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Sample records for globular cluster formation

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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

  7. 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.

  8. 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.

  9. 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.

  10. 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

  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. 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

  13. 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.

  14. 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.

  15. On the assembly of dwarf galaxies in clusters and their efficient formation of globular clusters

    NASA Astrophysics Data System (ADS)

    Mistani, Pouria A.; Sales, Laura V.; Pillepich, Annalisa; Sanchez-Janssen, Rubén; Vogelsberger, Mark; Nelson, Dylan; Rodriguez-Gomez, Vicente; Torrey, Paul; Hernquist, Lars

    2016-01-01

    Galaxy clusters contain a large population of low-mass dwarf elliptical galaxies whose exact origin is unclear: their colours, structural properties and kinematics differ substantially from those of dwarf irregulars in the field. We use the Illustris cosmological simulation to study differences in the assembly histories of dwarf galaxies (3 × 108 < M*/M⊙ < 1010) according to their environment. We find that cluster dwarfs achieve their maximum total and stellar mass on average ˜8 and ˜4.5 Gyr ago (or redshifts z = 1.0 and 0.4, respectively), around the time of infall into the clusters. In contrast, field dwarfs not subjected to environmental stripping reach their maximum mass at z = 0. These different assembly trajectories naturally produce a colour bimodality, with blue isolated dwarfs and redder cluster dwarfs exhibiting negligible star formation today. The cessation of star formation happens over median times 3.5-5 Gyr depending on stellar mass, and shows a large scatter (˜1-8 Gyr), with the lower values associated with starburst events that occur at infall through the virial radius or pericentric passages. We argue that such starbursts together with the early assembly of cluster dwarfs can provide a natural explanation for the higher specific frequency of globular clusters (GCs) in cluster dwarfs, as found observationally. We present a simple model for the formation and stripping of GCs that supports this interpretation. The origin of dwarf ellipticals in clusters is, therefore, consistent with an environmentally driven evolution of field dwarf irregulars. However, the z = 0 field analogues of cluster dwarf progenitors have today stellar masses a factor of ˜3 larger - a difference arising from the early truncation of star formation in cluster dwarfs.

  16. The formation of primordial binaries in globular clusters by star-disk interactions

    NASA Technical Reports Server (NTRS)

    Murray, Stephen D.; Clarke, C. J.; Pringle, J. E.

    1991-01-01

    The formation of primordial binaries in globular clusters is examined using simple numerical models. Clusters of protostars collapse until their velocity dispersion rises sufficiently to reverse the infall and the cluster reaches equilibrium. During the collapse, interactions between stars and protostellar disks lead to stellar capture. It is found that binary fraction of a few percent typically result. Binary formation is terminated when the velocity dispersion rises to a point at which most encounters result in disk destruction rather than capture. As a result, much gas is returned to the cluster ISM, limiting the star formation efficiency to a value significantly below 100 percent.

  17. 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.

  18. Modeling the formation of globular cluster systems in the Virgo cluster

    SciTech Connect

    Li, Hui; Gnedin, Oleg Y. E-mail: ognedin@umich.edu

    2014-11-20

    The mass distribution and chemical composition of globular cluster (GC) systems preserve fossil record of the early stages of galaxy formation. The observed distribution of GC colors within massive early-type galaxies in the ACS Virgo Cluster Survey (ACSVCS) reveals a multi-modal shape, which likely corresponds to a multi-modal metallicity distribution. We present a simple model for the formation and disruption of GCs that aims to match the ACSVCS data. This model tests the hypothesis that GCs are formed during major mergers of gas-rich galaxies and inherit the metallicity of their hosts. To trace merger events, we use halo merger trees extracted from a large cosmological N-body simulation. We select 20 halos in the mass range of 2 × 10{sup 12} to 7 × 10{sup 13} M {sub ☉} and match them to 19 Virgo galaxies with K-band luminosity between 3 × 10{sup 10} and 3 × 10{sup 11} L {sub ☉}. To set the [Fe/H] abundances, we use an empirical galaxy mass-metallicity relation. We find that a minimal merger ratio of 1:3 best matches the observed cluster metallicity distribution. A characteristic bimodal shape appears because metal-rich GCs are produced by late mergers between massive halos, while metal-poor GCs are produced by collective merger activities of less massive hosts at early times. The model outcome is robust to alternative prescriptions for cluster formation rate throughout cosmic time, but a gradual evolution of the mass-metallicity relation with redshift appears to be necessary to match the observed cluster metallicities. We also affirm the age-metallicity relation, predicted by an earlier model, in which metal-rich clusters are systematically several billion younger than their metal-poor counterparts.

