxy NGC 1317. Results: The GC sample based on radial velocities confirms the colour peaks already found in our photometric study. The bright clusters, which probably have their origin in a 2 Gyr old starburst and younger star formation events, avoid the systemic velocity. A Gaussian velocity distribution is found only for clusters fainter than about mR = 22 mag. The velocity distribution of clusters shows a pronounced peak at 1600 km s-1. These clusters populate a wide area in the south-western region which we suspect to be a disk population. Globular clusters or subsamples of them do not show a clear rotation signal. This is different from the galaxy light, where rotation along the major axis is

  10. Revised GALEX Ultraviolet Catalog of Globular Clusters in M31

    NASA Astrophysics Data System (ADS)

    Rey, Soo-Chang; Lee, K.; Kang, Y.; Sohn, T.; GALEX Science Team

    2010-01-01

    We present near-ultraviolet (NUV) and far-ultraviolet (FUV) photometry of the globular clusters (GCs) and globular cluster candidates in M31 from 23 mosaic observations of Galaxy Evolution Explorer (GALEX). We construct revised UV-optical merged catalog of GCs and GC candidates in M31 by cross-matching between UV photometry and optical and near-IR photometry from Revised Bolobna Catalog and Caldwell et al. (2009). The UV catalog of M31 GCs includes 626 GCs and 529 GC candidates. We explored the general UV properties and age distribution of GCs comparing with the stellar population models. We suggest UV color-color diagram has advantage of separating GCs very effectively from background galaxies and foreground stars than the optical one.

  11. GLOBULAR CLUSTERS AND DARK SATELLITE GALAXIES THROUGH THE STREAM VELOCITY

    SciTech Connect

    Naoz, Smadar; Narayan, Ramesh

    2014-08-10

    The formation of purely baryonic globular clusters with no gravitationally bound dark matter is still a theoretical challenge. We show that these objects might form naturally whenever there is a relative stream velocity between baryons and dark matter. The stream velocity causes a phase shift between linear modes of baryonic and dark matter perturbations, which translates to a spatial offset between the two components when they collapse. For a 2σ (3σ) density fluctuation, baryonic clumps with masses in the range 10{sup 5}-2.5 × 10{sup 6} M {sub ☉} (10{sup 5}-4 × 10{sup 6} M {sub ☉}) collapse outside the virial radii of their counterpart dark matter halos. These objects could survive as long-lived, dark-matter-free objects and might conceivably become globular clusters. In addition, their dark matter counterparts, which were deprived of gas, might become dark satellite galaxies.

  12. Helium abundance difference within globular clusters: NGC 2808.

    NASA Astrophysics Data System (ADS)

    Cacciari, C.; Pasquini, L.; Valenti, E.; Käufl, H. U.; Mauas, P.

    Multiple populations have been recently detected in most Galactic globular clusters, even with no significant spread in metallicity. Unusual features of the observed colour-magnitude diagrams can be explained by differences in the He content of the stars belonging to the sub-populations. We report on empirical evidence of He abundance spread in a few globular clusters, with particular attention to NGC 2808, where He abundance variation has been measured in a pair of otherwise identical red giant stars using the HeI 1083 nm line. A quantitative estimate of this difference has been derived by appropriate chromospheric modelling, in very good agreement with stellar evolution requirements. Partly based on observations collected at ESO VLT (Chile), under programme 384.D-0283.

  13. 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.

  14. An Archival Search For Young Globular Clusters in Galaxies

    NASA Astrophysics Data System (ADS)

    Whitmore, Brad

    1995-07-01

    One of the most intriguing results from HST has been the discovery of ultraluminous star clusters in interacting and merging galaxies. These clusters have the luminosities, colors, and sizes that would be expected of young globular clusters produced by the interaction. We propose to use the data in the HST Archive to determine how prevalent this phenomena is, and to determine whether similar clusters are produced in other environments. Three samples will be extracted and studied in a systematic and consistent manner: 1} interacting and merging galaxies, 2} starburst galaxies, 3} a control sample of ``normal'' galaxies. A preliminary search of the archives shows that there are at least 20 galaxies in each of these samples, and the number will grow by about 50 observations become available. The data will be used to determine the luminosity function, color histogram , spatial distribution, and structural properties of the clusters using the same techniques employed in our study of NGC 7252 {``Atoms -for-Peace'' galaxy} and NGC 4038/4039 {``The Antennae''}. Our ultimate goals are: 1} to understand how globular clusters form, and 2} to use the clusters as evolutionary tracers to unravel the histories of interacting galaxies.

  15. The Globular Cluster System of NGC 4636 and Formation of Globular Clusters in Giant Elliptical Galaxies

    NASA Astrophysics Data System (ADS)

    Park, Hong Soo; Lee, Myung Gyoon; Hwang, Ho Seong; Kim, Sang Chul; Arimoto, Nobuo; Yamada, Yoshihiko; Tamura, Naoyuki; Onodera, Masato

    2012-11-01

    We present a spectroscopic analysis of the metallicities, ages, and alpha-elements of the globular clusters (GCs) in the giant elliptical galaxy (gE) NGC 4636 in the Virgo Cluster. Line indices of the GCs are measured from the integrated spectra obtained with Faint Object Camera and Spectrograph on the Subaru 8.2 m Telescope. We derive [Fe/H] values of 59 GCs based on the Brodie & Huchra method, and [Z/H], age, and [α/Fe] values of 33 GCs from the comparison of the Lick line indices with single stellar population models. The metallicity distribution of NGC 4636 GCs shows a hint of a bimodality with two peaks at [Fe/H] = -1.23(σ = 0.32) and -0.35(σ = 0.19). The age spread is large from 2 Gyr to 15 Gyr and the fraction of young GCs with age <5 Gyr is about 27%. The [α/Fe] of the GCs shows a broad distribution with a mean value [α/Fe] ≈0.14 dex. The dependence of these chemical properties on the galactocentric radius is weak. We also derive the metallicities, ages, and [α/Fe] values for the GCs in other nearby gEs (M87, M49, M60, NGC 5128, NGC 1399, and NGC 1407) from the line index data in the literature using the same methods as used for NGC 4636 GCs. The metallicity distribution of GCs in the combined sample of seven gEs including NGC 4636 is found to be bimodal, supported by the KMM test with a significance level of >99.9%. All these gEs harbor some young GCs with ages less than 5 Gyr. The mean age of the metal-rich GCs ([Fe/H] >-0.9) is about 3 Gyr younger than that of the metal-poor GCs. The mean value of [α/Fe] of the gE GCs is smaller than that of the Milky Way GCs. We discuss these results in the context of GC formation in gEs. Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

  16. THE GLOBULAR CLUSTER SYSTEM OF NGC 4636 AND FORMATION OF GLOBULAR CLUSTERS IN GIANT ELLIPTICAL GALAXIES

    SciTech Connect

    Park, Hong Soo; Lee, Myung Gyoon; Hwang, Ho Seong; Kim, Sang Chul; Arimoto, Nobuo; Yamada, Yoshihiko; Tamura, Naoyuki; Onodera, Masato E-mail: mglee@astro.snu.ac.kr E-mail: sckim@kasi.re.kr E-mail: yoshihiko.yamada@nao.ac.jp E-mail: monodera@phys.ethz.ch

    2012-11-10

    We present a spectroscopic analysis of the metallicities, ages, and alpha-elements of the globular clusters (GCs) in the giant elliptical galaxy (gE) NGC 4636 in the Virgo Cluster. Line indices of the GCs are measured from the integrated spectra obtained with Faint Object Camera and Spectrograph on the Subaru 8.2 m Telescope. We derive [Fe/H] values of 59 GCs based on the Brodie and Huchra method, and [Z/H], age, and [{alpha}/Fe] values of 33 GCs from the comparison of the Lick line indices with single stellar population models. The metallicity distribution of NGC 4636 GCs shows a hint of a bimodality with two peaks at [Fe/H] = -1.23({sigma} = 0.32) and -0.35({sigma} = 0.19). The age spread is large from 2 Gyr to 15 Gyr and the fraction of young GCs with age <5 Gyr is about 27%. The [{alpha}/Fe] of the GCs shows a broad distribution with a mean value [{alpha}/Fe] Almost-Equal-To 0.14 dex. The dependence of these chemical properties on the galactocentric radius is weak. We also derive the metallicities, ages, and [{alpha}/Fe] values for the GCs in other nearby gEs (M87, M49, M60, NGC 5128, NGC 1399, and NGC 1407) from the line index data in the literature using the same methods as used for NGC 4636 GCs. The metallicity distribution of GCs in the combined sample of seven gEs including NGC 4636 is found to be bimodal, supported by the KMM test with a significance level of >99.9%. All these gEs harbor some young GCs with ages less than 5 Gyr. The mean age of the metal-rich GCs ([Fe/H] >-0.9) is about 3 Gyr younger than that of the metal-poor GCs. The mean value of [{alpha}/Fe] of the gE GCs is smaller than that of the Milky Way GCs. We discuss these results in the context of GC formation in gEs.

  17. Intermediate-Age Globular Clusters in M31

    NASA Astrophysics Data System (ADS)

    Puzia, Thomas

    2005-07-01

    We propose deep ACS/WFC imaging of four halo M31 globular clusters in order to derive their horizontal branch morphologies. Our spectroscopic investigation of their integrated light identifies them as members of an intermediate-age population of globular clusters in M31. Since our spectroscopic results are based on the analysis of Balmer absorption lines, we need to secure our results against an artificial juvenation due to extreme horizontal branch morphologies. The proposed observations will allow a clear-cut answer to the question of whether spectroscopically derived intermediate-age estimates are due to genuinely younger ages or are the result of anomalously hot horizontal branch morphologies. Either way, our results will have important implications for spectroscopically derived ages and metallicities of distant stellar populations. Because of the high spatial resolution of the proposed ACS/WFC observations we will also derive accurate surface brightness profiles of our target globular clusters and investigate the influence of stellar density on horizontal branch morphology. Moreover, together with deep parallel WFPC2 fields we will study the metallicity dispersion of the background stellar population in M31 as a function of galactocentric radius.

  18. The first Δa observations of three globular clusters

    NASA Astrophysics Data System (ADS)

    Paunzen, E.; Iliev, I. Kh.; Pintado, O. I.; Baum, H.; Maitzen, H. M.; Netopil, M.; Önehag, A.; Zejda, M.; Fraga, L.

    2014-09-01

    Globular clusters are main astrophysical laboratories to test and modify evolutionary models. Thought to be rather homogeneous in their local elemental distribution of members, results suggest a wide variety of chemical peculiarities. Besides different main sequences, believed to be caused by different helium abundances, peculiarities of blue horizontal-branch stars and on the red giant branch were found. This whole zoo of peculiar objects has to be explained in the context of stellar formation and evolution. The tool of Δa photometry is employed in order to detect peculiar stars in the whole spectral range. This three filter narrow-band system measures the flux distribution in the region from 4900 to 5600 Å in order to find any peculiarities around 5200 Å. It is highly efficient to detect classical chemically peculiar stars of the upper main sequence, Be/Ae, shell and metal-weak objects in the Milky Way and Magellanic Clouds. We present Δa photometry of 2266 stars from 109 individual frames for three globular clusters (NGC 104, NGC 6205, and NGC 7099). A comparison with published abundances, for three horizontal-branch stars, only, yields an excellent agreement. According to the 3σ detection limit of each globular cluster, about 3 per cent of the stars lie in abnormal regions in the diagnostic diagrams. The first observations of three widely different aggregates give very promising results, which will serve as a solid basis for follow-up observations including photometric as well as spectroscopic studies.

  19. A spectroscopic study of the globular Cluster NGC 4147

    NASA Astrophysics Data System (ADS)

    Villanova, S.; Monaco, L.; Moni Bidin, C.; Assmann, P.

    2016-08-01

    We present the abundance analysis for a sample of 18 red giant branch stars in the metal-poor globular cluster NGC 4147 based on medium- and high-resolution spectra. This is the first extensive spectroscopic study of this cluster. We derive abundances of C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y, Ba, and Eu. We find a metallicity of [Fe/H] = -1.84 ± 0.02 and an α-enhancement of +0.38 ± 0.05 (errors on the mean), typical of halo globular clusters in this metallicity regime. A significant spread is observed in the abundances of light elements C, N, O, Na, and Al. In particular, we found an Na-O anticorrelation and Na-Al correlation. The cluster contains only ˜15 per cent of stars that belong to the first generation (Na-poor and O-rich). This implies that it suffered a severe mass-loss during its lifetime. Its [Ca/Fe] and [Ti/Fe] mean values agree better with the Galactic halo trend than with the trend of extragalactic environments at the cluster metallicity. This possibly suggests that NGC 4147 is a genuine Galactic object at odd with what claimed by some author that proposed the cluster to be member of the Sagittarius dwarf galaxy. An antirelation between the light s-process element Y and Na may also be present.

  20. Luminosity Function of Faint Globular Clusters in M87

    SciTech Connect

    Waters, Christopher Z.; Zepf, Stephen E.; Lauer, Tod R.; Baltz, Edward A.; Silk, Joseph; /Oxford U.

    2006-07-14

    We present the luminosity function to very faint magnitudes for the globular clusters in M87, based on a 30 orbit Hubble Space Telescope (HST) WFPC2 imaging program. The very deep images and corresponding improved false source rejection allow us to probe the mass function further beyond the turnover than has been done before. We compare our luminosity function to those that have been observed in the past, and confirm the similarity of the turnover luminosity between M87 and the Milky Way. We also find with high statistical significance that the M87 luminosity function is broader than that of the Milky Way. We discuss how determining the mass function of the cluster system to low masses can constrain theoretical models of the dynamical evolution of globular cluster systems. Our mass function is consistent with the dependence of mass loss on the initial cluster mass given by classical evaporation, and somewhat inconsistent with newer proposals that have a shallower mass dependence. In addition, the rate of mass loss is consistent with standard evaporation models, and not with the much higher rates proposed by some recent studies of very young cluster systems. We also find that the mass-size relation has very little slope, indicating that there is almost no increase in the size of a cluster with increasing mass.

  1. Formation of multiple stellar populations in globular clusters

    NASA Astrophysics Data System (ADS)

    Jiang, Dengkai

    2014-09-01

    Observations reveal the presence of multiple stellar populations (MSPs) in globular clusters (GCs) that exhibit wide abundance variations and multiple sequences in their Hertzsprung-Russell diagrams. We present a scenario for the formation of MSPs in GCs. In this scenario, initial GCs are single-generation clusters, and our model predicts that the anomalous-abundance stars observed in GCs are the merged and accreted stars produced by binary interactions, which are rapidly rotating stars at the moment of their formation. A stellar population with binaries can reproduce two important observational pieces of evidence of MSPs, the Na-O anticorrelation and the multiple sequences in the HR diagram.

  2. Effects of cosmic string velocities and the origin of globular clusters

    NASA Astrophysics Data System (ADS)

    Lin, Ling; Yamanouchi, Shoma; Brandenberger, Robert

    2015-12-01

    With the hypothesis that cosmic string loops act as seeds for globular clusters in mind, we study the role that velocities of these strings will play in determining the mass distribution of globular clusters. Loops with high enough velocities will not form compact and roughly spherical objects and can hence not be the seeds for globular clusters. We compute the expected number density and mass function of globular clusters as a function of both the string tension and the peak loop velocity, and compare the results with the observational data on the mass distribution of globular clusters in our Milky Way. We determine the critical peak string loop velocity above which the agreement between the string loop model for the origin of globular clusters (neglecting loop velocities) and observational data is lost.

  3. The cataclysmic variable population in the globular cluster 47 Tucanae

    NASA Astrophysics Data System (ADS)

    Rivera Sandoval, Liliana E.; Grindlay, Jonathan; Van Den Berg, Maureen; Anderson, Jay; Heinke, Craig; Ivanova, Natalia; Cohn, Haldan N.; Lugger, Phyllis M.; Edmonds, Peter D.; Cool, Adrienne M.

    2016-07-01

    The cores of globular clusters are perfect places to study dynamical interactions between stars because of the high stellar densities (up to 10^6 stars/pc^3). In this work we use near-ultraviolet images of the globular cluster 47 Tucanae obtained with the Hubble Space Telescope (HST). In combination with deep optical HST and Chandra X-ray data, we study the cataclysmic variable (CVs) population of the cluster. With magnitudes down to Mv=11.6 mag, we are able to identify CVs at very low mass-accretion rates. This allows us to get the deepest measurements of the cluster CV luminosity function ever. Helping us to understand how the stellar interactions affect the creation and destruction of these binaries. Here I will show our results and I will compare them to the ones of other clusters. I will also discuss our results with respect to the models of formation and evolution of CVs, specifically to the predicted number of these binaries and their radial distribution.

  4. THE BLUE HOOK POPULATIONS OF MASSIVE GLOBULAR CLUSTERS

    SciTech Connect

    Brown, Thomas M.; Smith, Ed; Sweigart, Allen V.; Lanz, Thierry; Landsman, Wayne B.; Hubeny, Ivan E-mail: edsmith@stsci.ed E-mail: lanz@astro.umd.ed E-mail: hubeny@aegis.as.arizona.ed

    2010-08-01

    We present new Hubble Space Telescope ultraviolet color-magnitude diagrams of five massive Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These observations were obtained to investigate the 'blue hook' (BH) phenomenon previously observed in UV images of the globular clusters {omega} Cen and NGC 2808. Blue hook stars are a class of hot (approximately 35,000 K) subluminous horizontal branch stars that occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. By coupling new stellar evolution models to appropriate non-LTE synthetic spectra, we investigate various theoretical explanations for these stars. Specifically, we compare our photometry to canonical models at standard cluster abundances, canonical models with enhanced helium (consistent with cluster self-enrichment at early times), and flash-mixed models formed via a late helium-core flash on the white dwarf cooling curve. We find that flash-mixed models are required to explain the faint luminosity of the BH stars, although neither the canonical models nor the flash-mixed models can explain the range of color observed in such stars, especially those in the most metal-rich clusters. Aside from the variation in the color range, no clear trends emerge in the morphology of the BH population with respect to metallicity.

  5. An AO-assisted Variability Study of Four Globular Clusters

    NASA Astrophysics Data System (ADS)

    Salinas, R.; Contreras Ramos, R.; Strader, J.; Hakala, P.; Catelan, M.; Peacock, M. B.; Simunovic, M.

    2016-09-01

    The image-subtraction technique applied to study variable stars in globular clusters represented a leap in the number of new detections, with the drawback that many of these new light curves could not be transformed to magnitudes due to severe crowding. In this paper, we present observations of four Galactic globular clusters, M 2 (NGC 7089), M 10 (NGC 6254), M 80 (NGC 6093), and NGC 1261, taken with the ground-layer adaptive optics module at the SOAR Telescope, SAM. We show that the higher image quality provided by SAM allows for the calibration of the light curves of the great majority of the variables near the cores of these clusters as well as the detection of new variables, even in clusters where image-subtraction searches were already conducted. We report the discovery of 15 new variables in M 2 (12 RR Lyrae stars and 3 SX Phe stars), 12 new variables in M 10 (11 SX Phe and 1 long-period variable), and 1 new W UMa-type variable in NGC 1261. No new detections are found in M 80, but previous uncertain detections are confirmed and the corresponding light curves are calibrated into magnitudes. Additionally, based on the number of detected variables and new Hubble Space Telescope/UVIS photometry, we revisit a previous suggestion that M 80 may be the globular cluster with the richest population of blue stragglers in our Galaxy. Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).

  6. Pal 12 - A metal-rich globular cluster in the outer halo

    NASA Technical Reports Server (NTRS)

    Cohen, J. G.; Frogel, J. A.; Persson, S. E.; Zinn, R.

    1980-01-01

    New optical and infrared observations of several stars in the distant globular cluster Pal 12 show that they have CO strengths and heavy element abundances only slightly less than in M 71, one of the more metal-rich globular clusters. Pal 12 thus has a metal abundance near the high end of the range over which globular clusters exist and lies in the outer galactic halo. Its red horizontal branch is not anomalous in view of the abundance that has been found.

  7. The Compositin of the Bulge Globular Cluster NGC 6273

    NASA Astrophysics Data System (ADS)

    Pilachowski, Catherine A.; Johnson, Christian

    2015-08-01

    Observations of red giants in the Bulge globular cluster NGC 6273 with the Michigan/Magellan Fiber System (M2FS) mounted on the Nasmyth-East port of the Magellan-Clay 6.5m telescope at Las Campanas Observatory reveal a spread in metallicity. Members have been confirmed with radial velocity. NGC 6273 has at least two populations separated by 0.2-0.3 dex in [Fe/H]. The sodium and aluminum abundances are correlated while the magnesium and aluminum abundances are anti-correlated. The cluster also shows a rise in the abundance of the s-process element lanthanum with [Fe/H] similar to other massive clusters. The cluster contains a possible 3rd population depleted in most elements by 0.3 dex.

  8. The age of the LMC globular cluster NGC 1783

    NASA Technical Reports Server (NTRS)

    Mould, Jeremy; Kristian, Jerome; Nemec, James; Jensen, Joseph; Aaronson, Marc

    1989-01-01

    The age of the LMC red globular cluster NGC 1783 is estimated as 0.9 + or - 0.4 billion yr by photometry of the main-sequence turnoff. The accuracy of the estimate is limited chiefly by the uncertainty in the distance modulus of the cluster. At (m - M)0 = 18.2 the cluster is aged 1.1 + or - 0.2 Gyr; at (m - M)0 = 18.7 it is 0.7 + or - 0.2 Gyr. NGC 1783 is a sufficiently rich cluster that one can see the full development of red giants on the asymptotic giant branch from the M type, through S, to carbon-rich atmospheres.

  9. 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

  10. STRUCTURAL PARAMETERS FOR GLOBULAR CLUSTERS IN M31

    SciTech Connect

    Wang Song; Ma Jun

    2013-08-01

    In this paper, we present surface brightness profiles for 79 globular clusters in M31, using images observed with the Hubble Space Telescope, some of which are from new observations. The structural and dynamical parameters are derived from fitting the profiles to several different models for the first time. The results show that in the majority of cases, King models fit the M31 clusters just as well as Wilson models and better than Sersic models. However, there are 11 clusters best fitted by Sersic models with the Sersic index n > 2, meaning that they have cuspy central density profiles. These clusters may be the well-known core-collapsed candidates. There is a bimodality in the size distribution of M31 clusters at large radii, which is different from their Galactic counterparts. In general, the properties of clusters in M31 and the Milky Way fall in the same regions of parameter spaces. The tight correlations of cluster properties indicate a ''fundamental plane'' for clusters, which reflects some universal physical conditions and processes operating at the epoch of cluster formation.

  11. Spectroscopic study of globular clusters in the halo of M31 with the Xinglong 2.16 m telescope

    NASA Astrophysics Data System (ADS)

    Fan, Zhou; Huang, Ya-Fang; Li, Jin-Zeng; Zhou, Xu; Ma, Jun; Wu, Hong; Zhang, Tian-Meng; Zhao, Yong-Heng

    2011-11-01

    We present spectroscopic observations for 11 confirmed globular clusters (GCs) of M31 with the OMR spectrograph on the 2.16 m telescope at the Xinglong site of National Astronomical Observatories, Chinese Academy of Sciences. Nine of our sample clusters are located in the halo of M31 and the most remote one is out to a projected radius of 78.75 kpc from the galactic center. For all our sample clusters, we measured the Lick absorption-line indices and radial velocities. It is noted that most GCs in our sample are distinct from the HI rotation curve of galaxy M31, especially for B514, MCGC5, H12 and B517, suggesting that most of our sample clusters do not have a kinematic association with the star-forming young disk of the galaxy. We separately fitted the absorption line indices from the updated stellar population model of Thomas et al. with two different tracks of Cassisi and Padova, by applying the χ2—minimization method. The fitting results show that all our sample clusters are older than 10Gyr, and metal-poor (-2.38 <= [Fe/H] <= -0.91dex). After merging the spectroscopic metallicity of our work with the previously published ones, we extended the cluster sample out to a projected radius of 117 kpc from the galaxy's center. We found the metallicity gradient exists for all the confirmed clusters with a slope of -0.028 ± 0.001 dex kpc-1. However, the slope turns out to be -0.018 ± 0.001 dex kpc-1 for all the halo clusters, which is much shallower. If we only consider the outer halo clusters with rp > 25 kpc, the slope becomes -0.010 ± 0.002 dex kpc-1 and if one cluster G001 is excluded from the outer halo sample, the slope is -0.004 ± 0.002 dex kpc-1. Thus, we conclude that the metallicity gradient for M31's outer halo clusters is not significant, which agrees well with previous findings.

  12. Structure and Dynamics of the Globular Cluster Palomar 13

    NASA Astrophysics Data System (ADS)

    Bradford, J. D.; Geha, M.; Muñoz, R. R.; Santana, F. A.; Simon, J. D.; Côté, P.; Stetson, P. B.; Kirby, E.; Djorgovski, S. G.

    2011-12-01

    We present Keck/DEIMOS spectroscopy and Canada-France-Hawaii Telescope/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/High Resolution Echelle Spectrometer 21 star velocity dispersion of σ = 2.2 ± 0.4 km s-1. We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is σ = 0.7+0.6 -0.5 km s-1. Combining our DEIMOS data with literature values, our final velocity dispersion is σ = 0.4+0.4 -0.3 km s-1. We determine a spectroscopic metallicity of [Fe/H] = -1.6 ± 0.1 dex, placing a 1σ upper limit of σ[Fe/H] ~ 0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be MV = -2.8 ± 0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (Σvpropr η, η = -2.8 ± 0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of M 1/2 = 1.3+2: 7 -1.3 × 103 M ⊙ and a mass-to-light ratio of M/LV = 2.4+5.0 -2.4 M ⊙/L ⊙. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a

  13. Exploring the Formation of Galaxies through Metallicities of Globular Clusters

    NASA Astrophysics Data System (ADS)

    Kim, Sooyoung; Yoon, Suk-Jin; Chung, Chul; Caldwell, Nelson; Schiavon, Ricardo P.; Kang, Yong Beom; Rey, Soo-Chang; Lee, Young-Wook; Tamura, Naoyuki; Sohn, S. Tony; Arimoto, Nobuo; Kodama, Tadayuki; Yamada, Yoshihiko

    2014-06-01

    Globular clusters (GCs) are among the oldest stellar objects in the universe. They have long served the role of providing constraints on many aspects of galaxy evolution theory. Bimodal color distribution of GC systems in many luminous early-type galaxies is an observationally established phenomenon and has been interpreted as evidence of two GC subgroups with different metallicities. In this study, we use spectroscopic data on the GC systems of two giant galaxies, M31 (the Andromeda) and M87 (NGC 4486), to investigate the GC bimodality and the underlying metallicity distributions. Recent high signal-to-ratio spectroscopic data on M31 GCs revealed a clear bimodality in absorption-line index distributions of old GCs. Given that spectroscopy provides a more robust probe into stellar population than photometry, the reported spectral line index bimodality may indicate the presence of two distinct GC populations. However, here we show that the spectroscopic dichotomy of M31 GCs is due to the nonlinear nature of metallicity-to-index conversion and therefore one does not need two separate GC subsystems. We consider this as an analogy to the recent interpretation in which metallicity-color nonlinearity is the prime cause for observed GC color bimodality. We present spectra of ~130 old globular clusters (GCs) associated with the Virgo giant elliptical galaxy M87, obtained with the Multi-Object Spectrography (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the Subaru telescope. The fundamental properties of globular clusters such as age, metallicity and elemental abundance ratio are investigated by comparing with a set of Simple Stellar Population (SSP) models. M87 GCs with reliable metallicity measurements exhibit significant inflection along the color-metallicity relations, through which observed color bimodality is reproduced using a broad, unimodal metallicity distribution. Our findings lend further support to this new interpretation of the GC color

  14. Intermediate-age globular clusters in four galaxy merger remnants

    SciTech Connect

    Trancho, Gelys; Miller, Bryan W.; Schweizer, François; Burdett, Daniel P.; Palamara, David

    2014-08-01

    We present the results of combining Hubble Space Telescope optical photometry with ground-based K{sub s} -band photometry from the Gemini imagers NIRI and FLAMINGOS-I to study the globular cluster (GC) populations in four early-type galaxies that are candidate remnants of recent mergers (NGC 1700, NGC 2865, NGC 4382, and NGC 7727). These galaxies were chosen based on their blue colors and fine structure, such as shells and ripples that are indicative of past interactions. We fit the combined VIK{sub s} GC data with simple toy models of mixed cluster populations that contain three subpopulations of different age and metallicity. The fits, done via chi-squared mapping of the parameter space, yield clear evidence for the presence of intermediate-age clusters in each galaxy. We find that the ages of ∼1-2 Gyr for these GC subpopulations are consistent with the previously estimated merger ages for the host galaxies.

  15. Search for Variable Stars in Six Southern Globular Clusters

    NASA Astrophysics Data System (ADS)

    Lee, Dong-Joo; Lee, Chung-Uk; Kim, Dong-Jin; Hong, Kyeongsoo; Koo, Jae-Rim; Lee, Jae Woo; Ryu, Yoon-Hyun; Kim, Seung-Lee; Cha, Sang-Mok; Lee, Yongseok; Lim, Beomdu; Park, Byeong-Gon; Jeon, Young-Beom

    2015-08-01

    Korea Astronomy and Space Science Institute (KASI) has installed wide-field photometric survey systems called Korea Microlensing Telescope Network (KMTNet) at three Southern observatories of CTIO in Chile, SAAO in South Africa, and SSO in Australia. Each system consists of a wide-field optical telescope with 1.6m diameter and a mosaic CCD camera with 18k by 18k pixels, having a 2.0 by 2.0 square degree field of view. Its primary scientific goal is to search for extrasolar planets, especially Earth-mass planets in the habitable zone, by using the gravitational microlensing method. We have performed the pre-science run of these three telescopes, with attaching a 4k CCD camera for a few months at each site before installing the 18k mosaic CCD camera. Considering the rather small field of view of the 4k CCD camera, we chose six globular clusters NGC 288, NGC 1851, NGC 3201, NGC 4372, NGC 6752, and NGC 6809 to monitor photometrically. Difference Image Analysis (DIA) technique was applied to the time-series CCD images in order to minimize the blending effect in the crowded field of globular clusters. We will present the preliminary results to search for variable stars in the clusters.

  16. The Age of the Inner Halo Globular Cluster NGC 6652

    NASA Technical Reports Server (NTRS)

    Chaboyer, Brian; Sarajedini, Ata; Armandroff, Taft E.

    2000-01-01

    Hubble Space Telescope (HST) (V,I) photometry has been obtained for the inner halo globular cluster NGC 6652. The photometry reaches approximately 4 mag below the turn-off and includes a well populated horizontal branch (HB). This cluster is located close to the Galactic center at RGC approximately equal to 2.0 kpc with a reddening of E(V-I) = 0.15 +/- 0.02 and has a metallicity of [Fe/H] approximately equal to -0.85. Based upon DELTA V (sup SGB) (sub HB), NGC 6652 is 11.7 plus or minus 1.6 Gyr old. Using A HB precise differential ages for 47 Tuc (a thick disk globular), M107 and NGC 1851 (both halo clusters) were obtained. NGC 6652 appears to be the same age as 47 Tuc and NGC 1851 (within +/- 1.2 Gyr), while there is a slight suggestion that M107 is older than NGC 6652 by 2.3 +/- 1.5 Gyr. As this is a less than 2 sigma result, this issue needs to be investigated further before a definitive statement regarding the relative age of M107 and NGC 6652 may be made.

  17. Deep Mixing and Metallicity in Globular Cluster Red Giants

    NASA Astrophysics Data System (ADS)

    Martell, Sarah L.

    2007-12-01

    We present results from a study of carbon depletion and deep mixing in globular cluster red giants across a wide range of metallicity. CH bandstrengths are measured from low-resolution (R 1000) spectra and converted to [C/Fe] abundances by comparisons with synthetic spectra. Although some models of deep mixing predict that its efficiency will be reduced at high metallicity, no sign of such a cutoff is seen in our data, which span the range -2.29 < [Fe/H] < -1.29.

  18. Bayesian Analysis of Multiple Populations in Galactic Globular Clusters

    NASA Astrophysics Data System (ADS)

    Wagner-Kaiser, Rachel A.; Sarajedini, Ata; von Hippel, Ted; Stenning, David; Piotto, Giampaolo; Milone, Antonino; van Dyk, David A.; Robinson, Elliot; Stein, Nathan

    2016-01-01

    We use GO 13297 Cycle 21 Hubble Space Telescope (HST) observations and archival GO 10775 Cycle 14 HST ACS Treasury observations of Galactic Globular Clusters to find and characterize multiple stellar populations. Determining how globular clusters are able to create and retain enriched material to produce several generations of stars is key to understanding how these objects formed and how they have affected the structural, kinematic, and chemical evolution of the Milky Way. We employ a sophisticated Bayesian technique with an adaptive MCMC algorithm to simultaneously fit the age, distance, absorption, and metallicity for each cluster. At the same time, we also fit unique helium values to two distinct populations of the cluster and determine the relative proportions of those populations. Our unique numerical approach allows objective and precise analysis of these complicated clusters, providing posterior distribution functions for each parameter of interest. We use these results to gain a better understanding of multiple populations in these clusters and their role in the history of the Milky Way.Support for this work was provided by NASA through grant numbers HST-GO-10775 and HST-GO-13297 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. This material is based upon work supported by the National Aeronautics and Space Administration under Grant NNX11AF34G issued through the Office of Space Science. This project was supported by the National Aeronautics & Space Administration through the University of Central Florida's NASA Florida Space Grant Consortium.

  19. Variable Stars In the Unusual, Metal-Rich Globular Cluster

    NASA Technical Reports Server (NTRS)

    Pritzl, Barton J.; Smith, Horace A.; Catelan, Marcio; Sweigart, Allen V.; Oegerle, William R. (Technical Monitor)

    2002-01-01

    We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6388 using time-series BV photometry. Twenty-eight new variables were found in this survey, increasing the total number of variables found near NGC 6388 to approx. 57. A significant number of the variables are RR Lyrae (approx. 14), most of which are probable cluster members. The periods of the fundamental mode RR Lyrae are shown to be unusually long compared to metal-rich field stars. The existence of these long period RRab stars suggests that the horizontal branch of NGC 6388 is unusually bright. This implies that the metallicity-luminosity relationship for RR Lyrae stars is not universal if the RR Lyrae in NGC 6388 are indeed metal-rich. We consider the alternative possibility that the stars in NGC 6388 may span a range in [Fe/H]. Four candidate Population II Cepheids were also found. If they are members of the cluster, NGC 6388 would be the most metal-rich globular cluster to contain Population II Cepheids. The mean V magnitude of the RR Lyrae is found to be 16.85 +/- 0.05 resulting in a distance of 9.0 to 10.3 kpc, for a range of assumed values of (M(sub V)) for RR Lyrae. We determine the reddening of the cluster to be E(B - V) = 0.40 +/- 0.03 mag, with differential reddening across the face of the cluster. We discuss the difficulty in determining the Oosterhoff classification of NGC 6388 and NGC 6441 due to the unusual nature of their RR Lyrae, and address evolutionary constraints on a recent suggestion that they are of Oosterhoff type II.

  20. The Counterparts of the Luminous, Bursting X-ray Sources in Globular Clusters-LTSA98

    NASA Technical Reports Server (NTRS)

    Anderson, Scott F.

    2003-01-01

    Under the fifth year of the LTSA, we have extended our HST and Chandra work to a number of additional globular clusters. The remarkable sensitivity and positional accuracy of the Chandra observations are enabling us to maximally exploit HST for UV/optical identifications for X-ray binaries in the cores of multiple globular clusters. The dozens of lower-luminosity X-ray sources in each globular cluster deeply examined thus far have moved us firmly into the era of studies which encompass populations of close; the large range of cluster properties we are studying have, for the first tine, established a firm empirical confirmation of the (long-suspected theoretically) high importance that close binaries play in the dynamical stability and evolution of globular clusters. The LTSA support has been a cornerstone of our success over the past 5 years in studies of globular cluster X-ray sources and their counterparts.

  1. Measuring consistent masses for 25 Milky Way globular clusters

    SciTech Connect

    Kimmig, Brian; Seth, Anil; Ivans, Inese I.; Anderton, Tim; Gregersen, Dylan; Strader, Jay; Caldwell, Nelson

    2015-02-01

    We present central velocity dispersions, masses, mass-to-light ratios (M/Ls ), and rotation strengths for 25 Galactic globular clusters (GCs). We derive radial velocities of 1951 stars in 12 GCs from single order spectra taken with Hectochelle on the MMT telescope. To this sample we add an analysis of available archival data of individual stars. For the full set of data we fit King models to derive consistent dynamical parameters for the clusters. We find good agreement between single-mass King models and the observed radial dispersion profiles. The large, uniform sample of dynamical masses we derive enables us to examine trends of M/L with cluster mass and metallicity. The overall values of M/L and the trends with mass and metallicity are consistent with existing measurements from a large sample of M31 clusters. This includes a clear trend of increasing M/L with cluster mass and lower than expected M/Ls for the metal-rich clusters. We find no clear trend of increasing rotation with increasing cluster metallicity suggested in previous work.

  2. A New Database of Globular Clusters Parameters: Distributions of Cluster Properties and Correlations Between Them

    NASA Astrophysics Data System (ADS)

    Djorgovski, S.; Meylan, G.

    1993-05-01

    The forthcoming ASPCS volume, ``Structure and Dynamics of Globular Clusters'' (expected publication date: early summer of 1993) will contain a set of appendices with data resources on Galactic globular clusters; the authors of these papers include I.R. King, S. Peterson, C.T. Pryor, S.C. Trager, and ourselves. From these papers we have compiled a data base of various observed and derived parameters for globular clusters (143 of them at last count). Our main purpose is to use these data for correlative studies of globular cluster properties. Others may find it useful for similar purposes, for planning and support of observations, for testing of theoretical models, etc. We will describe the data base, and present some simple analysis of the cluster properties and correlations among them. The data will be made available to the community in a computer form, as ASCII files. Interested users should send an email message to the Internet address: george @ deimos.caltech.edu, and may also find the above mentioned ASPCS volume useful in their work. We thank our colleagues who contributed data for this compilation for their efforts. S.D. acknowledges a partial support from the NASA contract NAS5-31348, and the NSF PYI award AST-9157412.

  3. Mergers and ejections of black holes in globular clusters

    NASA Astrophysics Data System (ADS)

    Aarseth, Sverre J.

    2012-05-01

    We report on results of fully consistent N-body simulations of globular cluster models with N= 100 000 members containing neutron stars and black holes (BHs). Using the improved 'algorithmic regularization' method of Hellström & Mikkola for compact subsystems, the new code NBODY7 enables for the first time general relativistic coalescence to be achieved for post-Newtonian terms and realistic parameters. Following an early stage of mass segregation, a few BHs form a small dense core which usually leads to the formation of one dominant binary. The subsequent evolution by dynamical shrinkage involves the competing processes of ejection and mergers by radiation energy loss. Unless the binary is ejected, long-lived triple systems often exhibit Kozai cycles with extremely high inner eccentricity (e > 0.999) which may terminate in coalescence at a few Schwarzschild radii. A characteristic feature is that ordinary stars as well as BHs and even BH binaries are ejected with high velocities. On the basis of the models studied so far, the results suggest a limited growth of a few remaining stellar mass BHs in globular clusters.

  4. Globular clusters: DNA of early-type galaxies?

    NASA Astrophysics Data System (ADS)

    Forte, Juan C.; Vega, E. Irene; Faifer, Favio R.; Smith Castelli, Analía V.; Escudero, Carlos; González, Nélida M.; Sesto, Leandro

    2014-06-01

    This paper explores if the mean properties of early-type galaxies (ETGs) can be reconstructed from `genetic' information stored in their globular clusters (GCs; i.e. in their chemical abundances, spatial distributions and ages). This approach implies that the formation of each globular occurs in very massive stellar environments, as suggested by some models that aim at explaining the presence of multipopulations in these systems. The assumption that the relative number of GCs to diffuse stellar mass depends exponentially on chemical abundance, [Z/H], and the presence of two dominant GC subpopulations (blue and red), allows the mapping of low-metallicity haloes and of higher metallicity (and more heterogeneous) bulges. In particular, the masses of the low-metallicity haloes seem to scale up with dark matter mass through a constant. We also find a dependence of the GC formation efficiency with the mean projected stellar mass density of the galaxies within their effective radii. The analysis is based on a selected subsample of galaxies observed within the ACS Virgo Cluster Survey of the Hubble Space Telescope. These systems were grouped, according to their absolute magnitudes, in order to define composite fiducial galaxies and look for a quantitative connection with their (also composite) GCs systems. The results strengthen the idea that GCs are good quantitative tracers of both baryonic and dark matter in ETGs.

  5. A NEW ABUNDANCE SCALE FOR THE GLOBULAR CLUSTER 47 Tuc

    SciTech Connect

    Koch, Andreas; McWilliam, Andrew E-mail: andy@ociw.edu

    2008-04-15

    We present chemical abundances for O, Na, Mg, Al, Si, Ca, Ti, and Fe in eight red giants and one turnoff star in the metal-rich globular cluster 47 Tuc, based on spectroscopy with the Magellan Inamori Kyocera Echelle high-resolution spectrograph on the Magellan 6.5 m Clay telescope. A robust line by a line differential abundance analysis technique, relative to the K-giant Arcturus, was used to reduce systematic errors from atmospheric and atomic parameters. Our derived mean LTE [Fe/H] of -0.76 {+-} 0.01 {+-} 0.04 dex (random and systematic error, respectively) is more metal poor by about 0.1 dex than recent literature results. The chemical element ratios in this nearby globular cluster most closely resemble those of the Galactic bulge, although there is a non-negligible overlap with the composition of thick-disk stars. We find that the [Al/Fe] and [Na/Fe] ratios coincide with the upper boundary of the trends seen in the bulge and thick disk. There is only a small intrinsic scatter in the majority of the abundance ratios, indicating that 47 Tuc is mostly a rather chemically homogeneous system.

  6. THE FIRST CONFIRMED MICROLENS IN A GLOBULAR CLUSTER

    SciTech Connect

    Pietrukowicz, P.; Udalski, A.; Minniti, D.; Alonso-Garcia, J.; Jetzer, Ph.

    2012-01-10

    In 2000 July/August a microlensing event occurred at a distance of 2.'33 from the center of the globular cluster M22 (NGC 6656), observed against the dense stellar field of the Milky Way bulge. We have used the adaptive optics system NACO at the ESO Very Large Telescope to resolve the two objects that participated in the event: the lens and the source. The position of the objects measured in 2011 July is in agreement with the observed relative proper motion of M22 with respect to the background bulge stars. Based on the brightness of the microlens components we find that the source is a solar-type star located at a distance of 6.0 {+-} 1.5 kpc in the bulge, while the lens is a 0.18 {+-} 0.01 M{sub Sun} dwarf member of the globular cluster located at the known distance of 3.2 {+-} 0.2 kpc from the Sun.

  7. Multivariate Analysis of the Globular Clusters in M87

    NASA Astrophysics Data System (ADS)

    Das, Sukanta; Chattopadhayay, Tanuka; Davoust, Emmanuel

    2015-11-01

    An objective classification of 147 globular clusters (GCs) in the inner region of the giant elliptical galaxy M87 is carried out with the help of two methods of multivariate analysis. First, independent component analysis (ICA) is used to determine a set of independent variables that are linear combinations of various observed parameters (mostly Lick indices) of the GCs. Next, K-means cluster analysis (CA) is applied on the independent components (ICs), to find the optimum number of homogeneous groups having an underlying structure. The properties of the four groups of GCs thus uncovered are used to explain the formation mechanism of the host galaxy. It is suggested that M87 formed in two successive phases. First a monolithic collapse, which gave rise to an inner group of metal-rich clusters with little systematic rotation and an outer group of metal-poor clusters in eccentric orbits. In a second phase, the galaxy accreted low-mass satellites in a dissipationless fashion, from the gas of which the two other groups of GCs formed. Evidence is given for a blue stellar population in the more metal rich clusters, which we interpret by Helium enrichment. Finally, it is found that the clusters of M87 differ in some of their chemical properties (NaD, TiO1, light-element abundances) from GCs in our Galaxy and M31.

  8. CN ANOMALIES IN THE HALO SYSTEM AND THE ORIGIN OF GLOBULAR CLUSTERS IN THE MILKY WAY

    SciTech Connect

    Carollo, Daniela; Martell, Sarah L.; Beers, Timothy C.; Freeman, Ken C. E-mail: smartell@aao.gov.au E-mail: kcf@mso.anu.edu.au

    2013-06-01

    We explore the kinematics and orbital properties of a sample of red giants in the halo system of the Milky Way that are thought to have formed in globular clusters based on their anomalously strong UV/blue CN bands. The orbital parameters of the CN-strong halo stars are compared to those of the inner- and outer-halo populations as described by Carollo et al., and to the orbital parameters of globular clusters with well-studied Galactic orbits. The CN-strong field stars and the globular clusters both exhibit kinematics and orbital properties similar to the inner-halo population, indicating that stripped or destroyed globular clusters could be a significant source of inner-halo field stars, and suggesting that both the CN-strong stars and the majority of globular clusters are primarily associated with this population.