  19. Modeling the Formation of Globular Cluster Systems in the Virgo Cluster

    NASA Astrophysics Data System (ADS)

    Li, Hui; Gnedin, Oleg Y.

    2014-11-01

    The mass distribution and chemical composition of globular cluster (GC) systems preserve fossil record of the early stages of galaxy formation. The observed distribution of GC colors within massive early-type galaxies in the ACS Virgo Cluster Survey (ACSVCS) reveals a multi-modal shape, which likely corresponds to a multi-modal metallicity distribution. We present a simple model for the formation and disruption of GCs that aims to match the ACSVCS data. This model tests the hypothesis that GCs are formed during major mergers of gas-rich galaxies and inherit the metallicity of their hosts. To trace merger events, we use halo merger trees extracted from a large cosmological N-body simulation. We select 20 halos in the mass range of 2 × 1012 to 7 × 1013 M ⊙ and match them to 19 Virgo galaxies with K-band luminosity between 3 × 1010 and 3 × 1011 L ⊙. To set the [Fe/H] abundances, we use an empirical galaxy mass-metallicity relation. We find that a minimal merger ratio of 1:3 best matches the observed cluster metallicity distribution. A characteristic bimodal shape appears because metal-rich GCs are produced by late mergers between massive halos, while metal-poor GCs are produced by collective merger activities of less massive hosts at early times. The model outcome is robust to alternative prescriptions for cluster formation rate throughout cosmic time, but a gradual evolution of the mass-metallicity relation with redshift appears to be necessary to match the observed cluster metallicities. We also affirm the age-metallicity relation, predicted by an earlier model, in which metal-rich clusters are systematically several billion younger than their metal-poor counterparts.

  20. Formation of globular clusters induced by external ultraviolet radiation II: Three-dimensional radiation hydrodynamics simulations

    NASA Astrophysics Data System (ADS)

    Abe, Makito; Umemura, Masayuki; Hasegawa, Kenji

    2016-08-01

    We explore the possibility of the formation of globular clusters under ultraviolet (UV) background radiation. One-dimensional spherical symmetric radiation hydrodynamics (RHD) simulations by Hasegawa et al. have demonstrated that the collapse of low-mass (106-7 M⊙) gas clouds exposed to intense UV radiation can lead to the formation of compact star clusters like globular clusters (GCs) if gas clouds contract with supersonic infall velocities. However, three-dimensional effects, such as the anisotropy of background radiation and the inhomogeneity in gas clouds, have not been studied so far. In this paper, we perform three-dimensional RHD simulations in a semi-cosmological context, and reconsider the formation of compact star clusters in strong UV radiation fields. As a result, we find that although anisotropic radiation fields bring an elongated shadow of neutral gas, almost spherical compact star clusters can be procreated from a "supersonic infall" cloud, since photo-dissociating radiation suppresses the formation of hydrogen molecules in the shadowed regions and the regions are compressed by UV heated ambient gas. The properties of resultant star clusters match those of GCs. On the other hand, in weak UV radiation fields, dark matter-dominated star clusters with low stellar density form due to the self-shielding effect as well as the positive feedback by ionizing photons. Thus, we conclude that the "supersonic infall" under a strong UV background is a potential mechanism to form GCs.