  9. A DOUBLE MAIN SEQUENCE IN THE GLOBULAR CLUSTER NGC 6397

    SciTech Connect

    Milone, A. P.; Aparicio, A.; Marino, A. F.; Piotto, G.; Bedin, L. R.; Anderson, J.; Cassisi, S.; Rich, R. M. E-mail: aparicio@iac.es E-mail: giampaolo.piotto@unipd.it E-mail: bedin@stsci.edu E-mail: rmr@astro.ucla.edu

    2012-01-20

    High-precision multi-band Hubble Space Telescope (HST) photometry reveals that the main sequence of the globular cluster NGC 6397 splits into two components, containing {approx}30% and {approx}70% of the stars. This double sequence is consistent with the idea that the cluster hosts two stellar populations: (1) a primordial population that has a composition similar to field stars, containing {approx}30% of the stars, and (2) a second generation with enhanced sodium and nitrogen, depleted carbon and oxygen, and a slightly enhanced helium abundance ({Delta}Y {approx} 0.01). We examine the color difference between the two sequences across a variety of color baselines and find that the second sequence is anomalously faint in m{sub F336W}. Theoretical isochrones indicate that this could be due to NH depletion.

  10. The orbital eccentricities of binary millisecond pulsars in globular clusters

    NASA Technical Reports Server (NTRS)

    Rasio, Frederic A.; Heggie, Douglas C.

    1995-01-01

    Low-mass binary millisecond pulsars (LMBPs) are born with very small orbital eccentricities, typically of order e(sub i) approximately 10(exp -6) to 10(exp -3). In globular clusters, however, higher eccentricities e(sub f) much greater than e(sub i) can be induced by dynamical interactions with passing stars. Here we show that the cross section for this process is much larger than previously estimated. This is becuse, even for initially circular binaries, the induced eccentricity e(sub f) for an encounter with pericenter separation r(sub p) beyond a few times the binary semimajor axis a declines only as a power law (e(sub f) varies as (r(sub p)/a)(exp -5/2), and not as an exponential. We find that all currently known LMBPs in clusters were probably affected by interactions, with their current eccentricities typically greater than at birth by an order of magnitude or more.

  11. Where Are Most of the Globular Clusters in Today's Universe?

    NASA Astrophysics Data System (ADS)

    Harris, William E.

    2016-04-01

    The total number of globular clusters (GCs) in a galaxy rises continuously with the galaxy luminosity L, while the relative number of galaxies decreases with L following the Schechter function. The product of these two very nonlinear functions gives the relative number of GCs contained by all galaxies at a given L. It is shown that GCs, in this universal sense, are most commonly found in galaxies within a narrow range around L⋆. In addition, blue (metal-poor) GCs outnumber the red (metal-richer) ones globally by 4 to 1 when all galaxies are added, pointing to the conclusion that the earliest stages of galaxy formation were especially favorable to forming massive, dense star clusters.

  12. Binary interactions and multiple stellar populations in globular clusters

    NASA Astrophysics Data System (ADS)

    Jiang, Dengkai

    2015-08-01

    Observations revealed the presence of multiple stellar populations in globular clusters (GCs) that exhibit wide abundance variations and multiple sequences in Hertzsprung-Russell diagram. We present a scenario for the formation of multiple stellar populations in GCs. In this scenario, initial GCs are single-generation clusters, and our model predicts that the abundance anomalous stars observed in GCs are the merged stars and the accretor stars produced by binary interactions, which are rapidly rotating stars at the moment of their formation. The stellar population with binaries can reproduce two important observational evidences of multiple stellar populations, the Na-O anticorrelation and the multiple sequences in HR diagram. This suggests that binary interactions may be a possible scenario for the formation of multiple stellar populations in GCs.

  13. Soar adaptive optics observations of the globular cluster NGC 6496

    SciTech Connect

    Fraga, Luciano; Kunder, Andrea; Tokovinin, Andrei E-mail: lfraga@lna.br

    2013-06-01

    We present high-quality BVRI photometric data in the field of globular cluster NGC 6496 obtained with the SOAR Telescope Adaptive Module (SAM). Our observations were collected as part of the ongoing SAM commissioning. The distance modulus and cluster color excess as found from the red clump are (m – M) {sub V} = 15.71 ± 0.02 mag and E(V – I) = 0.28 ± 0.02 mag. An age of 10.5 ± 0.5 Gyr is determined from the difference in magnitude between the red clump and the subgiant branch. These parameters are in excellent agreement with the values derived from isochrone fitting. From the color-magnitude diagram we find a metallicity of [Fe/H] = –0.65 dex and hence support a disk classification for NGC 6496. The complete BVRI data set for NGC 6469 is made available in the electronic edition of the Journal.

  14. Dynamical evolution and spatial mixing of multiple population globular clusters

    NASA Astrophysics Data System (ADS)

    Vesperini, Enrico; McMillan, Stephen L. W.; D'Antona, Francesca; D'Ercole, Annibale

    2013-03-01

    Numerous spectroscopic and photometric observational studies have provided strong evidence for the widespread presence of multiple stellar populations in globular clusters. In this paper, we study the long-term dynamical evolution of multiple population clusters, focusing on the evolution of the spatial distributions of the first- (FG) and second-generation (SG) stars. In previous studies, we have suggested that SG stars formed from the ejecta of FG AGB stars are expected initially to be concentrated in the cluster inner regions. Here, by means of N-body simulations, we explore the time-scales and the dynamics of the spatial mixing of the FG and the SG populations and their dependence on the SG initial concentration. Our simulations show that, as the evolution proceeds, the radial profile of the SG/FG number ratio, NSG/NFG, is characterized by three regions: (1) a flat inner part; (2) a declining part in which FG stars are increasingly dominant and (3) an outer region where the NSG/NFG profile flattens again (the NSG/NFG profile may rise slightly again in the outermost cluster regions). Until mixing is complete and the NSG/NFG profile is flat over the entire cluster, the radial variation of NSG/NFG implies that the fraction of SG stars determined by observations covering a limited range of radial distances is not, in general, equal to the SG global fraction, (NSG/NFG)glob. The distance at which NSG/NFG equals (NSG/NFG)glob is approximately between 1 and 2 cluster half-mass radii. The time-scale for complete mixing depends on the SG initial concentration, but in all cases complete mixing is expected only for clusters in advanced evolutionary phases, having lost at least 60-70 per cent of their mass due to two-body relaxation (in addition to the early FG loss due to the cluster expansion triggered by SNII ejecta and gas expulsion).The results of our simulations suggest that in many Galactic globular clusters the SG should still be more spatially concentrated than the

  15. Dynamical friction in multi-component evolving globular clusters

    SciTech Connect

    Alessandrini, Emiliano; Lanzoni, Barbara; Miocchi, Paolo; Ciotti, Luca; Ferraro, Francesco R.

    2014-11-10

    We use the Chandrasekhar formalism and direct N-body simulations to study the effect of dynamical friction on a test object only slightly more massive than the field stars, orbiting a spherically symmetric background of particles with a mass spectrum. The main goal is to verify whether the dynamical friction time (t {sub DF}) develops a non-monotonic radial dependence that could explain the bimodality of the blue straggler radial distributions observed in globular clusters. In these systems, in fact, relaxation effects lead to a mass and velocity radial segregation of the different mass components, so that mass-spectrum effects on t {sub DF} are expected to be dependent on radius. We find that in spite of the presence of different masses, t {sub DF} is always a monotonic function of radius, at all evolutionary times and independently of the initial concentration of the simulated cluster. This is because the radial dependence of t {sub DF} is largely dominated by the total mass density profile of the background stars (which is monotonically decreasing with radius). Hence, a progressive temporal erosion of the blue straggler star (BSS) population at larger and larger distances from the cluster center remains the simplest and the most likely explanation of the shape of the observed BSS radial distributions, as suggested in previous works. We also confirm the theoretical expectation that approximating a multi-mass globular cluster as made of (averaged) equal-mass stars can lead to significant overestimations of t {sub DF} within the half-mass radius.

  16. Variable stars in the globular cluster M 28 (NGC 6626)

    NASA Astrophysics Data System (ADS)

    Prieto, G.; Catelan, M.; Contreras Ramos, R.; Pritzl, B. J.; Smith, H. A.; Alonso-García, J.

    2012-07-01

    Context. We present a new search for variable stars in the Galactic globular cluster M 28 (NGC 6626). Aims: The search is based on a series of BVI images obtained with the SMARTS Consortium's 1.3 m telescope at Cerro Tololo Inter-American Observatory, Chile. Methods: The search was carried out using the ISIS v2.2 image subtraction package. Results: We find a total of 25 variable stars in the field of the cluster, nine being new discoveries. Of the newly found variables, one is an ab-type RR Lyrae star, six are c-type RR Lyrae, and two are long-period/semi-regular variables. V22, previously classified as a type II Cepheid, appears as a bona-fide RRc in our data. In turn, V20, previously classified as an ab-type RR Lyrae, could not be properly phased with any reasonable period. Conclusions: The properties of the ab-type RR Lyrae stars in M 28 appear most consistent with an Oosterhoff-intermediate classification, which is unusual for bona-fide Galactic globulars clusters. However, the cluster's c-type variables do not clearly support such an Oosterhoff type, and a hybrid Oosterhoff I/II system is accordingly another possibility, thus raising the intriguing possibility of multiple populations being present in M 28. Coordinates, periods, and light curves in differential fluxes are provided for all the detected variables. Based on observations obtained with the SMARTS Consortium 1.3 m telescope at the Cerro Tololo Inter-American Observatory, Chile.Figures 2-5 are available in electronic form at http://www.aanda.org

  17. The Frequency of Lithium-Rich Giants in Globular Clusters

    NASA Astrophysics Data System (ADS)

    Kirby, Evan N.; Guhathakurta, Puragra; Zhang, Andrew J.; Hong, Jerry; Guo, Michelle; Guo, Rachel; Cohen, Judith G.; Cunha, Katia M. L.

    2016-01-01

    Although red giants destroy lithium, 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 consistent 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.

  18. Mass segregation in the outer halo globular cluster Palomar 14

    NASA Astrophysics Data System (ADS)

    Frank, Matthias J.; Grebel, Eva K.; Küpper, Andreas H. W.

    2014-09-01

    We present evidence for mass segregation in the outer halo globular cluster Palomar 14, which is intuitively unexpected since its present-day two-body relaxation time significantly exceeds the Hubble time. Based on archival Hubble Space Telescope imaging, we analyse the radial dependence of the stellar mass function in the cluster's inner 39.2 pc in the mass range of 0.53 ≤ m ≤ 0.80 M⊙, ranging from the main-sequence turn-off down to a V-band magnitude of 27.1 mag. The mass function at different radii is well approximated by a power law and rises from a shallow slope of 0.6 ± 0.2 in the cluster's core to a slope of 1.6 ± 0.3 beyond 18.6 pc. This is seemingly in conflict with the finding by Beccari et al., who interpret the cluster's non-segregated population of (more massive) blue straggler stars, compared to (less massive) red giants and horizontal branch stars, as evidence that the cluster has not experienced dynamical segregation yet. We discuss how both results can be reconciled. Our findings indicate that the cluster was either primordially mass segregated and/or used to be significantly more compact in the past. For the latter case, we propose tidal shocks as the mechanism driving the cluster's expansion, which would imply that Palomar 14 is on a highly eccentric orbit. Conversely, if the cluster formed already extended and with primordial mass segregation, this could support an accretion origin of the cluster.

  19. Chemical abundances in the old LMC globular cluster Hodge 11

    NASA Astrophysics Data System (ADS)

    Mateluna, R.; Geisler, D.; Villanova, S.; Carraro, G.; Grocholski, A.; Sarajedini, A.; Cole, A.; Smith, V.

    2012-12-01

    Context. The study of globular clusters is one of the most powerful ways to learn about a galaxy's chemical evolution and star formation history. They preserve a record of chemical abundances at the time of their formation and are relatively easy to age date. The most detailed knowledge of the chemistry of a star is given by high resolution spectroscopy, which provides accurate abundances for a wide variety of elements, yielding a wealth of information on the various processes involved in the cluster's chemical evolution. Aims: We studied red giant branch (RGB) stars in an old, metal-poor globular cluster of the Large Magellanic Cloud (LMC), Hodge 11 (H11), in order to measure as many elements as possible. The goal is to compare its chemical trends to those in the Milky Way halo and dwarf spheroidal galaxies in order to help understand the formation history of the LMC and our own Galaxy. Methods: We have obtained high resolution VLT/FLAMES spectra of eight RGB stars in H11. The spectral range allowed us to measure a variety of elements, including Fe, Mg, Ca, Ti, Si, Na, O, Ni, Cr, Sc, Mn, Co, Zn, Ba, La, Eu and Y. Results: We derived a mean [Fe/H] = -2.00 ± 0.04, in the middle of previous determinations. We found low [α/Fe] abundances for our targets, more comparable to values found in dwarf spheroidal galaxies than in the Galactic halo, suggesting that if H11 is representative of its ancient populations then the LMC does not represent a good halo building block. Our [Ca/Fe] value is about 0.3 dex less than that of halo stars used to calibrate the Ca IR triplet technique for deriving metallicity. A hint of a Na abundance spread is observed. Its stars lie at the extreme high O, low Na end of the Na:O anti-correlation displayed by Galactic and LMC globular clusters. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile (proposal ID 082.B-0458).Table 4 is only available in electronic form at http://www.aanda.org

  20. Radial velocities in the globular cluster ω Centauri

    NASA Astrophysics Data System (ADS)

    Reijns, R. A.; Seitzer, P.; Arnold, R.; Freeman, K. C.; Ingerson, T.; van den Bosch, R. C. E.; van de Ven, G.; de Zeeuw, P. T.

    2006-01-01

    We have used the ARGUS multi-object spectrometer at the CTIO 4 m Blanco telescope to obtain 2756 radial velocity measurements for 1966 individual stars in the globular cluster ω Centauri brighter than blue photographic magnitude of about 16.5. Of these, 1589 stars are cluster members. A comparison with two independent radial velocity studies, carried out by Suntzeff & Kraft and by Mayor et al., demonstrates that the median error of our measurements is below 2 km s-1 for the stars brighter than B-magnitude 15, which constitute the bulk of the sample. The observed velocity dispersion decreases from about 15 km s-1 in the inner few arcmin to about 6 km s-1 at a radius of 25'. The cluster shows significant rotation, with a maximum amplitude of about 6 km s-1 in the radial zone between 6' and 10'. In a companion paper by van de Ven et al., we correct these radial velocities for the perspective rotation caused by the space motion of the cluster, and combine them with the internal proper motions of nearly 8000 cluster members measured by van Leeuwen et al., to construct a detailed dynamical model of ω Centauri and to measure its distance.

  1. Discovery of Remote Globular Cluster Satellites of M87

    NASA Astrophysics Data System (ADS)

    Sparkman, Lea; Guo, Rachel; Toloba, Elisa; Guhathakurta, Puragra; Peng, Eric W.; Ferrarese, Laura; Cote, Patrick; NGVS Collaboration

    2016-01-01

    We present the discovery of several tens of globular clusters (GCs) in the outer regions of the giant elliptical M87, the brightest galaxy in the Virgo Cluster. These M87 GC satellites were discovered in the course of Keck/DEIMOS spectroscopic follow up of GC candidates that were identified in the Next Generation Virgo cluster Survey (NGVS). Specifically, the primary targets of this Keck spectroscopic campaign were GC satellites of early-type dwarf (dE) galaxies. However, we found that our sample contained a subset of GCs for which M87 is the most likely host. This subset is consistent with having an r^-1 power-law surface density distribution and a radial velocity distribution both centered on M87. The remote M87 GC satellites span the radial range 140 to 900 kpc, out to about a third of the Virgo Cluster's virial radius (for comparison, M87's effective radius is only 8 kpc). These M87 GC satellites are probably former satellites of other Virgo Cluster galaxies that have subsequently been cannibalized by M87.This research was supported by the National Science Foundation and the UC Santa Cruz Science Internship Program.

  2. GEMS Observations of Obscured Galactic Bulge Globular Clusters

    NASA Astrophysics Data System (ADS)

    Geisler, Douglas; Saracino, Sara; Dalessandro, Emanuele; Ferraro, Francesco; Lanzoni, Barbara; Mauro, Francesco; Villanova, Sandro; Moni Bidin, Christian; Miocchi, Paolo; Massari, Davide

    2016-01-01

    We will present results for several heavily obscured Galactic globular clusters lying in the bulge, including Liller 1 and NGC 6624. The observations were obtained exploiting the exceptional high-resolution capabilities of the near-IR camera GSAOI combined with the Gemini Multi-Conjugate Adaptive Optics System at the GEMINI South Telescope. The images in the J and K bands are generally sub-0.1", only slightly larger than the diffraction limit of the telescope, yielding the deepest and most accurate color-magnitude diagrams obtained so far from the ground for these clusters. We derived the structural and physical properties of both clusters, supplementing the GEMS data with data from the Vista Variables in the Via Lactea project. We were also able to investigate the age of NGC 6624. We find that Liller 1 is significantly less concentrated and less extended than previously thought. We estimated the mass of Liller 1 to be 2.3 million solar masses, comparable to that of the most massive clusters in the Galaxy. Also, Liller 1 has the second-highest collision rate among all star clusters in the Galaxy, thus confirming that it is an ideal environment for the formation of collisional objects (such as millisecond pulsars). The NGC 6624 CMD reveals the second knee of the mainsequence and allows us to determine a very accurate age of 12.0 +-0.5 Gyr.

  3. MULTIWAVELENGTH PHOTOMETRY IN THE GLOBULAR CLUSTER M2

    SciTech Connect

    Dalessandro, E.; Lanzoni, B.; Ferraro, F.R.; Beccari, G.; Schiavon, R.; Rood, R.T.

    2009-06-15

    We present a multiwavelength photometric analysis of the globular cluster M2. The data set has been obtained by combining high-resolution (Hubble Space Telescope/WFPC2 and ACS) and wide-field (Galaxy Evolution Explorer) space observations and ground-based (MEGACAM-CFHT, EMMI-NTT) images. The photometric sample covers the entire cluster extension from the very central regions up to the tidal radius and beyond. It allows an accurate determination of the cluster center of gravity and other structural parameters derived from the star count density profile. Moreover, we study the Blue Straggler Star (BSS) population and its radial distribution. A total of 123 BSSs have been selected, and their radial distribution has been found to be bimodal (highly peaked in the center, decreasing at intermediate radii, and rising outward), as already found in a number of other clusters. The radial position of the minimum of the BSS distribution is consistent with the radius of avoidance caused by the dynamical friction of massive (1.2 M {sub sun}) objects over the cluster age. We also searched for gradients in the red giant branch (RGB) and the asymptotic giant branch (AGB) populations. At the 2{sigma} level, we found an overabundance of AGB stars within the core radius and confirmed the result of Sohn et al. that the central region of M2 is bluer than the outer part. We show that the latter is due to a deficit of very luminous RGB stars in the central region.

  4. CN and CH Bandstrengths in Bright Globular Cluster Red Giants

    NASA Astrophysics Data System (ADS)

    Martell, Sarah L.; Smith, G. H.

    2006-12-01

    We present preliminary results from a survey of CN and CH bandstrengths in bright red giant stars (MV -1.5) in Galactic globular clusters. Our cluster sample spans a wide metallicity range, from M92 ([Fe/H]=-2.28) to M71 ([Fe/H]=-0.73). The data were all taken using the Shane 120-inch telescope and the Kast spectrograph at Lick Observatory; the homogeneity of the sample makes it ideal for a comparative study of carbon depletion (and therefore deep mixing rate) as a function of stellar metallicity. Thus far we have measured molecular bandstrength indices for CH and CN, as well as indices for Ca and Mg lines; the task of converting the index measurements to carbon and nitrogen abundances will require comparisons with synthetic spectra. The molecular CN index behaves as expected from a study of the literature: within individual clusters, it varies significantly from star to star. The data also allow us to examine the dependence of the Ca and Mg indices on cluster metallicity at a given MV. The index MHK shows clear sensitivity to [Fe/H] across the full metallicity range of our sample. A similar study is also in progress involving analogous stars in the open clusters NGC 188, NGC 2158, NGC 6791, and NGC 7789 (-0.3 < [Fe/H] < +0.3).

  5. The Main Sequence Luminosity Function of Low-Mass Globular Clusters

    NASA Astrophysics Data System (ADS)

    Smith, Graeme

    2009-07-01

    Theoretical work indicates that the dynamical evolution of globular clusters of low mass and low central concentration is strongly determined by mass-loss processes, such as stellar evaporation and tidal stripping, that can eventually lead to cluster dissolution. In fact, mass loss and cluster disruption is now considered to be a viable explanation for the form of the faint end of the Milky Way globular cluster luminosity function. A clear observational demonstration of the prevalence of cluster mass-loss would have ramifications not only for the dynamical evolution of individual globular clusters and their internal stellar mass distributions, but also for the relationships between halo field and cluster stars and the properties of globular cluster systems in galaxies. Our previous WFPC2 imaging of the low-mass diffuse halo cluster Palomar 5 revealed a main sequence deficient in stars compared to other low-concentration globular clusters of much higher mass, consistent with there having been a considerable loss of stars from this system. But is Pal 5 typical of low-mass, low-concentration halo clusters? We propose to place the mass-loss scenario on a firm observational footing {or otherwise} by using WFC3 imaging to measure the main-sequence stellar mass functions of two of the lowest-mass lowest-concentration globular clusters in the Milky Way, AM-4 and Palomar 13, in order to search for analogous evidence of stellar depletion.

  6. WFPC2 Observations of the Sagittarius Globular Cluster Arp 2

    NASA Astrophysics Data System (ADS)

    Dueñas, E. N.; Mighell, K. J.

    2002-12-01

    We present our preliminary analysis of archival Hubble Space Telescope WFPC2 observations in the F555W (V) and F814W (I) filters of the metal-poor globular cluster Arp 2 in the nearby Sagittarius dwarf spheroidal galaxy. The deep V vs. (V-I) color-magnitude diagram reaches 3.5 magnitudes below the main sequence turnoff with a signal-to-noise ratio of 10 or greater in both filters; it also features a well-defined subgiant branch as well as a few blue stragglers. The upper red giant branch is not analyzed due to photometric saturation. Most previous studies of Arp 2 analyzed the ground-based BV observations of Buonanno et al. (1995) or the ground-based VI observations of Sarajedini & Layden (1997). Buonanno et al. found that Arp 2 is a metal-poor, intermediate age cluster, while Layden & Sarajedini (2000) determined that it is coeval with other ancient Galactic globular clusters, e.g. 13.1 +/- 0.9 Gyr. We have analyzed our space-based observations with the Yonsei-Yale (Y2) isochrones of Yi et al. (2001) and present evidence that confirms the Layden & Sarajedini's ancient-age hypothesis for Arp 2. Dueñas was supported, in part, by awards to South Carolina State University from NASA/MU-SPIN (NCC 5-534) and NASA/OSS (NAG 5-10145), and the NOAO/KPNO REU Program, funded by the National Science Foundation. Mighell was supported by a grant from the National Aeronautics and Space Administration (NASA), S-67046-F, awarded by the Long-Term Space Astrophysics program of NASA's Office of Space Science.

  7. The gamma-ray pulsar population of globular clusters: implications for the GeV excess

    NASA Astrophysics Data System (ADS)

    Hooper, Dan; Linden, Tim

    2016-08-01

    It has been suggested that the GeV excess, observed from the region surrounding the Galactic Center, might originate from a population of millisecond pulsars that formed in globular clusters. With this in mind, we employ the publicly available Fermi data to study the gamma-ray emission from 157 globular clusters, identifying a statistically significant signal from 25 of these sources (ten of which are not found in existing gamma-ray catalogs). We combine these observations with the predicted pulsar formation rate based on the stellar encounter rate of each globular cluster to constrain the gamma-ray luminosity function of millisecond pulsars in the Milky Way's globular cluster system. We find that this pulsar population exhibits a luminosity function that is quite similar to those millisecond pulsars observed in the field of the Milky Way (i.e. the thick disk). After pulsars are expelled from a globular cluster, however, they continue to lose rotational kinetic energy and become less luminous, causing their luminosity function to depart from the steady-state distribution. Using this luminosity function and a model for the globular cluster disruption rate, we show that millisecond pulsars born in globular clusters can account for only a few percent or less of the observed GeV excess. Among other challenges, scenarios in which the entire GeV excess is generated from such pulsars are in conflict with the observed mass of the Milky Way's Central Stellar Cluster.

  8. The gamma-ray pulsar population of globular clusters: Implications for the GeV excess

    DOE PAGESBeta

    Hooper, Dan; Linden, Tim

    2016-08-09

    In this study, it has been suggested that the GeV excess, observed from the region surrounding the Galactic Center, might originate from a population of millisecond pulsars that formed in globular clusters. With this in mind, we employ the publicly available Fermi data to study the gamma-ray emission from 157 globular clusters, identifying a statistically significant signal from 25 of these sources (ten of which are not found in existing gamma-ray catalogs). We combine these observations with the predicted pulsar formation rate based on the stellar encounter rate of each globular cluster to constrain the gamma-ray luminosity function of millisecondmore » pulsars in the Milky Way's globular cluster system. We find that this pulsar population exhibits a luminosity function that is quite similar to those millisecond pulsars observed in the field of the Milky Way (i.e. the thick disk). After pulsars are expelled from a globular cluster, however, they continue to lose rotational kinetic energy and become less luminous, causing their luminosity function to depart from the steady-state distribution. Using this luminosity function and a model for the globular cluster disruption rate, we show that millisecond pulsars born in globular clusters can account for only a few percent or less of the observed GeV excess. Among other challenges, scenarios in which the entire GeV excess is generated from such pulsars are in conflict with the observed mass of the Milky Way's Central Stellar Cluster.« less

  9. Globular Cluster Star Classification: Application to M13

    NASA Astrophysics Data System (ADS)

    Caimmi, R.

    2013-06-01

    Starting from recent determination of Fe, O, Na abundances on a restricted sample (N=67) of halo and thick disk stars, a natural and well motivated selection criterion is defined for the classification globular cluster stars. An application is performed to M13 using a sample (N=113) for which Fe, O, Na abundances have been recently inferred from observations. A comparison is made between the current and earlier M13 star classifications. Both O and Na empirical differential abundance distributions are determined for each class and for the whole sample (with the addition of Fe in the last case) and compared with their theoretical counterparts due to cosmic scatter obeying a Gaussian distribution whose parameters are inferred from related subsamples. The occurrence of an agreement between the empirical and theoretical distributions is interpreted as absence of significant chemical evolution and vice versa. The procedure is repeated with regard to four additional classes depending on whether oxygen and sodium abundance is above (stage CE) or below (stage AF) a selected threshold. Both O and Na empirical differential abundance distributions, related to the whole sample, exhibit a linear fit for the AF and CE stage. Within the errors, the oxygen slope for the CE stage is equal and of opposite sign with respect to the sodium slope for AF stage, while the contrary holds when dealing with the oxygen slope for the AF stage with respect to the sodium slope for the CE stage. In the light of simple models of chemical evolution applied to M13, oxygen depletion appears to be mainly turned into sodium enrichment for [O/H]≥-1.35 and [Na/H]≤-1.45, while one or more largely preferred channels occur for [O/H]<-1.35 and [Na/H]>-1.45. In addition, the primordial to the current M13 mass ratio can be inferred from the true sodium yield in units of the sodium solar abundance. Though the above results are mainly qualitative due to large (∓.5 dex) uncertainties in abundance

  10. New EGRET/COMPTEL studies of globular clusters.

    NASA Astrophysics Data System (ADS)

    Manandhar, R. P.; Grindlay, J. E.; Thompson, D. J.

    1996-12-01

    We present results of our Phase 2 program of observations of two nearby globular clusters, NGC 6397 and NGC 6752, with EGRET and COMPTEL to search for the integrated emission from their populations of millisecond pulsars, LMXBs (in quiescence), and CVs. Upper limits were derived for each cluster that are a factor of 2 more sensitive than in the general cluster survey of Michelson et al. (1994ApJ...435..218M) using EGRET. Our COMPTEL limits are the first reported for the 1-30MeV band for these clusters and are comparably sensitive to the EGRET limits for an assumed Crab-like spectrum yet can also constrain source spectra if they break at energies below 30MeV. These limits are better than the COMPTEL limits recently reported for 47 Tuc by O'Flaherty et al. (1995A&A...297L..29O). The lowest upper limits, for NGC 6397, restrict the integrated luminosity to be below ~10^35^ergs/s in either the COMPTEL or EGRET energy bands. A serendipitous source, GRO J1718-60, was detected (4.1σ) with EGRET, near (but not coincident with) NGC 6397. Although this source would be consistent with the nearby (1.7°) pulsar J1704-60, the published spindown luminosity is too low for it to be a plausible counterpart. Constraints on the typical gamma-ray luminosity for millisecond pulsars and quiescent LMXBs are derived and discussed.

  11. Effect of dynamical interactions on integrated properties of globular clusters

    NASA Astrophysics Data System (ADS)

    Zhuang, Yulong; Zhang, Fenghui; Anders, Peter; Ruan, Zhifeng; Cheng, Liantao; Kang, Xiaoyu

    2015-02-01

    Globular clusters (GCs) are generally treated as natural validators of simple stellar population (SSP) models. However, there are still some differences between real GCs and SSPs. In this work, we use a direct N-body simulation code NBODY6 to study the influences of dynamical interactions, metallicity and primordial binaries on Milky Way GCs' integrated properties. Our models start with N = 100 000 stars, covering a metallicity range Z = 0.0001 ˜ 0.02, a subset of our models contain primordial binaries, resulting in a binary fraction as currently observed at a model age of GCs. Stellar evolution and external tidal field representative for an average Milky Way GC are taken into consideration. The integrated colours and Lick indices are calculated using BaSeL and Bluered stellar spectral libraries separately. By including dynamical interactions, our model clusters show integrated features (i.e. colours up to 0.01 mag bluer, Hβ up to 0.1 Å greater and [MgFe]' 0.05 Å smaller) making the clusters appear slightly younger than the model clusters without dynamical interactions. This effect is caused mainly by the preferential loss of low-mass stars which have a stronger contribution to redder passbands as well as different spectral features compared to higher mass stars. In addition, this effect is larger at lower metallicities. On the contrary, the incorporation of primordial binaries reduces this effect.

  12. Homogeneous Photometry. V. The Globular Cluster NGC 4147

    NASA Astrophysics Data System (ADS)

    Stetson, Peter B.; Catelan, M.; Smith, Horace A.

    2005-12-01

    New BVRI broadband photometry and astrometry are presented for the globular cluster NGC 4147, based upon measurements derived from 524 ground-based CCD images mostly either donated by colleagues or retrieved from public archives. We have also reanalyzed five exposures of the cluster obtained with WFPC2 on the Hubble Space Telescope in the F439W and F555W (B and V) filters. We present calibrated color-magnitude and color-color diagrams. Analysis of the color-magnitude diagram reveals morphological properties generally consistent with published metal-abundance estimates for the cluster, and an age typical of other Galactic globular clusters of similar metallicity. We have also redetermined the periods and mean magnitudes for the RR Lyrae variables, including a new c-type variable reported here for the first time. Our data do not show clear evidence for photometric variability in candidate V18, recently reported by Arellano Ferro et al. (2004, Rev. Mex. A&A, 40, 209). These observations also support the nonvariable status of candidates V5, V9, and V15. The union of our light-curve data with those of Newburn (1957, AJ, 62, 197), Mannino (1957, Mem. Soc. Astron. Italiana, 28, 285), and Arellano Ferro et al. (op. cit.) permits the derivation of significantly improved periods. The mean periods and the Bailey period-amplitude diagrams support the classification of the cluster as Oosterhoff I, despite its predominantly blue horizontal branch. The number ratio of c- to ab-type RR Lyrae stars, on the other hand, is unusually high for an Oosterhoff I cluster. The calibrated results have been made available through the first author's Web site. Based in part on archival observations made with ESO telescopes at the La Silla and Paranal Observatory under program ID 60.A-9050(A). This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center

  13. SHRINKING THE BRANEWORLD: BLACK HOLE IN A GLOBULAR CLUSTER

    SciTech Connect

    Gnedin, Oleg Y.; Maccarone, Thomas J.; Psaltis, Dimitrios; Zepf, Stephen E. E-mail: tjm@astro.soton.ac.u E-mail: zepf@pa.msu.ed

    2009-11-10

    Large extra dimensions have been proposed as a possible solution to the hierarchy problem in physics. In one of the suggested models, the RS2 braneworld model, black holes may evaporate by Hawking radiation faster than in general relativity, on a timescale that depends on the black hole mass and on the asymptotic radius of curvature of the extra dimensions. Thus the size of the extra dimensions can be constrained by astrophysical observations. Here we point out that the black hole, recently discovered in an extragalactic globular cluster, places the strongest upper limit on the size of the extra dimensions in the RS2 model, L approx< 0.003 mm. This black hole has the virtues of old age and relatively small mass. The derived upper limit is within an order of magnitude of the absolute limit afforded by astrophysical observations of black holes.

  14. Dark matter and the age of globular clusters

    NASA Technical Reports Server (NTRS)

    Dearborn, David; Raffelt, Georg; Salati, Pierre; Silk, Joseph; Bouquet, Alain

    1990-01-01

    The ability of weakly interacting particles called cosmions to suppress convection in the cores of horizontal-branch stars is investigated numerically. It is found that such convection-breaking in horizontal-branch stars by cosmions or by any other novel mode of energy transfer induces thermal relaxation oscillations with a period of about 500,000 yr, corresponding to the core Kelvin-Helmholtz time scale. These thermal pulses can be understood analytically in terms of a simple two-zone model. Observationally, the brightness and brightness dispersion of horizontal-branch stars increases, the periods of RR Lyrae stars change over a pulsation time scale, and the duration of central helium burning slightly decreases. None of these effects is in conflict with observations but, on the contrary, point to a speculative resolution of the age problem for globular clusters and to an alternative explanation of the period fluctuations of RR Lyrae stars.

  15. DO INTERMEDIATE-MASS BLACK HOLES EXIST IN GLOBULAR CLUSTERS?

    SciTech Connect

    Sun, Mou-Yuan; Jin, Ya-Ling; Gu, Wei-Min; Liu, Tong; Lin, Da-Bin; Lu, Ju-Fu

    2013-10-20

    The existence of intermediate-mass black holes (IMBHs) in globular clusters (GCs) remains a crucial problem. Searching for IMBHs in GCs reveals a discrepancy between radio observations and dynamical modelings: the upper mass limits constrained by radio observations are systematically lower than that of dynamical modelings. One possibility for such a discrepancy is that, as we suggest in this work, there exist outflows in accretion flows. Our results indicate that, for most sources, current radio observations cannot rule out the possibility that IMBHs may exist in GCs. In addition, we adopt an M-dot -L{sub R} relation to revisit this issue, which confirms the results obtained by the fundamental plane relation.

  16. On the Galactic globular cluster Delta VHBbump parameter .

    NASA Astrophysics Data System (ADS)

    Di Cecco, A.; Bono, G.; Pietrinferni, A.; Becucci, R.; Stetson, P. B.; Cassisi, S.; Degl'Innocenti, S.; Prada Moroni, P.; Monelli, M.; Buonanno, R.; Corsi, C. E.; Caputo, F.; Iannicola, G.; Ferraro, I.; Pulone, L.; Walker, A. R.

    We calculated new estimates of the Delta VHBbump parameter for 15 Galactic Globular Clusters (GGCs) using accurate ground-based photometric data. We enlarged our sample with literature data and we obtained a sample of 62 GGCs covering a wide metallicity range (-2.16le [M/H]le -0.58 dex). To compare the data with the theory we used theoretical models from \\citet{pie04} and two different metallicity scales. We found that the observed values are higher (˜0.4 mag) than the canonical predictions. Moreover, the discepancy increases in the \\citet{ki03} metallicity scale and it decreases in the \\citet{cg97} scale. We investigated also the impact of CNONa extreme mixture and higher He-enhanced abundance (Y=0.30). The use of these models is not in the direction to explain the observed discrepancy.

  17. X-ray illumination of globular cluster puzzles. [globular cluster X ray sources as clues to Milky Way Galaxy age and evolution

    NASA Technical Reports Server (NTRS)

    Lightman, A. P.; Grindlay, J. E.

    1982-01-01

    Globular clusters are thought to be among the oldest objects in the Galaxy, and provide, in this connection, important clues for determining the age and process of formation of the Galaxy. The present investigation is concerned with puzzles relating to the X-ray emission of globular clusters, taking into account questions regarding the location of X-ray emitting clusters (XEGC) unusually near the galactic plane and/or galactic center. An adopted model is discussed for the nature, formation, and lifetime of X-ray sources in globular clusters. An analysis of the available data is conducted in connection with a search for correlations between binary formation time scales, central relaxation times, galactic locations, and X-ray emission. The positive correlation found between distance from galactic center and two-body binary formation time for globular clusters, explanations for this correlation, and the hypothesis that X-ray sources in globular clusters require binary star systems provide a possible explanation of the considered puzzles.

  18. Giant Rapid X-ray Flares in Extragalactic Globular Clusters

    NASA Astrophysics Data System (ADS)

    Irwin, Jimmy; Maksym, W. Peter; Romanowsky, Aaron J.; Strader, Jay; Lin, Dacheng

    2016-01-01

    There is only one known class of non-destructive, highly energetic astrophysical object in the Universe whose energy emission varies by more than a factor of 100 on time scales of less than a minute -- soft gamma repeaters/anomalous X-ray pulsars, whose flares are believed to be caused by the energy release from the cracking of a neutron star's surface by very strong magnetic fields. All other known violent, rapid explosions, including gamma-ray bursts and supernovae, are believed to destroy the object in the process. Here, we report the discovery of a second class of non-destructive, highly energetic rapidly flaring X-ray object located within two nearby galaxies with fundamentally different properties than soft gamma repeaters/anomalous X-ray pulsars. One source is located within a suspected globular cluster of the host galaxy and flared one time, while the other source is located in either a globular cluster of the host galaxy or the core of a stripped dwarf companion galaxy that flared on five occasions over a seven year time span. When not flaring, the sources appear as normal accreting neutron star or black hole X-ray binaries, indicating that the flare event does not significantly disrupt the host system. While the nature of these sources is still unclear, the discovery of these sources in decade-old archival Chandra X-ray Observatory data illustrates the under-utilization of X-ray timing as a means to discover new classes of explosive events in the Universe.

  19. Uncovering Multiple Populations in Globular Clusters with Washington Photometry

    NASA Astrophysics Data System (ADS)

    Geisler, Douglas; Cummings, Jeff; Villanova, Sandro; Carraro, Giovanni

    2015-01-01

    Globular Clusters (GCs), long considered as ideal Simple Stellar Populations, are now known to harbor a wide variety of chemical inhomogeneities. Multiple populations (MP) are being found in a growing number of Galactic globular clusters (GCs) via both photometric and spectroscopic techniques. Indeed, it has been suggested that a GC is an object that possesses MP. A definitive investigation of MP in GCs will undoubtedly provide a profound improvement in our understanding of their formation and evolution.However, most studies employ either high resolution VLT spectroscopy, HST photometry or inefficient filters from the ground. A ground-based photometric system which is both efficient and effective would be especially excellent for uncovering MP. We demonstrate that the Washington system meets these goals. The Washington C filter, in addition to being specifically designed for the purpose of detecting MPs, is both much broader and redder than competing UV filters, making it far more efficient at detecting MPs and much less sensitive to reddening and extinction.Our analysis of the well-studied GC NGC 1851 shows indeed that the C filter is both very efficient and effective at detecting its previously discovered MPs in the RGB and SGB, using relatively little telescope time on only a 1-meter telescope. Remarkably, we have also detected an intrinsically broad MS best characterized by two distinct but heavily overlapping populations that cannot be explained by binaries, field stars, or photometric errors. Detailed analysis shows that the MS distribution is in very good agreement with that seen on the RGB. This is the first time MPs in a MS have been discovered from the ground, and just as strikingly, using only a 1-meter telescope. The Washington system thus proves to be a very powerful tool for investigating MPs, and holds particular promise for extragalactic objects where photons are limited.

  20. A CHANDRA STUDY OF THE GALACTIC GLOBULAR CLUSTER OMEGA CENTAURI

    SciTech Connect

    Haggard, Daryl; Cool, Adrienne M.; Davies, Melvyn B. E-mail: cool@sfsu.edu

    2009-05-20

    We analyze a {approx}70 ks Chandra Advanced CCD Imaging Spectrometer exposure of the globular cluster {omega} Cen (NGC 5139). The {approx}17' x 17' field of view fully encompasses three core radii and almost twice the half-mass radius. We detect 180 sources to a limiting flux of {approx}4.3 x 10{sup -16} erg cm{sup -2} s{sup -1} (L{sub x} = 1.2 x 10{sup 30} erg s{sup -1} at 4.9 kpc). After accounting for the number of active galactic nuclei and possible foreground stars, we estimate that 45-70 of the sources are cluster members. Four of the X-ray sources have previously been identified as compact accreting binaries in the cluster-three cataclysmic variables (CVs) and one quiescent neutron star. Correlating the Chandra positions with known variable stars yields eight matches, of which five are probable cluster members that are likely to be binary stars with active coronae. Extrapolating these optical identifications to the remaining unidentified X-ray source population, we estimate that 20-35 of the sources are CVs and a similar number are active binaries. This likely represents most of the CVs in the cluster, but only a small fraction of all the active binaries. We place a 2{sigma} upper limit of L{sub x} < 3 x 10{sup 30} erg s{sup -1} on the integrated luminosity of any additional faint, unresolved population of sources in the core. We explore the significance of these findings in the context of primordial versus dynamical channels for CV formation. The number of CVs per unit mass in {omega} Cen is at least 2-3 times lower than in the field, suggesting that primordial binaries that would otherwise lead to CVs are being destroyed in the cluster environment.

  1. An Abundance Analysis of Red Giant Stars in the Retrograde Galactic Globular Cluster NGC 3201: Implications for Cluster Formation Scenarios

    NASA Astrophysics Data System (ADS)

    Simmerer, Jennifer A.; Ivans, I. I.

    2011-01-01

    Globular clusters have long been central to the study of Galactic Chemical Evolution. They serve as laboratories for stellar physics, evolution, and nucleosynthesis as well as representing fossil remnants of Galactic assembly processes. Our work addresses two recent areas of interest: globular clusters as accreted objects and globular clusters as hosts for multiple stellar populations. The globular cluster NGC 3201 is a curious object on a retrograde orbit. Some studies suggest that it contains stars of more than one metallicity, a property seen only in the peculiar globular cluster Omega Centauri. Both properties hint at an extra-Galactic origin. We present an elemental abundance pattern for NGC 3201 based on high resolution, high signal-to-noise spectra of red giant stars. We present abundance patterns of similar stars from the globular cluster M5 for comparison. Interpretation of our results is complicated by the discovery that at least two of our giants are variable stars. Though we can derive adequate stellar parameter solutions for both stars in every stage of variability and heavy element abundances do not change with the stellar phase, the abundances of the light elements O, Na, Mg, and Al are extremely unstable and vary greatly. Our inability to correctly model light element line formation in the atmosphere of variable red giant stars has significant implications for studies of star to star abundance variations in exactly these elements in globular clusters, which rely on stars at the same evolutionary stage as the variables in NGC 3201.

  2. A proper motion study of the globular cluster M55

    NASA Astrophysics Data System (ADS)

    Zloczewski, K.; Kaluzny, J.; Thompson, I. B.

    2011-07-01

    We have derived the absolute proper motion (PM) of the globular cluster M55 using a large set of CCD images collected with the du Pont telescope between 1997 and 2008. We find (μα cos δ, μδ) = (-3.31 ± 0.10, -9.14 ± 0.15) mas yr-1 relative to background galaxies. Membership status was determined for 16 945 stars with 14 < V < 21 from the central part of the cluster. The PM catalogue includes 52 variables, of which 43 are probable members of M55. This sample not only is dominated by pulsating blue straggler stars, but also includes five eclipsing binaries, three of which are main-sequence objects. The survey also identified several candidate blue, yellow and red straggler stars belonging to the cluster. We detected 15 likely members of the Sgr dSph galaxy located behind M55. The average PM for these stars was measured to be (μα cos δ, μδ) = (-2.23 ± 0.14, -1.83 ± 0.24) mas yr-1.

  3. The Chemical Evolution of Heavy Elements in Globular Clusters

    NASA Astrophysics Data System (ADS)

    Shingles, Luke J.; Karakas, Amanda I.; Hirschi, Raphael

    2014-01-01

    We present preliminary results from a chemical evolution model that tracks the composition of heavy elements beyond iron in a globular cluster. The heavy elements can be used as tracers of the nucleosynthetic events that defined the formation and evolution of star clusters in the early Universe. In particular, the chemical evolution model focuses on the hypothesis that rapidly-rotating massive stars produced the heavy elements via the slow neutron-capture process and seeded the proto-cluster while the stars we see today were still forming. We compare our model with heavy element abundances in M4 and M5, and M22. Our results are strongly dependent on the highly uncertain rate of the 17O(α,γ)21Ne reaction, which determines the strength of 16O as a neutron poison. We find that the [Pb/Ba] ratio is too low to match the empirical value, which might suggest that a contribution from AGB stars is required.

  4. AN ECLIPSING BLUE STRAGGLER IN THE GLOBULAR CLUSTER {omega} CENTAURI

    SciTech Connect

    Li Kai; Qian Shengbang

    2012-12-01

    {omega} Centauri is the largest globular cluster in the Milky Way and hence contains the largest number of variable stars within a single cluster. The results of photometric solutions are presented for the EA-type binary V239 in this cluster. According to our analysis, V239 is a typical Algol-type binary. We obtain M = 1.20 {+-} 0.10 M{sub Sun }, R = 1.21 {+-} 0.03 R{sub Sun }, and L = 13.68 {+-} 0.63 L{sub Sun} for the primary component. The secondary component has M = 0.07 {+-} 0.02 M{sub Sun }, R = 0.90 {+-} 0.03 R{sub Sun }, and L = 2.17 {+-} 0.14 L{sub Sun }. The binary system is located in the blue straggler region on the color-magnitude diagram of {omega} Centauri and the mass of the primary component exceeds the mass of a turnoff star. Therefore, we think that V239 is a blue straggler and that V239 was formed by mass transfer from the present secondary component to the present primary.

  5. Stromvil CCD Photometry in Globular Cluster M 3

    NASA Astrophysics Data System (ADS)

    Boyle, Richard P.; Janusz, R.; Philip, A. G. D.; Straizys, V.; Vrba, F.

    2007-12-01

    We observed the globular cluster M 3 in the 7-band Stromvil system plus Vilnius X-band at the 1.8-m Vatican Advanced Technology Telescope with a 2K CCD giving a 6-arcmin field. We observed the open cluster M 67 in the same run. Here from the residuals of many stars fit to quality CCD Vilnius photometry (Laugalys et al., 2004, Baltic Astronomy, 13, 1) we reshape and thus correct the initial flatfields. M 67 with a wide color base gives the color transformations of the run by calibrating from about 12 photoelectric standards in the Stromvil and similar Vilnius systems. In M 3 six photoelectric standards of moderate quality, all red stars of 13th magnitude in the Vilnius system calibrate the zero-point magnitude scale. Point-spread-function fitting to the stars in the crowded M 3 field resolves blends. With relatively short exposures of minutes a limiting magnitude V=15 is obtained with a signal/noise ratio about 100. So from this new photometry we subsequently can classify all types of stars and treat questions of reddening, distances, membership and metallicity at least at the horizontal branch of the cluster.

  6. Probing the faintest stars in a globular star cluster.

    PubMed

    Richer, Harvey B; Anderson, Jay; Brewer, James; Davis, Saul; Fahlman, Gregory G; Hansen, Brad M S; Hurley, Jarrod; Kalirai, Jasonjot S; King, Ivan R; Reitzel, David; Rich, R Michael; Shara, Michael M; Stetson, Peter B

    2006-08-18

    NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days of time on the Hubble Space Telescope, we have constructed an ultradeep color-magnitude diagram for this cluster. We see a clear truncation in each of its two major stellar sequences. Faint red main-sequence stars run out well above our observational limit and near to the theoretical prediction for the lowest mass stars capable of stable hydrogen burning in their cores. We also see a truncation in the number counts of faint blue stars, namely white dwarfs. This reflects the limit to which the bulk of the white dwarfs can cool over the lifetime of the cluster. There is also a turn toward bluer colors in the least luminous of these objects. This was predicted for the very coolest white dwarfs with hydrogen-rich atmospheres as the formation of H(2) and the resultant collision-induced absorption cause their atmospheres to become largely opaque to infrared radiation. PMID:16917054

  7. DYNAMICAL FORMATION OF MILLISECOND PULSARS IN GLOBULAR CLUSTERS

    SciTech Connect

    Hui, C. Y.; Cheng, K. S.; Taam, Ronald E.

    2010-05-10

    The cumulative luminosity distribution functions (CLFs) of radio millisecond pulsars (MSPs) in globular clusters (GCs) and in the Galactic field at a frequency of 1.4 GHz have been examined. Assuming a functional form, N {proportional_to} L{sup q} where N is the number of MSPs and L is the luminosity at 1.4 GHz, it is found that the CLFs significantly differ with a steeper slope, q = -0.83 {+-} 0.05, in GCs than in the Galactic field (q = -0.48 {+-} 0.04), suggesting a different formation or evolutionary history of MSPs in these two regions of the Galaxy. To probe the production mechanism of MSPs in clusters, a search of the possible relationships between the MSP population and cluster properties was carried out. The results of an investigation of nine GCs indicate positive correlations between the MSP population and the stellar encounter rate and metallicity. This provides additional evidence suggesting that stellar dynamical interactions are important in the formation of the MSP population in GCs.

  8. On the nature of the globular cluster X-ray sources

    NASA Technical Reports Server (NTRS)

    Silk, J.; Arons, J.

    1975-01-01

    It is suggested that the globular cluster X-ray sources can be interpreted in terms of mass shed by evolving postmain-sequence stars in the central regions of the globular cluster and accreting onto a massive (approximately 100-1000 solar masses) central black hole. Tentative indications that support this hypothesis include the high central escape velocities and the distribution of metallicities among the four globular clusters containing luminous X-ray sources. We argue that these X-ray sources cannot be explained as a consequence of the close capture of a nuclear burning star by a compact stellar remnant.

  9. New Breakthroughs in the Battle of the Bulge Using Globular Clusters

    NASA Astrophysics Data System (ADS)

    Geisler, D.; Mauro, F.; Bidin, C. M.; Cohen, R.; Chené, A.; Villanova, S.; Cummings, J.; Gormaz, A.; Minniti, D.; Alonso-García, J.; Hempel, M.; VVV Team

    2015-05-01

    We present some recent work undertaken mostly at the Universidad de Concepción using bulge globular clusters to better understand this important but poorly studied Galactic component, especially based on data from the VVV Survey. This includes discovering new bulge globulars, investigating dual HBs, and obtaining Ca triplet metallicities and velocities.

  10. A novel look at energy equipartition in globular clusters

    NASA Astrophysics Data System (ADS)

    Bianchini, P.; van de Ven, G.; Norris, M. A.; Schinnerer, E.; Varri, A. L.

    2016-06-01

    Two-body interactions play a major role in shaping the structural and dynamical properties of globular clusters (GCs) over their long-term evolution. In particular, GCs evolve towards a state of partial energy equipartition that induces a mass dependence in their kinematics. By using a set of Monte Carlo cluster simulations evolved in quasi-isolation, we show that the stellar mass dependence of the velocity dispersion σ(m) can be described by an exponential function σ2 ∝ exp (-m/meq), with the parameter meq quantifying the degree of partial energy equipartition of the systems. This simple parametrization successfully captures the behaviour of the velocity dispersion at lower as well as higher stellar masses, that is, the regime where the system is expected to approach full equipartition. We find a tight correlation between the degree of equipartition reached by a GC and its dynamical state, indicating that clusters that are more than about 20 core relaxation times old, have reached a maximum degree of equipartition. This equipartition-dynamical state relation can be used as a tool to characterize the relaxation condition of a cluster with a kinematic measure of the meq parameter. Vice versa, the mass dependence of the kinematics can be predicted knowing the relaxation time solely on the basis of photometric measurements. Moreover, any deviations from this tight relation could be used as a probe of a peculiar dynamical history of a cluster. Finally, our novel approach is important for the interpretation of state-of-the-art Hubble Space Telescope proper motion data, for which the mass dependence of kinematics can now be measured, and for the application of modelling techniques which take into consideration multimass components and mass segregation.

  11. POTASSIUM IN GLOBULAR CLUSTER STARS: COMPARING NORMAL CLUSTERS TO THE PECULIAR CLUSTER NGC 2419

    SciTech Connect

    Carretta, E.; Bragaglia, A.; Sollima, A.; Gratton, R. G.; Lucatello, S.; D'Orazi, V.; Sneden, C. E-mail: angela.bragaglia@oabo.inaf.it E-mail: raffaele.gratton@oapd.inaf.it E-mail: valentina.dorazi@mq.edu.au

    2013-05-20

    Two independent studies recently uncovered two distinct populations among giants in the distant, massive globular cluster (GC) NGC 2419. One of these populations has normal magnesium (Mg) and potassium (K) abundances for halo stars: enhanced Mg and roughly solar K. The other population has extremely depleted Mg and very enhanced K. To better anchor the peculiar NGC 2419 chemical composition, we have investigated the behavior of K in a few red giant branch stars in NGC 6752, NGC 6121, NGC 1904, and {omega} Cen. To verify that the high K abundances are intrinsic and not due to some atmospheric features in giants, we also derived K abundances in less evolved turn-off and subgiant stars of clusters 47 Tuc, NGC 6752, NGC 6397, and NGC 7099. We normalized the K abundance as a function of the cluster metallicity using 21 field stars analyzed in a homogeneous manner. For all GCs of our sample, the stars lie in the K-Mg abundance plane on the same locus occupied by the Mg-normal population in NGC 2419 and by field stars. This holds for both giants and less-evolved stars. At present, NGC 2419 seems unique among GCs.

  12. A study of rotating globular clusters. The case of the old, metal-poor globular cluster NGC 4372

    NASA Astrophysics Data System (ADS)

    Kacharov, N.; Bianchini, P.; Koch, A.; Frank, M. J.; Martin, N. F.; van de Ven, G.; Puzia, T. H.; McDonald, I.; Johnson, C. I.; Zijlstra, A. A.

    2014-07-01

    Context. NGC 4372 is a poorly studied old, very metal-poor globular cluster (GC) located in the inner Milky Way halo. Aims: We present the first in-depth study of the kinematic properties and derive the structural parameters of NGC 4372 based on the fit of a Plummer profile and a rotating, physical model. We explore the link between internal rotation to different cluster properties and together with similar studies of more GCs, we put these in the context of globular cluster formation and evolution. Methods: We present radial velocities for 131 cluster member stars measured from high-resolution FLAMES/GIRAFFE observations. Their membership to the GC is additionally confirmed from precise metallicity estimates. We build a velocity dispersion profile and a systemic rotation curve using this kinematic data set. Additionally, we obtain an elliptical number density profile of NGC 4372 based on optical images using a Markov chain Monte Carlo fitting algorithm. From this, we derive the cluster's half-light radius and ellipticity as rh = 3.44' ± 0.04' and ɛ = 0.08 ± 0.01. Finally, we give a physical interpretation of the observed morphological and kinematic properties of this GC by fitting an axisymmetric, differentially rotating, dynamical model. Results: Our results show that NGC 4372 has an unusually high ratio of rotation amplitude to velocity dispersion (1.2 vs. 4.5 km s-1) for its metallicity. This puts it in line, however, with two other exceptional, very metal-poor GCs: M 15 and NGC 4590. We also find a mild flattening of NGC 4372 in the direction of its rotation. Given its old age, this suggests that the flattening is indeed caused by the systemic rotation rather than tidal interactions with the Galaxy. Additionally, we estimate the dynamical mass of the GC Mdyn = 2.0 ± 0.5 × 105M⊙ based on the dynamical model, which constrains the mass-to-light ratio of NGC 4372 between 1.4 and 2.3 M⊙/L⊙, representative of an old, purely stellar population. Based on

  13. The Rich Globular Cluster System of Abell 1689 and the Radial Dependence of the Globular Cluster Formation Efficiency

    NASA Astrophysics Data System (ADS)

    Alamo-Martínez, K. A.; Blakeslee, J. P.; Jee, M. J.; Côté, P.; Ferrarese, L.; González-Lópezlira, R. A.; Jordán, A.; Meurer, G. R.; Peng, E. W.; West, M. J.

    2013-09-01

    We study the rich globular cluster (GC) system in the center of the massive cluster of galaxies Abell 1689 (z = 0.18), one of the most powerful gravitational lenses known. With 28 Hubble Space Telescope/Advanced Camera for Surveys orbits in the F814W bandpass, we reach a magnitude I 814 = 29 with gsim90% completeness and sample the brightest ~5% of the GC system. Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we estimate a total population of N^total_GC = 162{,}850^{+75,450}_{-51,310} GCs within a projected radius of 400 kpc. As many as half of the GCs may comprise an intracluster component. Even with the sizable uncertainties, which mainly result from the uncertain GCLF parameters, this system is by far the largest GC population studied to date. The specific frequency SN is high, but not uncommon for central galaxies in massive clusters, rising from SN ≈ 5 near the center to ~12 at large radii. Passive galaxy fading would increase SN by ~20% at z = 0. We construct the radial mass profiles of the GCs, stars, intracluster gas, and lensing-derived total mass, and we compare the mass fractions as a function of radius. The estimated mass in GCs, {M}_GC^total = 3.9 × 1010 M ⊙, is comparable to ~80% of the total stellar mass of the Milky Way. The shape of the GC mass profile appears intermediate between those of the stellar light and total cluster mass. Despite the extreme nature of this system, the ratios of the GC mass to the baryonic and total masses, and thus the GC formation efficiency, are typical of those in other rich clusters when comparing at the same physical radii. The GC formation efficiency is not constant, but varies with radius, in a manner that appears similar for different clusters; we speculate on the reasons for this similarity in profile.

  14. The colour-magnitude relation of globular clusters in Centaurus and Hydra. Constraints on star cluster self-enrichment with a link to massive Milky Way globular clusters

    NASA Astrophysics Data System (ADS)

    Fensch, J.; Mieske, S.; Müller-Seidlitz, J.; Hilker, M.

    2014-07-01

    Aims: We investigate the colour-magnitude relation of metal-poor globular clusters, the so-called blue tilt, in the Hydra and Centaurus galaxy clusters and constrain the primordial conditions for star cluster self-enrichment. Methods: We analyse U,I photometry for about 2500 globular clusters in the central regions of Hydra and Centaurus, based on VLT/FORS1 data. We measure the relation between mean colour and luminosity for the blue and red subpopulation of the globular cluster samples. We convert these relations into mass-metallicity space and compare the obtained GC mass-metallicity relation with predictions from the star cluster self-enrichment model by Bailin & Harris (2009, ApJ, 695, 1082). For this we include effects of dynamical and stellar evolution and a physically well motivated primordial mass-radius scaling. Results: We obtain a mass-metallicity scaling of Z ∝ M0.27 ± 0.05 for Centaurus GCs and Z ∝ M0.40 ± 0.06 for Hydra GCs, consistent with the range of observed relations in other environments. We find that the GC mass-metallicity relation already sets in at present-day masses of a few and is well established in the luminosity range of massive MW clusters like ω Centauri. The inclusion of a primordial mass-radius scaling of star clusters significantly improves the fit of the self-enrichment model to the data. The self-enrichment model accurately reproduces the observed relations for average primordial half-light radii rh ~ 1-1.5 pc, star formation efficiencies f⋆ ~ 0.3-0.4, and pre-enrichment levels of [Fe/H] - 1.7 dex. The slightly steeper blue tilt for Hydra can be explained either by a ~30% smaller average rh at fixed f⋆ ~ 0.3, or analogously by a ~20% smaller f⋆ at fixed rh ~ 1.5 pc. Within the self-enrichment scenario, the observed blue tilt implies a correlation between GC mass and width of the stellar metallicity distribution. We find that this implied correlation matches the trend of width with GC mass measured in Galactic GCs

  15. STELLAR COLLISIONS AND BLUE STRAGGLER STARS IN DENSE GLOBULAR CLUSTERS

    SciTech Connect

    Chatterjee, Sourav; Rasio, Frederic A.; Sills, Alison; Glebbeek, Evert

    2013-11-10

    Blue straggler stars (BSSs) are abundantly observed in all Galactic globular clusters (GGCs) where data exist. However, observations alone cannot reveal the relative importance of various formation channels or the typical formation times for this well-studied population of anomalous stars. Using a state-of-the-art Hénon-type Monte Carlo code that includes all relevant physical processes, we create 128 models with properties typical of the observed GGCs. These models include realistic numbers of single and binary stars, use observationally motivated initial conditions, and span large ranges in central density, concentration, binary fraction, and mass. Their properties can be directly compared with those of observed GGCs. We can easily identify the BSSs in our models and determine their formation channels and birth times. We find that for central densities above ∼10{sup 3} M{sub ☉} pc{sup –3}, the dominant formation channel is stellar collisions, while for lower density clusters, mass transfer in binaries provides a significant contribution (up to 60% in our models). The majority of these collisions are binary-mediated, occurring during three-body and four-body interactions. As a result, a strong correlation between the specific frequency of BSSs and the binary fraction in a cluster can be seen in our models. We find that the number of BSSs in the core shows only a weak correlation with the collision rate estimator Γ traditionally used by observers, in agreement with the latest Hubble Space Telescope Advanced Camera for Surveys data. Using an idealized 'full mixing' prescription for collision products, our models indicate that the BSSs observed today may have formed several Gyr ago. However, denser clusters tend to have younger (∼1 Gyr) BSSs.

  16. HST Images of Seyfert's Sextet: The Candidate Globular Cluster Population

    NASA Astrophysics Data System (ADS)

    Palma, C.; Zonak, S. G.; Hunsberger, S. D.; Durrell, P. R.; Gallagher, S. C.; Charlton, J. C.; English, J.

    2001-12-01

    We present Hubble Space Telescope images of the dense galaxy group, Seyfert's Sextet (Hickson Compact Group 79). This group includes four galaxies with concordant redshifts, a fifth background galaxy, and a bright tidal feature. From UBVI (F336W, F439W, F555W, and F814W) images, we identify globular cluster candidates (GCCs) associated with both the interacting galaxies and the lower surface brightness tidal features. The population of GCCs is compared to both stellar population models and to other interacting groups of galaxies, such as Stephan's Quintet, the Antennae, and other Toomre sequence mergers. Few point sources are detected in the U filter, which indicates that it is unlikely for there to have been much star cluster formation within the past 100 Myr or so. However, a number of sources are detected in B, V, and I. The GCC colors are used to constrain their ages and to elicit information on the star formation history of this interacting group. We gratefully acknowledge support from the NSF (grant AST 00-71223) and from STScI (grant HST-GO-08717.04-A).

  17. Formation of Short-Period Binary Pulsars in Globular Clusters.

    PubMed

    Rasio; Pfahl; Rappaport

    2000-03-20

    We present a new dynamical scenario for the formation of short-period binary millisecond pulsars in globular clusters. Our work is motivated by the recent observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc, most neutron stars acquire binary companions through exchange interactions with primordial binaries. The resulting systems have semimajor axes in the range approximately 0.1-1 AU and neutron star companion masses approximately 1-3 M middle dot in circle. For many of these systems, we find that when the companion evolves off the main sequence and fills its Roche lobe, the subsequent mass transfer is dynamically unstable. This leads to a common envelope phase and the formation of short-period neutron star-white dwarf binaries. For a significant fraction of these binaries, the decay of the orbit due to gravitational radiation will be followed by a period of stable mass transfer driven by a combination of gravitational radiation and tidal heating of the companion. The properties of the resulting short-period binaries match well those of observed binary pulsars in 47 Tuc. PMID:10702129

  18. The metal content of the bulge globular cluster NGC 6528

    NASA Astrophysics Data System (ADS)

    Zoccali, M.; Barbuy, B.; Hill, V.; Ortolani, S.; Renzini, A.; Bica, E.; Momany, Y.; Pasquini, L.; Minniti, D.; Rich, R. M.

    2004-08-01

    High resolution spectra of five stars in the bulge globular cluster NGC 6528 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph. Out of the five stars, two of them showed evidence of binarity. The target stars belong to the horizontal and red giant branch stages, at 4000 < Tefflt; 4800 K. Multiband V, I, J, H, Ks photometry was used to derive initial effective temperatures and gravities. The main purpose of this study is the determination of metallicity and elemental ratios for this template bulge cluster, as a basis for the fundamental calibration of metal-rich populations. The present analysis provides a metallicity [Fe/H] = -0.1±0.2 and the α-elements O, Mg and Si, show [α/Fe] ≈ +0.1, whereas Ca and Ti are around the solar value or below, resulting in an overall metallicity Z ≈ Z⊙. Observations collected both at the European Southern Observatory, Paranal and La Silla, Chile (ESO programme 65.L-0340) and with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, operated by AURA Inc. under contract to NASA. Tables \\ref{targets}, \\ref{logobs}, \\ref{tablines} and Fig. \\ref{chart} are only available in electronic form at http://www.edpsciences.org

  19. The ATCA Survey for Black Holes in Southern Globular Clusters

    NASA Astrophysics Data System (ADS)

    Strader, Jay; Miller-Jones, James; Maccarone, Tom; Chomiuk, Laura

    2013-10-01

    Hundreds of stellar-mass black holes (BHs) are expected to form in the early lives of most globular star clusters (GCs), mass segregate to the cluster center, and largely be ejected through mutual gravitational interactions. Observations by our group have provided the first strong evidence for stellar-mass BHs in Galactic GCs. The key advance has been the use of radio continuum observations sensitive to BHs with low mass accretion rates. The existence of stellar-mass BHs in GCs would have broad implications: (i) GCs would become important hunting grounds for stellar-mass BHs; (ii) BHs in GCs would offer tests of the physics of low-luminosity accretion; (iii) GCs might offer a less biased view of the BH mass function than the field; and (iv) the prospects for detecting gravitational wave sources such as BH--BH or BH--pulsar binaries would be improved. The same radio observations can also reveal the presence of intermediate-mass black holes (IMBHs) in the centers of GCs. The dynamical evidence for IMBHs is hard to interpret uniquely; IMBHs should also be detectable through accretion. Our radio observations have placed the strongest limits to date on the presence of IMBHs in several Galactic GCs. We have begun a deep, systematic radio continuum survey for BHs in Galactic GCs. Here we propose ATCA data for a pilot sample of five southern GCs. With well-defined selection criteria, our sample will allow the first statistical determination of the presence of BHs in GCs.

  20. 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

  1. The extended stellar substructures of four metal-poor globular clusters in the Galactic bulge

    NASA Astrophysics Data System (ADS)

    Chun, Sang-Hyun; Sohn, Young-Jong

    2016-08-01

    We investigated the stellar density substructures around four metal-poor globular clusters (NGC 6266, NGC 6626, NGC 6642, and NGC 6723) in the Galactic bulge. Wide-field near-infrared (JHK s ) imaging data were obtained from WFCAM of UKIRT telescope. Field stars contamination around the globular clusters was reduced by using a statistical weighted filtering algorithm. Tidal stripping stellar substructures in the form of tidal tail (NGC 6266 and NGC 6626) or small density lobes/chunk (NGC 6642 and NGC 6723) were found around the four globular clusters in the two-dimensional density contour maps. We also find the overdensity features, which deviate from the theoretical models, in the outer region of radial density profiles. The observed results imply that the four globular clusters have experienced a strong tidal force or the bulge/disk shock effect of the Galaxy.

  2. 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.

  3. Giant X-ray Flares From Suspected Black Holes in Extragalactic Globular Clusters

    NASA Astrophysics Data System (ADS)

    Irwin, Jimmy; Speegle, Tyler; Prado, Ian; Mildebrath, David; Romanowsky, Aaron J.; Strader, Jay

    2014-08-01

    The existence of both stellar- and intermediate-mass black holes within globular clusters has been the subject of intense debate for decades. The rich globular cluster populations of nearby elliptical galaxies provide much more fertile hunting grounds over the meager globular cluster population of the Milky Way to search for accreting black holes emitting near their Eddington limit. Extreme X-ray variability of >1e39 ergs/s sources provide the best means of identifying such black holes. We present results from our search for short-term (< few hours) X-ray flares from extragalactic globular clusters. Interesting candidates include a source that flared to 8e40 ergs/s on a ~1 minute time scale, and another source that flared by a factor of 50 to 2e39 ergs/s for several hundred seconds in four different Chandra epochs.

  4. The Universe and Globular Clusters: An Age Conflict?

    NASA Astrophysics Data System (ADS)

    D'Antona, F.; Caloi, V.; Mazzitelli, I.

    1997-03-01

    Chaboyer (1995) has recently shown that the range of possible ages of globular clusters, τ ~ 14-18 Gyr, corresponding to different choices of distance scales, is further enlarged mainly by uncertainties in modeling convection. Within the mixing-length theory (MLT) the result is τ = 11-21 Gyr. Bolte & Hogan (1995) adopted the field subdwarfs as indicators of the distance scale and, assuming that τ is insensitive to the treatment of convection, found that τ = (15.8 +/- 2.1) Gyr. Within Chaboyer's findings, it is marginally possible to avoid an ``age conflict'' with the expansion age of the universe, even if this latter is t0 = 8-11 Gyr, while this is not the case with the Bolte & Hogan (1995) conclusion. Mazzitelli, D'Antona, & Caloi (1995) have implemented updated macro- and microphysics inputs and determined the distance scale by fitting the horizontal-branch models to the horizontal branch of metal-poor globular clusters. Within the MLT, they find τ ~= 14 Gyr. Switching from MLT to a full spectrum of turbulence (FST) model of convection (Canuto & Mazzitelli 1991) lowers the value of τ by 1-2 Gyr. Since the Mazzitelli et al. (1995) results would alleviate the age conflict to the point of making it disappear, it is important to assess their validity by answering the following questions. Independently of the assumptions on the distance scale: 1. What is the role of convection near turnoff on both its shape and luminosity? Chaboyer (1995) and Mazzitelli et al. (1995) find an appreciable effect, while Bolte & Hogan (1995) a priori assume no such effect. 2. How meaningful is the fit of the red giant branch in the determination of τ?In this paper, we answer question (1) carrying out extensive new stellar model calculations. When we assume the long distance scale, the FST results differ from the MLT values both in the shape and in the luminosity of the turnoff, while there is no appreciable difference if one employs the short distance scale. Helium sedimentation

  5. Estimated number of field stars toward Galactic globular clusters and Local Group Galaxies

    NASA Technical Reports Server (NTRS)

    Ratnatunga, K. U.; Bahcall, J. N.

    1985-01-01

    Field star densities are estimated for 89 fields with /b/ greater than 10 degrees based on the Galaxy model of Bahcall and Soneira (1980, 1984; Bahcall et al. 1985). Calculated tables are presented for 76 of the fields toward Galactic globular clusters, and 16 Local Group Galaxies in 13 fields. The estimates can be used as an initial guide for planning both ground-based and Space Telescope observations of globular clusters at intermediate-to-high Galactic latitudes.

  6. Bayesian Analysis of Two Stellar Populations in Galactic Globular Clusters III: Analysis of 30 Clusters

    NASA Astrophysics Data System (ADS)

    Wagner-Kaiser, R.; Stenning, D. C.; Sarajedini, A.; von Hippel, T.; van Dyk, D. A.; Robinson, E.; Stein, N.; Jefferys, W. H.

    2016-09-01

    We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival ACS Treasury observations of 30 Galactic Globular Clusters to characterize two distinct stellar populations. A sophisticated Bayesian technique is employed to simultaneously sample the joint posterior distribution of age, distance, and extinction for each cluster, as well as unique helium values for two populations within each cluster and the relative proportion of those populations. We find the helium differences among the two populations in the clusters fall in the range of ˜0.04 to 0.11. Because adequate models varying in CNO are not presently available, we view these spreads as upper limits and present them with statistical rather than observational uncertainties. Evidence supports previous studies suggesting an increase in helium content concurrent with increasing mass of the cluster and also find that the proportion of the first population of stars increases with mass as well. Our results are examined in the context of proposed globular cluster formation scenarios. Additionally, we leverage our Bayesian technique to shed light on inconsistencies between the theoretical models and the observed data.

  7. The incidence of binaries in globular cluster stellar populations

    NASA Astrophysics Data System (ADS)

    Lucatello, S.; Sollima, A.; Gratton, R.; Vesperini, E.; D'Orazi, V.; Carretta, E.; Bragaglia, A.

    2015-12-01

    Binary fraction and orbital characteristics provide indications on the conditions of star formation, as they shed light on the environment they were born in. Multiple systems are more common in low density environments than in higher density environments. In the current debate about the formation of globular clusters and their multiple populations, studying the binary incidence in the populations they host offers a crucial piece of information on the environment of their birth and their subsequent dynamical evolution. Through a multiyear observational campaign using FLAMES at VLT, we monitored the radial velocity of 968 red-giant-branch stars located around the half-light radii in a sample of ten Galactic globular clusters. We found a total of 21 radial velocity variables identified as bona fide binary stars, for a binary fraction of 2.2% ± 0.5%. When separating the sample into first generation and second generation stars, we find a binary fraction of 4.9% ± 1.3% and 1.2% ± 0.4%, respectively. Through simulations that take possible sources of bias into account in detecting radial velocity variations in the two populations, we show that the difference is significant and only marginally affected by these effects. This kind of different binary fraction strongly suggests different conditions in the environment of formation and evolution of first and second generations stars, with the latter being born in a much denser environment. Our result hence strongly supports the idea that the second generation forms in a dense subsystem at the center of the loosely distributed first generation, where (loose) binaries are efficiently destroyed. Based on data obtained with the Very Large Telescope at the European Southern Observatory, programs: 073.D-0100, 073.D-0211 and 083.D-0208.Full Tables 1, 3, and table of the individual radial velocities are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc

  8. Application of three-body stability to globular clusters - I. The stability radius

    NASA Astrophysics Data System (ADS)

    Kennedy, Gareth F.

    2014-11-01

    The tidal radius is commonly determined analytically by equating the tidal field of the galaxy to the gravitational potential of the cluster. Stars crossing this radius can move from orbiting the cluster centre to independently orbiting the galaxy. In this paper, the stability radius of a globular cluster is estimated using a novel approach from the theoretical standpoint of the general three-body problem. This is achieved by an analytical formula for the transition radius between stable and unstable orbits in a globular cluster. A stability analysis, outlined by Mardling, is used here to predict the occurrence of unstable stellar orbits in the outermost region of a globular cluster in a distant orbit around a galaxy. It is found that the eccentricity of the cluster-galaxy orbit has a far more significant effect on the stability radius of globular clusters than previous theoretical results of the tidal radius have found. A simple analytical formula is given for determining the transition between stable and unstable orbits, which is analogous to the tidal radius for a globular cluster. The stability radius estimate is interior to tidal radius estimates and gives the innermost region from which stars can random walk to their eventual escape from the cluster. The time-scale for this random walk process is also estimated using numerical three-body scattering experiments.

  9. Mapping Tidal Streams and Tails around Galactic Globular Clusters using RAVE

    NASA Astrophysics Data System (ADS)

    Kunder, Andrea; Steinmetz, M.; RAVE Collaboration

    2014-01-01

    Stellar population studies of globular clusters have suggested that the brightest globular clusters in the Galaxy are actually the remnant nuclei of dwarf spheroidal galaxies. If present Galactic globular clusters formed within larger stellar systems, they are likely surrounded by extra-tidal halos and tails made up of stars that were tidally stripped from their parent systems. Also, they would have lost the majority fraction of the initial mass due to their internal and external dynamical effects, such as tidal heating and stripping. This information suggests that surroundings around globular clusters can provide an excellent example of such a structure. We use the Radial Velocity Experiment (RAVE) to search for signatures of tidal tails around the globular clusters prominently featured in the extensive RAVE footprint. Stars with RAVE metallicities, radial velocities and proper motions consistent with the abundance patterns and properties of the cluster are presented for Omega Centauri, NGC 3201, NGC 362, NGC 2808 and NGC 1851. The bright magnitudes of these stars make them easy targets for high resolution follow-up observations, allowing us to carry out chemical tagging to identify (or exclude) stars outside the tidal radius of the cluster as tidal debris. As these clusters are well studied with accurate abundances and distances, the RAVE stars located within the tidal radius of these clusters will also aid in the improvement of the stellar parameters and abundances extracted from the RAVE spectra.

  10. The onset of gravothermal oscillations in globular cluster evolution

    NASA Technical Reports Server (NTRS)

    Breeden, Joseph L.; Cohn, Haldan N.; Hut, Piet

    1994-01-01

    We have carried out an extensive set of Fokker-Planck simulations of the evolution of globular clusters on very long timescales, up to 600 times the initial core collapse time t(sub cc). We consider an idealized equal mass star cluster, with a wide range of values for the total number of stars, 7000 less than N less than 2 x 10(exp 6). Our models include the heating effect of compact binaries formed in three-body encounters, which halts the initial core collapse and drives a core reexpansion. Postcollapse gravothermal oscillations of the cluster core are found to occur for all N approximately greater than or equal to 8000. For 8000 approximately less than or equal to N approximately less than or equal to 11,000, the oscillation has a simple, regular waveform with a single, well-defined period. For N approximately equals 12,000, the oscillations become nonlinear in a process resembling a period doubling. For N approximately greater than or equal to 14,000, the waveform of the oscillations becomes increasingly more irregular with increasing N, resembling chaotic behavior for N approximately greater than or equal to 15,000. During the oscillations, the core radius and core mass vary dramatically: by more than a factor of 10 for N greater than 15,000, by more than a factor of 100 for N greater than 5 x 10(exp 4), and by more than a factor of 1000 for N greater than 5 x 10(exp 5). However, even during the times of maximum expansion, the core contains only a small fraction of the cluster mass. For most N values, the maximum core mass at any time after core collapse is less than 1% of the cluster mass. The exceptions lie in the range 5 x 10(exp 4) approximately less than or equal to N approximately equal to or less than 2 x 10(exp 5), where the maximum post-collapse core mass reaches approximately 2% of the cluster mass. We discuss the observational implications of these predictions.

  11. ON THE FORMATION OF MULTIPLE STELLAR POPULATIONS IN GLOBULAR CLUSTERS

    SciTech Connect

    Charlie, Conroy; Spergel, David N.

    2011-01-01

    Nearly all globular clusters (GCs) studied to date show evidence for multiple stellar populations, in stark contrast to the conventional view that GCs are a mono-metallic, coeval population of stars. This generic feature must therefore emerge naturally within massive star cluster formation. Building on earlier work, we propose a simple physical model for the early evolution (several 10{sup 8} yr) of GCs. We consider the effects of stellar mass loss, Type II supernovae (SNe II) and prompt Type Ia supernovae (SNe Ia), ram pressure, and accretion from the ambient interstellar medium (ISM) on the development of a young GC's own gas reservoir. In our model, SNe II from a first generation of star formation clears the GC of its initial gas reservoir. Over the next several 10{sup 8} yr, mass lost from asymptotic giant branch stars and matter accreted from the ambient ISM collect at the center of the GC. This material must remain quite cool (T {approx} 10{sup 2} K), but does not catastrophically cool on a crossing time because of the high Lyman-Werner flux density in young GCs. The collection of gas within the GC must compete with ram pressure from the ambient ISM. After several 10{sup 8} yr, the Lyman-Werner photon flux density drops by more than three orders of magnitude, allowing molecular hydrogen and then stars to form. After this second generation of star formation, SNe II from the second generation and then prompt SNe Ia associated with the first generation maintain a gas-free GC, thereby ending the cycle of star formation events. Our model makes clear predictions for the presence or absence of multiple stellar populations within GCs as a function of GC mass and formation environment. While providing a natural explanation for the approximately equal number of first- and second-generation stars in GCs, substantial accretion from the ambient ISM may produce fewer chemically peculiar second-generation stars than are observed. Analyzing intermediate-age LMC clusters, we

  12. Galactic bulge X-ray burst sources from disrupted globular clusters?

    NASA Technical Reports Server (NTRS)

    Grindlay, J. E.; Hertz, P.

    1985-01-01

    The origin of the bright galactic bulge X-ray sources, or GX sources, is unclear despite intensive study for the past 15 years. It is suggested that the fact that many (or most) of the GX sources are X-ray burst sources (GXRBS) and are otherwise apparently identical to the luminous X-ray sources found in globular cluster cores implies that they too may have a globular cluster origin. The possibility that the compact X-ray binaries found in globulars are ejected is constrained by observations of CVs in and out of clusters. The GXRBS are instead hypothesized to have been formed by capture processes in globular clusters which have now largely been disrupted by repeated tidal stripping and shocking in the galactic plane. A statistical analysis of the 12 GXRBS which have precise positions from Einstein and/or optical (or radio) observations indicate that it is probably significant that a bright, of less than about 19, G or K star is found within the error circle (3 arcmin radius) in four cases. These may be surviving giants in a disrupted globular cluster core. Implications for globular cluster evolution and the GXRBS themselves are discussed.

  13. The Dynamical Evolution of Stellar Black Holes in Globular Clusters

    NASA Astrophysics Data System (ADS)

    Morscher, Meagan; Pattabiraman, Bharath; Rodriguez, Carl; Rasio, Frederic A.; Umbreit, Stefan

    2015-02-01

    Our current understanding of the stellar initial mass function and massive star evolution suggests that young globular clusters (GCs) may have formed hundreds to thousands of stellar-mass black holes (BHs), the remnants of stars with initial masses from ~20-100 M ⊙. Birth kicks from supernova explosions may eject some BHs from their birth clusters, but most should be retained. Using a Monte Carlo method we investigate the long-term dynamical evolution of GCs containing large numbers of stellar BHs. We describe numerical results for 42 models, covering a broad range of realistic initial conditions, including up to 1.6 × 106 stars. In almost all models we find that significant numbers of BHs (up to ~103) are retained all the way to the present. This is in contrast to previous theoretical expectations that most BHs should be ejected dynamically within a few gigayears The main reason for this difference is that core collapse driven by BHs (through the Spitzer "mass segregation instability") is easily reverted through three-body processes, and involves only a small number of the most massive BHs, while lower-mass BHs remain well-mixed with ordinary stars far from the central cusp. Thus the rapid segregation of stellar BHs does not lead to a long-term physical separation of most BHs into a dynamically decoupled inner core, as often assumed previously. Combined with the recent detections of several BH X-ray binary candidates in Galactic GCs, our results suggest that stellar BHs could still be present in large numbers in many GCs today, and that they may play a significant role in shaping the long-term dynamical evolution and the present-day dynamical structure of many clusters.

  14. THE DYNAMICAL EVOLUTION OF STELLAR BLACK HOLES IN GLOBULAR CLUSTERS

    SciTech Connect

    Morscher, Meagan; Pattabiraman, Bharath; Rodriguez, Carl; Rasio, Frederic A.; Umbreit, Stefan

    2015-02-10

    Our current understanding of the stellar initial mass function and massive star evolution suggests that young globular clusters (GCs) may have formed hundreds to thousands of stellar-mass black holes (BHs), the remnants of stars with initial masses from ∼20-100 M {sub ☉}. Birth kicks from supernova explosions may eject some BHs from their birth clusters, but most should be retained. Using a Monte Carlo method we investigate the long-term dynamical evolution of GCs containing large numbers of stellar BHs. We describe numerical results for 42 models, covering a broad range of realistic initial conditions, including up to 1.6 × 10{sup 6} stars. In almost all models we find that significant numbers of BHs (up to ∼10{sup 3}) are retained all the way to the present. This is in contrast to previous theoretical expectations that most BHs should be ejected dynamically within a few gigayears The main reason for this difference is that core collapse driven by BHs (through the Spitzer {sup m}ass segregation instability{sup )} is easily reverted through three-body processes, and involves only a small number of the most massive BHs, while lower-mass BHs remain well-mixed with ordinary stars far from the central cusp. Thus the rapid segregation of stellar BHs does not lead to a long-term physical separation of most BHs into a dynamically decoupled inner core, as often assumed previously. Combined with the recent detections of several BH X-ray binary candidates in Galactic GCs, our results suggest that stellar BHs could still be present in large numbers in many GCs today, and that they may play a significant role in shaping the long-term dynamical evolution and the present-day dynamical structure of many clusters.

  15. Terzan 8: a Sagittarius-flavoured globular cluster

    NASA Astrophysics Data System (ADS)

    Carretta, E.; Bragaglia, A.; Gratton, R. G.; D'Orazi, V.; Lucatello, S.; Sollima, A.

    2014-01-01

    Massive globular clusters (GCs) contain at least two generations of stars with slightly different ages and clearly distinct light element abundances. The Na-O anticorrelation is the best studied chemical signature of multiple stellar generations. Instead, low-mass clusters usually appear to be chemically homogeneous. We are investigating low-mass GCs to understand what the lower mass limit is where multiple populations can form, mainly using the Na and O abundance distribution. We used VLT/FLAMES spectra of giants in the low-mass, metal-poor GC Terzan 8 that belongs to the Sagittarius dwarf galaxy to determine abundances of Fe, O, Na, α-, Fe-peak, and neutron-capture elements in six stars observed with UVES and 14 observed with GIRAFFE. The average metallicity is [Fe/H] = -2.27 ± 0.03 (rms = 0.08), based on the six high-resolution UVES spectra. Only one star, observed with GIRAFFE, shows an enhanced abundance of Na and we tentatively assign it to the second generation. In this cluster, unlike what happens in more massive GCs, the second generation seems to represent at most a minority fraction. We discuss the implications of our findings, comparing Terzan 8 with the other Sgr dSph GCs, and to GCs and field stars in the Large Magellanic Cloud, Fornax, and in other dwarfs galaxies. Based on observations collected at ESO telescopes under programme 087.B-0086Tables 2, 3, 7-10 are available in electronic form at http://www.aanda.orgFull 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/561/A87

  16. Ruprecht 106: The First Single Population Globular Cluster?

    NASA Astrophysics Data System (ADS)

    Villanova, S.; Geisler, D.; Carraro, G.; Moni Bidin, C.; Muñoz, C.

    2013-12-01

    All old Galactic globular clusters (GCs) studied in detail to date host at least two generations of stars, where the second is formed from gas polluted by processed material produced by massive stars of the first. This process can happen if the initial mass of the cluster exceeds a threshold above which ejecta are retained and a second generation is formed. A determination of this mass threshold is mandatory in order to understand how GCs form. We analyzed nine red giant branch stars belonging to the cluster Ruprecht 106. Targets were observed with the UVES@VLT2 spectrograph. Spectra cover a wide range and allowed us to measure abundances for light (O, Na, Mg, Al), α (Si, Ca, Ti), iron-peak (Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), and neutron-capture (Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Dy, Pb) elements. Based on these abundances, we show that Ruprecht 106 is the first convincing example of a single-population GC (i.e., a true simple stellar population), although the sample is relatively small. This result is supported also by an independent photometric test and by the horizontal branch morphology and the dynamical state. It is old (~12 Gyr) and, at odds with other GCs, has no α-enhancement. The material it formed from was contaminated by both s- and r-process elements. The abundance pattern points toward an extragalactic origin. Its present-day mass (M = 104.83 M ⊙) can be assumed as a strong lower limit for the initial mass threshold below which no second generation is formed. Clearly, its initial mass must have been significantly greater, but we have no current constraints on the amount of mass loss during its evolution.

  17. On the Density Profile of the Globular Cluster M92

    NASA Astrophysics Data System (ADS)

    Di Cecco, A.; Zocchi, A.; Varri, A. L.; Monelli, M.; Bertin, G.; Bono, G.; Stetson, P. B.; Nonino, M.; Buonanno, R.; Ferraro, I.; Iannicola, G.; Kunder, A.; Walker, A. R.

    2013-04-01

    We present new number density and surface brightness profiles for the globular cluster M92 (NGC 6341). These profiles are calculated from optical images collected with the CCD mosaic camera MegaCam at the Canada-France-Hawaii Telescope and with the Advanced Camera for Surveys on the Hubble Space Telescope. The ground-based data were supplemented with the Sloan Digital Sky Survey photometric catalog. Special care was taken to discriminate candidate cluster stars from field stars and to subtract the background contamination from both profiles. By examining the contour levels of the number density, we found that the stellar distribution becomes clumpy at radial distances larger than ~13', and there is no preferred orientation of contours in space. We performed detailed fits of King and Wilson models to the observed profiles. The best-fit models underestimate the number density inside the core radius. Wilson models better represent the observations, in particular in the outermost cluster regions: the good global agreement of these models with the observations suggests that there is no need to introduce an extra-tidal halo to explain the radial distribution of stars at large radial distances. The best-fit models for the number density and the surface brightness profiles are different, even though they are based on the same observations. Additional tests support the evidence that this fact reflects the difference in the radial distribution of the stellar tracers that determine the observed profiles (main-sequence stars for the number density, bright evolved stars for the surface brightness). Based in part on data obtained from the ST-ECF Science Archive Facility. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency.

  18. Ruprecht 106: The first single population globular cluster?

    SciTech Connect

    Villanova, S.; Geisler, D.; Muñoz, C.; Carraro, G.; Moni Bidin, C.

    2013-12-01

    All old Galactic globular clusters (GCs) studied in detail to date host at least two generations of stars, where the second is formed from gas polluted by processed material produced by massive stars of the first. This process can happen if the initial mass of the cluster exceeds a threshold above which ejecta are retained and a second generation is formed. A determination of this mass threshold is mandatory in order to understand how GCs form. We analyzed nine red giant branch stars belonging to the cluster Ruprecht 106. Targets were observed with the UVES@VLT2 spectrograph. Spectra cover a wide range and allowed us to measure abundances for light (O, Na, Mg, Al), α (Si, Ca, Ti), iron-peak (Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), and neutron-capture (Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Dy, Pb) elements. Based on these abundances, we show that Ruprecht 106 is the first convincing example of a single-population GC (i.e., a true simple stellar population), although the sample is relatively small. This result is supported also by an independent photometric test and by the horizontal branch morphology and the dynamical state. It is old (∼12 Gyr) and, at odds with other GCs, has no α-enhancement. The material it formed from was contaminated by both s- and r-process elements. The abundance pattern points toward an extragalactic origin. Its present-day mass (M = 10{sup 4.83} M {sub ☉}) can be assumed as a strong lower limit for the initial mass threshold below which no second generation is formed. Clearly, its initial mass must have been significantly greater, but we have no current constraints on the amount of mass loss during its evolution.

  19. The Stellar Populations of Nuclei, Globular Clusters, and Stars in dE Galaxies in Virgo and Fornax

    NASA Astrophysics Data System (ADS)

    Whitfield Miller, Bryan; Hyazinth Puzia, Thomas; Hilker, Michael; Sanchez-Janssen, Ruben; Kissler-Patig, Markus

    2015-08-01

    We present ages and metallicities for globular clusters, nuclei, and underlying stars in nucleated dwarf elliptical galaxies (dE,N) in the Virgo and Fornax Cluster based on Lick/IDS index measurements and SSP models. Gemini/GMOS spectroscopy shows that the globular clusters are mostly old and metal-poor, very similar to the globular clusters in the Milky Way halo. The nuclei and underlying stars tend to be more metal-rich than the globular clusters and have a wide range of ages. The [α/Fe] ratios for both the globular clusters and nuclei range between 0.0 and 0.3. Formation scenarios for globular clusters and nuclei will be discussed.

  20. THE OBSERVATIONAL AND THEORETICAL TIDAL RADII OF GLOBULAR CLUSTERS IN M87

    SciTech Connect

    Webb, Jeremy J.; Sills, Alison; Harris, William E.

    2012-02-10

    Globular clusters have linear sizes (tidal radii) which theory tells us are determined by their masses and by the gravitational potential of their host galaxy. To explore the relationship between observed and expected radii, we utilize the globular cluster population of the Virgo giant M87. Unusually deep, high signal-to-noise images of M87 are used to measure the effective and limiting radii of approximately 2000 globular clusters. To compare with these observations, we simulate a globular cluster population that has the same characteristics as the observed M87 cluster population. Placing these simulated clusters in the well-studied tidal field of M87, the orbit of each cluster is solved and the theoretical tidal radius of each cluster is determined. We compare the predicted relationship between cluster size and projected galactocentric distance to observations. We find that for an isotropic distribution of cluster velocities, theoretical tidal radii are approximately equal to observed limiting radii for R{sub gc} < 10 kpc. However, the isotropic simulation predicts a steep increase in cluster size at larger radii, which is not observed in large galaxies beyond the Milky Way. To minimize the discrepancy between theory and observations, we explore the effects of orbital anisotropy on cluster sizes, and suggest a possible orbital anisotropy profile for M87 which yields a better match between theory and observations. Finally, we suggest future studies which will establish a stronger link between theoretical tidal radii and observed radii.

  1. GLOBULAR CLUSTER SYSTEMS IN BRIGHTEST CLUSTER GALAXIES: A NEAR-UNIVERSAL LUMINOSITY FUNCTION?

    SciTech Connect

    Harris, William E.; O'Halloran, Heather; Cockcroft, Robert E-mail: ohallohm@mcmaster.ca; and others

    2014-12-20

    We present the first results from our Hubble Space Telescope brightest cluster galaxy (BCG) survey of seven central supergiant cluster galaxies and their globular cluster (GC) systems. We measure a total of 48,000 GCs in all seven galaxies, representing the largest single GC database. We find that a log-normal shape accurately matches the observed the luminosity function (LF) of the GCs down to the globular cluster luminosity function turnover point, which is near our photometric limit. In addition, the LF has a virtually identical shape in all seven galaxies. Our data underscore the similarity in the formation mechanism of massive star clusters in diverse galactic environments. At the highest luminosities (L ≳ 10{sup 7} L {sub ☉}), we find small numbers of ''superluminous'' objects in five of the galaxies; their luminosity and color ranges are at least partly consistent with those of ultra-compact dwarfs. Last, we find preliminary evidence that in the outer halo (R ≳ 20 kpc), the LF turnover point shows a weak dependence on projected distance, scaling as L {sub 0} ∼ R {sup –0.2}, while the LF dispersion remains nearly constant.

  2. Globular Cluster Systems in Brightest Cluster Galaxies: A Near-universal Luminosity Function?

    NASA Astrophysics Data System (ADS)

    Harris, William E.; Morningstar, Warren; Gnedin, Oleg Y.; O'Halloran, Heather; Blakeslee, John P.; Whitmore, Bradley C.; Côté, Patrick; Geisler, Douglas; Peng, Eric W.; Bailin, Jeremy; Rothberg, Barry; Cockcroft, Robert; Barber DeGraaff, Regina

    2014-12-01

    We present the first results from our Hubble Space Telescope brightest cluster galaxy (BCG) survey of seven central supergiant cluster galaxies and their globular cluster (GC) systems. We measure a total of 48,000 GCs in all seven galaxies, representing the largest single GC database. We find that a log-normal shape accurately matches the observed the luminosity function (LF) of the GCs down to the globular cluster luminosity function turnover point, which is near our photometric limit. In addition, the LF has a virtually identical shape in all seven galaxies. Our data underscore the similarity in the formation mechanism of massive star clusters in diverse galactic environments. At the highest luminosities (L >~ 107 L ⊙), we find small numbers of "superluminous" objects in five of the galaxies; their luminosity and color ranges are at least partly consistent with those of ultra-compact dwarfs. Last, we find preliminary evidence that in the outer halo (R >~ 20 kpc), the LF turnover point shows a weak dependence on projected distance, scaling as L 0 ~ R -0.2, while the LF dispersion remains nearly constant.

  3. Globular Clusters and Spur Clusters in NGC 4921, the Brightest Spiral Galaxy in the Coma Cluster

    NASA Astrophysics Data System (ADS)

    Lee, Myung Gyoon; Jang, In Sung

    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 × 105 M⊙. 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. 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 MI (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, H0 = 77.9 ± 3.6 km s-1 Mpc-1. We estimate the GC specific frequency of NGC 4921 to be SN = 1.29 ± 0.25, close to the values for early-type galaxies. This indicates that NGC 4921 is in the transition phase to S0s.

  4. Are Some Milky Way Globular Clusters Hosted by Undiscovered Galaxies?

    NASA Astrophysics Data System (ADS)

    Zaritsky, Dennis; Crnojević, Denija; Sand, David J.

    2016-07-01

    The confirmation of a globular cluster (GC) in the recently discovered ultrafaint galaxy Eridanus II (Eri II) motivated us to examine the question posed in the title. After estimating the halo mass of Eri II using a published stellar mass—halo mass relation, the one GC in this galaxy supports extending the relationship between the number of GCs hosted by a galaxy and the galaxy’s total mass about two orders of magnitude in stellar mass below the previous limit. For this empirically determined specific frequency of between 0.06 and 0.39 GCs per 109 M ⊙ of total mass, the surviving Milky Way (MW) subhalos with masses smaller than 1010 M ⊙ could host as many as 5–31 GCs, broadly consistent with the actual population of outer halo MW GCs, although matching the radial distribution in detail remains a challenge. Using a subhalo mass function from published high-resolution numerical simulations and a Poissonian model for populating those halos with the aforementioned empirically constrained frequency, we find that about 90% of these GCs lie in lower-mass subhalos than that of Eri II. From what we know about the stellar mass–halo mass function, the subhalo mass function, and the mass-normalized GC specific frequency, we conclude that some of the MW’s outer halo GCs are likely to be hosted by undetected subhalos with extremely modest stellar populations.

  5. Detail Chemical Composition of M33 Globular Clusters

    NASA Astrophysics Data System (ADS)

    McWilliam, Andrew

    2008-08-01

    I propose to perform the first high-resolution detailed chemical abundance study of globular clusters (GCs) in the Local Group spiral galaxy M33 with Keck HIRES. My integrated-light technique permits detailed chemical abundance measurement (and approximate ages) for old populations at larger distance than ever done before. The basic goal is to accurately define the chemical abundance properties of the M33 GC system, for a comparison with the Milky Way, LMC, SMC and Local Group dwarf galaxies. Abundances of Fe, (alpha)-elements, Na and Al will constrain the relative contributions of Type Ia and Type II SNe and probe ~1 Gyr enrichment timescales. The s-process elements (e.g. Zr, Y, Ba, La), made by AGB stars, will probe timescales of several Gyr. These elements are sensitive to whether chemical enrichment occurred slowly, or in a burst, and whether the enrichment was global, or occurred in disparate systems, such as dwarf galaxies that were later accreted. The project will provide basic information to advance an understanding of chemical enrichment and nucleosynthesis, and galaxy evolution.

  6. Chemical Abundances of Multiple Stellar Populations in Massive Globular Clusters

    NASA Astrophysics Data System (ADS)

    Marino, Anna

    2015-08-01

    Recent discoveries have revealed that, contrary to expectations, some of the most massive globular clusters (GCs) in the Milky Way show variations in the heavy element content, including s-process elements and iron, and that these stellar groups populate different sequences on the color-magnitude diagram. We refer to these objects as anomlous GCs, as discussed in Marino et al. (2015, arXiv150207438M). On the other hand, typical Galactic GCs show just variations in the elements involved in the hot H-burning, resulting in well-known chemical patterns such as the C-N/O-Na/Mg-Al anticorrelations.Interestingly, the chemical analogies of these newly-discovered anomalous GCs with the most massive Milky Way GC, Omega Centauri, considered the nuclear relict of a dwarf galaxy, suggest the fascinating idea that they could be the survived nuclei of more massive systems.Hence, the most massive GCs could be the ''bridge'' between *normal* mono-metallic GCs, those showing chemical variations only in the light elements, and more massive systems like dwarf galaxies.I will present the spectroscopic and photometric current obsevational scenario for the anomalous GCs, focusing on Omega Centauri, M22, NGC5286 and NGC1851; and discuss the observational scenario in the context of the possible origin of these objects as nuclei of dwarf galaxies.

  7. Metallicity distributions of globular cluster systems in galaxies

    NASA Astrophysics Data System (ADS)

    Eerik, H.; Tenjes, P.

    We collected a sample of 100 galaxies for which different observers have determined colour indices of globular cluster candidates. The sample includes representatives of galaxies of various morphological types and different luminosities. Colour indices (in most cases (V-I), but also (B-I) and (C-T_1)) were transformed into metallicities [Fe/H] according to a relation by Kissler-Patig (1998). These data were analysed with the KMM software in order to estimate similarity of the distribution with uni- or bimodal Gaussian distribution. We found that 45 of 100 systems have bimodal metallicity distributions. Mean metallicity of the metal-poor component for these galaxies is < [Fe/H]> = -1.40 +/- 0.02, of the metal-rich component < [Fe/H]> = -0.69 +/- 0.03. Dispersions of the distributions are 0.15 and 0.18, respectively. Distribution of unimodal metallicities is rather wide. These data will be analysed in a subsequent paper in order to find correlations with parameters of galaxies and galactic environment.

  8. Multicolor NTT Photometry of the Anomalous Globular Cluster NGC 288

    NASA Astrophysics Data System (ADS)

    Alcaino, Gonzalo; Liller, William; Alvarado, Franklin

    1997-12-01

    We present UBV photometry of 2494 stars in a 38 arcmin square field in the anomalous high latitude globular cluster NGC 288 observed with a CCD camera and the 3.5 m NTT at ESO/La Silla. Our results can be summarized as follows: VTO=18.90±0.l4, and turnoff colors at (B-V)TO=0.46±0.05, (U - B)TO= -0.02±0.05 and independently (U - V)TO=0.45±0.07 (estimated external uncertainties). By fitting the resulting B - V ridge line to the theoretical isochrones of Bergbusch & VandenBerg (1992, ApJS, 81, 163) with [Fe/H]=-1.26, Y=0.2358, and [O/Fe]=+0.55, we derive the following values: the apparent distance modulus, (m - M)V = 14.85±0.10 and the interstellar reddening E(B - V) =0.04±0.02, and an age of 14±2 Gyr. We note the presence of approximately 17 blue stragglers, 4 hot faint BHB stars, and 5 faint possibly variable stars.

  9. ROTATION AND MULTIPLE STELLAR POPULATION IN GLOBULAR CLUSTERS

    SciTech Connect

    Bekki, Kenji

    2010-11-20

    We investigate structure and kinematics of the second generation of stars (SG) formed from gaseous ejecta of the first generation of stars (FG) in forming globular clusters (GCs). We consider that SG can be formed from gaseous ejecta from asymptotic giant branch stars of FG with the initial total mass of 10{sup 6} M {sub sun}-10{sup 8} M {sub sun} to explain the present masses of the Galactic GCs. Our three-dimensional hydrodynamical simulations with star formation show that SG formed in the central regions of FG can have a significant amount of rotation (V/{sigma}{approx} 0.8-2.5). The rotational amplitude of SG can depend strongly on the initial kinematics of FG. We thus propose that some GCs composed of FG and SG had a significant amount of rotation when they were formed. We also suggest that although later long-term ({approx}10 Gyr) dynamical evolution of stars can smooth out the initial structural and kinematical differences between FG and SG to a large extent, initial flattened structures and rotational kinematics of SG can be imprinted on shapes and internal rotation of the present GCs. We discuss these results in terms of internal rotation observed in the Galactic GCs.

  10. Atmosphere composition of quiescent accreting neutron stars in globular clusters

    NASA Astrophysics Data System (ADS)

    Servillat, M.

    2012-12-01

    Through the study of the quiescent X-ray emission of neutron stars in low-mass X-ray binaries it is possible to constrain the equation of state of dense matter. However, the chemical composition of the neutron star atmosphere is still uncertain. Using deep Chandra observations, we report the detailed spectral analysis of a neutron star in the globular cluster M28. For the first time for this kind of object, different atmosphere models composed of hydrogen, helium or carbon are used. The carbon model can be ruled out, and the derived mass and radius are clearly distinct depending on the composition of the atmosphere, leading to different constraints on the equation of state. We compare those results with the other similar neutron stars studied with a hydrogen atmosphere model only and show that a helium model could be relevant in many cases. Measurements of neutron star masses/radii by spectral fitting should consider the possibility of heavier element atmospheres, which produce larger masses/radii for the same data, unless the composition of the accretor is known independently.

  11. New SX Phe variables in the globular cluster NGC 288

    NASA Astrophysics Data System (ADS)

    Martinazzi, E.; Kepler, S. O.; Costa, J. E. S.; Pieres, A.; Bonatto, C.; Bica, E.; Fraga, L.

    2015-03-01

    We report the discovery of two new variable stars in the metal-poor globular cluster NGC 288, found by means of time series CCD photometry. We classified the new variables as SX Phoenicis (SX Phe) due to their characteristic fundamental mode periods (1.02 ± 0.01 and 0.69 ± 0.01 h), and refine the period estimates for other six known variables. SX Phe stars are known to follow a well-defined period-luminosity (P-L) relation and, thus, can be used for determining distances; they are more numerous than RR Lyraes in NGC 288. We obtain the P-L relation for the fundamental mode MV = (-2.59 ± 0.18) log P0(d) + (-0.34 ± 0.24) and for the first-overtone mode MV = (-2.59 ± 0.18) log P1(d) + (0.50 ± 0.25). Multichromatic isochrone fits to our UBV colour-magnitude diagrams, based on the Dartmouth Stellar Evolution Database, provide <[Fe/H]> = -1.3 ± 0.1, E(B - V) = 0.02 ± 0.01 and absolute distance modulus (m - M)0 = 14.72 ± 0.01 for NGC 288.

  12. Spectroscopy of chromospheric lines of giants in the globular cluster

    NASA Technical Reports Server (NTRS)

    Dupree, A. K.; Hartmann, Lee; Smith, Graeme H.; Rodgers, A. W.; Roberts, W. H.; Zucker, D. B.

    1994-01-01

    Spectroscopic observations of chromospheric transitions (Mg II, H-alpha, and Ca II K) from two red giants (A31 and A59) in the globular cluster NGC 6572 were made with the Goddard High Resolution Spectrograph on the Hubble Space Telescope and the coude spectrograph of the 1.9 m telescope at the Mount Stromlo Observatory. These measurements give evidence for chromospheric activity and outward motions within the atmospheres. The surface flux of the Mg II emission is comparable to that in disk population giants of similar (B-V) color. The Mg II profiles are asymmetric, which is most likely caused by absorption in an expanding stellar atmosphere and/or by possible interstellar features. Notches are found in the core of the H-alpha line of A59, which are similar to those found in Cepheids. This suggests that shocks are present in the atmosphere of A59 and indicates that hydrodynamic phenomena are influencing the levvel of chromospheric emission and producing upper atmospheric motions which may lead to mass loss.

  13. Globular clusters as tracers of the halo assembly of nearby central cluster galaxies

    NASA Astrophysics Data System (ADS)

    Hilker, Michael; Richtler, Tom

    2016-08-01

    The properties of globular cluster systems (GCSs) in the core of the nearby galaxy clusters Fornax and Hydra I are presented. In the Fornax cluster we have gathered the largest radial velocity sample of a GCS system so far, which enables us to identify photometric and kinematic sub-populations around the central galaxy NGC 1399. Moreover, ages, metallicities and [α/Fe] abundances of a sub-sample of 60 bright globular clusters (GCs) with high S/N spectroscopy show a multi-modal distribution in the correlation space of these three parameters, confirming heterogeneous stellar populations in the halo of NGC 1399. In the Hydra I cluster very blue GCs were identified. They are not uniformly distributed around the central galaxies. 3-color photometry including the U-band reveals that some of them are of intermediate age. Their location coincides with a group of dwarf galaxies under disruption. This is evidence of a structurally young stellar halo ``still in formation'', which is also supported by kinematic measurements of the halo light that point to a kinematically disturbed system. The most massive GCs divide into generally more extended ultra-compact dwarf galaxies (UCDs) and genuine compact GCs. In both clusters, the spatial distribution and kinematics of UCDs are different from those of genuine GCs. Assuming that some UCDs represent nuclei of stripped galaxies, the properties of those UCDs can be used to trace the assembly of nucleated dwarf galaxies into the halos of central cluster galaxies. We show via semi-analytical approaches within a cosmological simulation that only the most massive UCDs in Fornax-like clusters can be explained by stripped nuclei, whereas the majority of lower mass UCDs belong to the star cluster family.

  14. PEERING INTO THE CORE OF A GLOBULAR CLUSTER

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Astronomers have used NASA's Hubble Space Telescope to peer into the center of a dense swarm of stars called Omega Centauri. Located some 17,000 light-years from Earth, Omega Centauri is a massive globular star cluster, containing several million stars swirling in locked orbits around a common center of gravity. The stars are packed so densely in the cluster's core that it is difficult for ground-based telescopes to make out individual stars. Hubble's high resolution is able to pick up where ground-based telescopes leave off, capturing distinct points of light from stars at the very center of the cluster. Omega Centauri is so large in our sky that only a small part of it fits within the field of view of the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. Yet even this tiny patch contains some 50,000 stars, all packed into a region only about 13 light-years wide. For comparison, a similarly sized region centered on the Sun would contain about a half dozen stars. The vast majority of stars in this Hubble image are faint, yellow-white dwarf stars similar to our Sun. The handful of bright yellow-orange stars are red giants that have begun to exhaust their nuclear fuel and have expanded to diameters about a hundred times that of the Sun. A number of faint blue stars are also visible in the image. These are in a brief phase of evolution between the dwarf stage and the red-giant stage, during which the surface temperature is high. The stars in Omega Centauri are all very old, about 12 billion years. Stars with a mass as high as that of our Sun have already completed their evolution and have faded away as white dwarfs, too faint to be seen even in the Hubble image. The stars in the core of Omega Centauri are so densely packed that occasionally one of them will actually collide with another one. Even in the dense center of Omega Centauri, stellar collisions will be infrequent. But the cluster is so old that many thousands of collisions have occurred

  15. THE RICH GLOBULAR CLUSTER SYSTEM OF ABELL 1689 AND THE RADIAL DEPENDENCE OF THE GLOBULAR CLUSTER FORMATION EFFICIENCY

    SciTech Connect

    Alamo-Martínez, K. A.; González-Lópezlira, R. A.; Blakeslee, J. P.; Côté, P.; Ferrarese, L.; Jee, M. J.; Jordán, A.; Meurer, G. R.; Peng, E. W.; West, M. J.

    2013-09-20

    We study the rich globular cluster (GC) system in the center of the massive cluster of galaxies Abell 1689 (z = 0.18), one of the most powerful gravitational lenses known. With 28 Hubble Space Telescope/Advanced Camera for Surveys orbits in the F814W bandpass, we reach a magnitude I{sub 814} = 29 with ∼>90% completeness and sample the brightest ∼5% of the GC system. Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we estimate a total population of N{sup total}{sub GC}= 162,850{sup +75,450}{sub -51,310} GCs within a projected radius of 400 kpc. As many as half of the GCs may comprise an intracluster component. Even with the sizable uncertainties, which mainly result from the uncertain GCLF parameters, this system is by far the largest GC population studied to date. The specific frequency S{sub N} is high, but not uncommon for central galaxies in massive clusters, rising from S{sub N} ≈ 5 near the center to ∼12 at large radii. Passive galaxy fading would increase S{sub N} by ∼20% at z = 0. We construct the radial mass profiles of the GCs, stars, intracluster gas, and lensing-derived total mass, and we compare the mass fractions as a function of radius. The estimated mass in GCs, M{sub GC}{sup total} = 3.9 × 10{sup 10} M{sub ☉}, is comparable to ∼80% of the total stellar mass of the Milky Way. The shape of the GC mass profile appears intermediate between those of the stellar light and total cluster mass. Despite the extreme nature of this system, the ratios of the GC mass to the baryonic and total masses, and thus the GC formation efficiency, are typical of those in other rich clusters when comparing at the same physical radii. The GC formation efficiency is not constant, but varies with radius, in a manner that appears similar for different clusters; we speculate on the reasons for this similarity in profile.

  16. HST Search for Planetary Nebulae in Local Group Globular Clusters

    NASA Astrophysics Data System (ADS)

    Bond, Howard E.

    2015-01-01

    If every star of about solar mass produces a planetary nebula (PN) near the end of its life, there should be several dozen PNe in the globular clusters (GCs) of the Local Group. However, ground-based surveys of Milky Way GCs have revealed only 4 PNe. A converse argument is that it is likely that the remnants of stars now evolving in ancient GCs leave the AGB so slowly that any ejected PN dissipates long before the star becomes hot enough to ionize it. Thus there should not be any PNe in Milky Way GCs--but there are four! It has been suggested that these PNe are the result of binary mergers of binary stars within GCs, i.e., that they are descendants of blue stragglers. To explore these issues and extend them beyond the Milky Way, I carried out a Snapshot imaging survey of GCs throughout the Local Group with the Hubble Space Telescope. Observations were made with the WFPC2 camera in 2007-2008, and with WFC3 in 2009-2011. Frames were obtained in a narrow-band [O III] 5007 filter and in a broad V filter (F555W). In this filter combination, a PN will have a comparable signal in both bandpasses, but stars will be much brighter in the V filter. I surveyed 41 GCs in M31, 4 in M33, 8 in the Magellanic Clouds, 2 in Fornax, and 1 each in NGC 6822, WLM, and NGC 147. Only one candidate PN was found, in the M31 GC B086. My results appear to be consistent with a ground-based spectroscopic survey for PNe in the M31 GCs by Jacoby et al. (2013), which found only 3 PN candidates in 274 clusters. PNe are very rare in GCs, but a few do exist, and they may require binary interactions for their formation.

  17. A search for eclipsing binaries in galactic globular clusters

    NASA Astrophysics Data System (ADS)

    von Braun, Kaspar

    2002-09-01

    We report on the discovery and analysis of short-period (0.1 days < P < 5 days), photometrically varying binary stars around and below the main-sequence turnoff of the globular clusters (GCs) NGC 3201, M10, & M12. These eclipsing binaries (EBs) may be used to determine directly the distances to GCs and constrain the Population II stellar main-sequence masses. During our search for binaries, we discovered the signature of differential reddening across the cluster fields which was especially strong for NGC 3201 and M10. We correct for this differential reddening by calculating average EV-I values for stars in small subregions of the field with respect to a fiducial region, which significantly improves the appearance of the GC color-magnitude diagrams (CMDs). The reddening zero point to be added to the differential value is determined by isochrone fitting. The results of our differential dereddening are presented in the form of high-resolution extinction maps. Our search for EBs returned 14 variable stars (11 EBs) in the field of NGC 3201, 3 variables (1 EB) in M10, and 2 EBs in M12. Of these variables, only one EB in NGC 3201 (a blue straggler W Ursa Majoris contact system) is a definite GC-member, based on spectroscopic observations. Another W UMa contact EB in M12 is most likely a member of M12, based on its location in the color-magnitude diagram (CMD) and its empirically calculated absolute magnitude. We present the phased lightcurves for all variables, estimate their distances and GC membership, and show their locations in the GC fields and CMDs, as well as the spectra of the NGC 3201 EBs. Finally, we discuss the implications of our results and outline future work.

  18. GALEX Grism Spectroscopy of the Globular Cluster Omega Centauri

    NASA Astrophysics Data System (ADS)

    Sweigart, Allen

    We propose to obtain GALEX FUV-only grism spectroscopy of the hot stars in omega Centauri, the most massive globular cluster in our Galaxy. Previous UIT imagery of omega Cen showed that it contains about 2000 hot horizontal branch (HB) stars, and we estimate that GALEX spectra can be obtained for about 500 of these stars in the outer regions of the cluster, including about 50 of the hot ``blue hook'' stars discovered with UIT. The blue hook stars appear to be both hotter (35,000 K) and less luminous in the UIT color-magnitude diagram than predicted by canonical HB models and, indeed, are unexplained by standard evolutionary theory. Brown et al. (2001) have suggested that the blue hook stars are the progeny of stars which mixed their surface hydrogen into their hot He-burning interior during a delayed helium flash subsequent to leaving the red giant branch. This ``flash-mixing'' results in a hot hydrogen-deficient star with a typical surface abundance of 96% He and 4% C by mass. The GALEX spectral region includes the strong lines of C III 1426, 1578 A, C IV 1550 A, and He II 1640 A which will allow this predicted carbon and helium enrichment to be detected. These observations will therefore provide a crucial test of the Brown et al. flash-mixing hypothesis and will determine if flash mixing represents a new evolutionary channel for populating the hot HB. The GALEX spectra will also address other questions concerning the hot HB in omega Cen including (1) the metallicity distribution of HB stars with 9,000 K < Teff < 11,000 K, (2) the effect of radiative levitation on the UV spectra of stars with Teff > 11,000 K, and (3) the origin of the subluminous HB stars found in the UIT photometry with 15,000K < Teff < 30,000 K.

  19. BVRI CCD photometry of the globular cluster NGC 6362

    NASA Astrophysics Data System (ADS)

    Alcaino, G.; Liller, W.

    1986-02-01

    We have obtained 78 BVRI CCD frames with the 1.54 m Danish telescope at ESO, La Silla, and have constructed V vs B - V, V vs V - R, V vs R - I, V vs V - I, and V vs B - I color-magnitude diagrams in a 4' × 2.'5 field of the globular cluster NGC 6362. From these five CMDs we find that the main-sequence turnoffs are all close to the same magnitude, namely VTO = 18.75 ± 0.1, and the color turn-offs at B-V = 0.50 ± 0.02, V-R = 0.31 ± 0.02, R-I = 0.35 ± 0.02, V-I = 0.68 ± 0.02, and B - I = 1.18 ± 0.03. The magnitude difference between the turnoff and the horizontal branch for the five diagrams is Δ Mv = 3.40 ± 0.15 in excellent agreement with the value given by Sandage (1982). Using Y = 0.2, Z = 0.001 ([Fe/H] = - 1.27), a = 1.65, a distance modulus of (m -M)v = 14.74, and E(B - V) = 0.10, we find that the VandenBerg and Bell isochrones (1985) yield a consistent age for NGC 6362 in all colors indexes of 16 ± 1.5 × 100 yr. The solar distance to the cluster is 7.7 kpc and the galactic distance is 5.6 kpc assuming R0 = 9 kpc.

  20. CN and CH variations in the globular cluster M55

    NASA Astrophysics Data System (ADS)

    Briley, Michael M.; Smith, Graeme H.; Hesser, James E.; Bell, R. A.

    1993-07-01

    Blue spectra of a sample of 24 bright M55 red giants within the magnitude range of V between 11.8 and 14 have been obtained with the CTIO 4 m telescope and Argus multifiber spectrograph. Indices sensitive to absorption by the 3883 A CN band were measured from these spectra and combined with the CN data of Smith and Norris (1982) to yield CN band strength measurements for 47 stars. Aside from the CH star L2406, no stars with unusually strong 3883 A CN bands were detected. Analysis with the aid of synthetic spectra indicates a star-to-star spread in (N/A) of less than 0.3 dex. A similar treatment of the CTIO G-band (CH) strengths suggests a decrease in C abundance of 0.1 to 0.3 dex with increasing luminosity on the red giant branch from M(V) about -0.1 to -2.3. Similar trends have also been noted in other metal-poor clusters and are believed to be the result of the progressive mixing of CN-processed material into the stellar atmosphere. The (C/A) abundances of the asymptotic giant branch stars are consistent with those of the brightest red giants in our sample, implying little mixing during horizontal and asymptotic branch evolution. The overall star-to-star variation in (C/A) on the red giant branch for M(V) less than -0.8 appears to be 0.2 dex. From these data it appears that M55 may be atypical of the Galactic globular clusters in terms of its homogeneity of star-to-star surface abundances of both C and N.

  1. The DRAGON simulations: globular cluster evolution with a million stars

    NASA Astrophysics Data System (ADS)

    Wang, Long; Spurzem, Rainer; Aarseth, Sverre; Giersz, Mirek; Askar, Abbas; Berczik, Peter; Naab, Thorsten; Schadow, Riko; Kouwenhoven, M. B. N.

    2016-05-01

    Introducing the DRAGON simulation project, we present direct N-body simulations of four massive globular clusters (GCs) with 106 stars and 5 per cent primordial binaries at a high level of accuracy and realism. The GC evolution is computed with NBODY6++GPU and follows the dynamical and stellar evolution of individual stars and binaries, kicks of neutron stars and black holes (BHs), and the effect of a tidal field. We investigate the evolution of the luminous (stellar) and dark (faint stars and stellar remnants) GC components and create mock observations of the simulations (i.e. photometry, colour-magnitude diagrams, surface brightness and velocity dispersion profiles). By connecting internal processes to observable features, we highlight the formation of a long-lived `dark' nuclear subsystem made of BHs, which results in a two-component structure. The inner core is dominated by the BH subsystem and experiences a core-collapse phase within the first Gyr. It can be detected in the stellar (luminous) line-of-sight velocity dispersion profiles. The outer extended core - commonly observed in the (luminous) surface brightness profiles - shows no collapse features and is continuously expanding. We demonstrate how a King model fit to observed clusters might help identify the presence of post core-collapse BH subsystems. For global observables like core and half-mass radii, the direct simulations agree well with Monte Carlo models. Variations in the initial mass function can result in significantly different GC properties (e.g. density distributions) driven by varying amounts of early mass-loss and the number of forming BHs.

  2. Are Young Massive Star Clusters in the Local Universe Analogous to Globular Clusters Progenitors?

    NASA Astrophysics Data System (ADS)

    Charbonnel, Corinne

    2015-08-01

    Several models do compete to reproduce the present-day characteristics of globular clusters (GC) and to explain the origin of the multiple stellar populations these systems are hosting.In parallel, independent clues on GC early evolution may be derived from observations of young massive clusters (YMC) in the Local Group.But are these two populations of clusters related? In this talk, we discuss how and if GC and YMC data can be reconciled.We revisit in particular the impact of massive stars on the early evolution of massive star clusters, as well as the question of early gas expulsion.We propose several tests to probe whether the YMC we are observing today can be considered as the analogues of GC progenitors.

  3. Formation of globular clusters induced by external ultraviolet radiation

    NASA Astrophysics Data System (ADS)

    Hasegawa, Kenji; Umemura, Masayuki; Kitayama, Tetsu

    2009-08-01

    We present a novel scenario for globular cluster (GC) formation, where the ultraviolet (UV) background radiation effectively works so as to produce compact star clusters. Recent observations on the age distributions of GCs indicate that many GCs formed even after the cosmic reionization epoch. This implies that a significant fraction of GCs formed in UV background radiation fields. Also, the star formation in an early-generation of subgalactic objects may be affected by strong UV radiation from pre-formed massive stars, e.g. Population III stars. Here, we explore the formation of GCs in UV radiation fields. For this purpose, we calculate baryon and dark matter (DM) dynamics in spherical symmetry, incorporating the self-shielding effects by solving the radiative transfer of UV radiation. In addition, we prescribe the star formation in cooled gas components and pursue the dynamics of formed stars. As a result, we find that the evolution of subgalactic objects in UV background radiation is separated into three types: (i) prompt star formation, where less massive clouds (~105-8Msolar) are promptly self-shielded and undergo star formation, (ii) delayed star formation, where photoionized massive clouds (>~108Msolar) collapse despite high thermal pressure and are eventually self-shielded to form stars in a delayed fashion, and (iii) supersonic infall, where photoionized less massive clouds (~105-8Msolar) contract with supersonic infall velocity and are self-shielded when a compact core forms. In particular, the type (iii) is a novel type found in the present simulations, and eventually produces a very compact star cluster. The resultant mass-to-light ratios, half-mass radii and velocity dispersions for the three types are compared to the observations of GCs, dwarf spheroidals (dSphs) and ultracompact dwarfs (UCDs). It turns out that the properties of star clusters resulting from supersonic infall match well with those of observed GCs, whereas the other two types are

  4. Hunting for Optical Companions to Binary Msps in Globular Clusters

    NASA Astrophysics Data System (ADS)

    Ferraro, Francesco

    2009-07-01

    Here we present a proposal which exploits the re-newed potential of HST after the Service Mission 4 for probing the population of binary Millisecond Pulsars {MSPs} in Globular Clusters. In particular we intend to: {1} extend the search for optical counterparts in Terzan 5, by pushing the performance of the WFC3 IR channel to sample the entire MS extension down to M=0.1 Mo; {2} perform a deep multi-band search of MSP companions with the WFC3, in 3 clusters {namely NGC6440, M28 and M5}, where recent radio observations have found particularly interesting objects; {3} derive an accurate radial velocity {with STIS} of the puzzling optical companion COM6266B recently discovered by our group, to firmly assess its cluster membership.This program is the result of a large collaboration among the three major groups {lead by Freire, Ransom and Possenti} which are performing extensive MSP search in GCs in the radio bands, and our group which has a large experience in performing accurate stellar photometry in crowded environments. This collaboration has produced a number of outstanding discoveries. In fact, three of the 6 optical counterparts to binary MSP companions known to date in GCs have been discovered by our group. The observations here proposed would easily double/triple the existing sample of known MSP companions, allowing the first meaningful approach to the study of the formation, evolution and recycling process of pulsar in GCs. Moreover, since most of binary MSPs in GCs are thought to form via stellar interactions in the high density core regions, the determination of the nature of the companion and the incidence of this collisionally induced population has a significant impact on our knowledge of the cluster dynamics. Even more interesting, the study of the optical companions to NSs in GCs allows one to derive tighter constraints {than those obtainable for NS binaries in the Galactic field} on the system properties. This has, in turn, an intrisic importance for

  5. Stable Carbon Isotope Ratios for Giant Stars in the Globular Cluster M13

    NASA Astrophysics Data System (ADS)

    Rhee, Jaehyon; Pilachowski, C. A.

    2013-01-01

    Recently, our paradigm for the formation and evolution of globular clusters has shifted. We now understand that the majority of present-day stars in globular clusters formed as second-generation stars, primarily from the ejecta of first-generation AGB stars, while the majority of first generation, less centrally concentrated stars, have been dynamically lost to the cluster (D'Ercole et al. 2011). This paradigm explains the observed star-to-star variations in the abundances of light element observed in globular clusters, and suggests that the carbon isotope ratio should be similarly differentiated between first and second generation stars. In an effort to verify this scenario, we have recently utilized the Gemini/NIFS to determine carbon isotope abundances (12C and 13C) for 18 giant stars in the globular clusters M13 through medium-resolution (R ˜ 5300) infrared spectroscopy of the first-overtone CO bands near 2.3 μm. Our program stars are distributed from the tip of the RGB to the BLF (the bump in the luminosity function) of M13, and their Na, Mg, and Al abundances are already known from homogeneous data set analysis. Therefore, adding reliable abundances of the stable carbon isotopes to this homogeneous spectroscopic sample permits systematic tests of cluster chemical evolution models. We report preliminary results of the carbon abundance analysis for our NIFS K-band spectra and present an overview of our ongoing effort with other globular clusters.

  6. The Chemical Properties of Milky Way and M31 Globular Clusters. I. A Comparative Study

    NASA Astrophysics Data System (ADS)

    Beasley, Michael A.; Brodie, Jean P.; Strader, Jay; Forbes, Duncan A.; Proctor, Robert N.; Barmby, Pauline; Huchra, John P.

    2004-10-01

    A comparative analysis is performed between high-quality integrated spectral indices of 30 M31 globular clusters, 20 Milky Way globular clusters, and a sample of field and cluster elliptical galaxies. We find that the Lick CN indices in the M31 and Galactic clusters are enhanced relative to the bulges of the Milky Way, M31, and elliptical spheroids, in agreement with Burstein and coworkers. Although not particularly evident in the Lick CN indices, the near-UV cyanogen feature (λ3883) is strongly enhanced with respect to the Galactic globular clusters at metallicities -1.5<[Fe/H]<-0.3. Carbon shows signs of varying among these two groups. For [Fe/H]>-0.8, we observe no systematic differences in the Hδ, Hγ, or Hβ indices between the M31 and Galactic globular clusters, in contrast to previous studies. The elliptical galaxy sample lies offset from the loci of the globular clusters in both the cyanogen-[MgFe] and Balmer-line-[MgFe] planes. Six of the M31 clusters appear young and are projected onto the M31 disk. Population synthesis models suggest that these are metal-rich clusters with ages 100-800 Myr, metallicities -0.20<=[Fe/H]<=0.35, and masses 0.7-~7.0×104 Msolar. Two other young clusters are Hubble V in NGC 205, observed as a template, and an older (~3 Gyr) cluster some 7 kpc away from the plane of the disk. The six clusters projected onto the disk show signs of rotation similar to the H I gas in M31, and three clusters exhibit thin disk kinematics, according to Morrison and coworkers. Dynamical mass estimates and detailed structural parameters are required for these objects to determine whether they are massive open clusters or globular clusters. If they are the latter, our findings suggest globular clusters may trace the buildup of galaxy disks. In either case, we conclude that these clusters are part of a young, metal-rich disk cluster system in M31, possibly as young as 1 Gyr old.

  7. ON THE BIRTH MASSES OF THE ANCIENT GLOBULAR CLUSTERS

    SciTech Connect

    Conroy, Charlie

    2012-10-10

    All globular clusters (GCs) studied to date show evidence for internal (star-to-star) variation in their light-element abundances (including Li, C, N, O, F, Na, Mg, Al, and probably He). These variations have been interpreted as evidence for multiple star formation episodes within GCs, with secondary episodes fueled, at least in part, by the ejecta of asymptotic giant branch (AGB) stars from a first generation of stars. A major puzzle emerging from this otherwise plausible scenario is that the fraction of stars associated with the second episode of star formation is observed to be much larger than expected for a standard initial mass function. The present work investigates this tension by modeling the observed anti-correlation between [Na/Fe] and [O/Fe] for 20 Galactic GCs. If the abundance pattern of the retained AGB ejecta does not depend on GC mass at fixed [Fe/H], then a strong correlation is found between the fraction of current GC stellar mass composed of pure AGB ejecta, f{sub p} , and GC mass. This fraction varies from 0.20 at low masses (10{sup 4.5} M{sub Sun }) to 0.45 at high masses (10{sup 6.5} M{sub Sun }). The fraction of mass associated with pure AGB ejecta is directly related to the total mass of the cluster at birth; the ratio between the initial and present mass in stars can therefore be derived. Assuming a star formation efficiency of 50%, the observed Na-O anti-correlations imply that GCs were at least 10-20 times more massive at birth, a conclusion that is in qualitative agreement with previous work. These factors are lower limits because any mass-loss mechanism that removes first- and second-generation stars equally will leave f{sub p} unchanged. The mass dependence of f{sub p} probably arises because lower mass GCs are unable to retain all of the AGB ejecta from the first stellar generation. Recent observations of elemental abundances in intermediate-age Large Magellanic Cloud clusters are re-interpreted and shown to be consistent with this

  8. On the Birth Masses of the Ancient Globular Clusters

    NASA Astrophysics Data System (ADS)

    Conroy, Charlie

    2012-10-01

    All globular clusters (GCs) studied to date show evidence for internal (star-to-star) variation in their light-element abundances (including Li, C, N, O, F, Na, Mg, Al, and probably He). These variations have been interpreted as evidence for multiple star formation episodes within GCs, with secondary episodes fueled, at least in part, by the ejecta of asymptotic giant branch (AGB) stars from a first generation of stars. A major puzzle emerging from this otherwise plausible scenario is that the fraction of stars associated with the second episode of star formation is observed to be much larger than expected for a standard initial mass function. The present work investigates this tension by modeling the observed anti-correlation between [Na/Fe] and [O/Fe] for 20 Galactic GCs. If the abundance pattern of the retained AGB ejecta does not depend on GC mass at fixed [Fe/H], then a strong correlation is found between the fraction of current GC stellar mass composed of pure AGB ejecta, fp , and GC mass. This fraction varies from 0.20 at low masses (104.5 M ⊙) to 0.45 at high masses (106.5 M ⊙). The fraction of mass associated with pure AGB ejecta is directly related to the total mass of the cluster at birth; the ratio between the initial and present mass in stars can therefore be derived. Assuming a star formation efficiency of 50%, the observed Na-O anti-correlations imply that GCs were at least 10-20 times more massive at birth, a conclusion that is in qualitative agreement with previous work. These factors are lower limits because any mass-loss mechanism that removes first- and second-generation stars equally will leave fp unchanged. The mass dependence of fp probably arises because lower mass GCs are unable to retain all of the AGB ejecta from the first stellar generation. Recent observations of elemental abundances in intermediate-age Large Magellanic Cloud clusters are re-interpreted and shown to be consistent with this basic scenario. The small scatter in fp at

  9. THE ORIGIN OF GAMMA RAYS FROM GLOBULAR CLUSTERS

    SciTech Connect

    Cheng, K. S.; Chernyshov, D. O.; Dogiel, V. A.; Hui, C. Y.; Kong, A. K. H.

    2010-11-10

    Fermi has detected gamma-ray emission from eight globular clusters (GCs). It is commonly believed that the energy sources of these gamma rays are millisecond pulsars (MSPs) inside GCs. Also it has been standard to explain the spectra of most Fermi Large Area Telescope pulsars including MSPs resulting from the curvature radiation (CR) of relativistic electrons/positrons inside the pulsar magnetosphere. Therefore, gamma rays from GCs are expected to be the collection of CR from all MSPs inside the clusters. However, the angular resolution is not high enough to pinpoint the nature of the emission. In this paper, we calculate the gamma rays produced by the inverse Compton (IC) scattering between relativistic electrons/positrons in the pulsar wind of MSPs in the GCs and background soft photons including cosmic microwave/relic photons, background star lights in the clusters, the galactic infrared photons, and the galactic star lights. We show that the gamma-ray spectrum from 47 Tucanae can be explained equally well by upward scattering of either the relic photons, the galactic infrared photons, or the galactic star lights, whereas the gamma-ray spectra from the other seven GCs are best fitted by the upward scattering of either the galactic infrared photons or the galactic star lights. We also find that the observed gamma-ray luminosity is correlated better with the combined factor of the encounter rate and the background soft photon energy density. Therefore, the IC scattering may also contribute to the observed gamma-ray emission from GCs detected by Fermi in addition to the standard CR process. Furthermore, we find that the emission region of high-energy photons from GCs produced by the IC scattering is substantially larger than the cores of GCs with a radius >10 pc. The diffuse radio and X-rays emitted from GCs can also be produced by the synchrotron radiation and IC scattering, respectively. We suggest that future observations including radio, X-rays, and gamma rays

  10. 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.