  1. The Celestial Buffet: multiple populations and globular cluster formation in dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Maxwell, Aaron J.; Wadsley, James; Couchman, H. M. P.; Sills, Alison

    2014-04-01

    We present a framework that explains the commonly observed variation in light element abundances in globular clusters. If globular clusters form in the centres of dwarf galaxies, they will be pumped on to larger orbits as star formation progresses. The potential well will only retain the moderate velocity asymptotic giant branch (AGB) ejecta, the expected source of enrichment, but not supernova ejecta. There is no need to increase the initial cluster mass, a requirement of self-enrichment scenarios, as all the stars within the dwarf can contribute. As the clusters move through the dwarf centre they sweep up a mix of AGB ejecta and in-falling pristine gas to form a second generation of stars. The specific mix will vary in time and is thus able to explain the spread in second generation abundances observed in different clusters. The globular clusters will survive to the present day or be stripped as part of the hierarchical merging process of larger galaxies. We illustrate how this process may operate using a high-resolution simulation of a dwarf galaxy at high redshift.

  2. Dynamical Formation of Black Hole Binaries in Globular Clusters and the Origins of GW150914

    NASA Astrophysics Data System (ADS)

    Rasio, Frederic A.; Chatterjee, Sourav; Rodriguez, Carl L.

    2016-06-01

    We show that GW150914, the binary black hole merger detected last year by LIGO, could easily have been formed dynamically through interactions in the dense core of an old globular cluster. Using models of globular clusters with detailed N-body dynamics and stellar evolution, we show that a typical cluster can very naturally form a binary black hole with "heavy" components that will merge at low redshift, like GW150914. We describe in some detail the dynamical interaction processes that could form such a system. Finally, we also show that theoretical predictions for this dynamical formation channel are in general far more robust than those from "population synthesis" studies for isolated massive binaries in the field.

  3. 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.

  4. From Globular Clusters to Tidal Dwarfs: Structure Formation in Tidal Tails

    NASA Astrophysics Data System (ADS)

    Knierman, K.; Hunsberger, S.; Gallagher, S.; Charlton, J.; Whitmore, B.; Hibbard, J.; Kundu, A.; Zaritsky, D.

    1999-12-01

    Galaxy interactions trigger star formation in tidal debris. How does this star formation depend on the local and global physical conditions? Using WFPC2/HST images, we investigate the range of structure within tidal tails of four classic ``Toomre Sequence'' mergers: NGC 4038/9 (``Antennae''), NGC 7252 (``Atoms for Peace''), NGC 3921, and NGC 3256. These tails contain a variety of stellar associations with sizes from globular clusters up to dwarf Irregulars. We explore whether there is a continuum between the two extremes. Our eight fields sample seven tidal tails at a variety of stages in the evolutionary sequence. Some of these tails are rich in HI while others are HI poor. Large tidal dwarfs are embedded in three of the tails. Using V and I WFPC2 images, we measure luminosities and colors of substructures within the tidal tails. The properties of globular cluster candidates in the tails will be contrasted with those of the hundreds of young clusters in the central regions of these mergers. We address whether globular clusters form and survive in the tidal tails and whether tidal dwarfs are composed of only young stars. By comparing the properties of structures in the tails of the four mergers with different ages, we examine systematic evolution of structure along the evolutionary sequence and as a function of HI content. We acknowledge support from NASA through STScI, and from NSF for an REU supplement for Karen Knierman.

  5. Constraining globular cluster formation through studies of young massive clusters - IV. Testing the fast rotating massive star scenario

    NASA Astrophysics Data System (ADS)

    Bastian, N.; Hollyhead, K.; Cabrera-Ziri, I.

    2014-11-01

    One of the leading models for the formation of multiple stellar populations within globular clusters is the `fast rotating massive star' (FRMS) scenario, where the ejecta of rapidly rotating massive stars is mixed with primordial material left over from the star formation process, to form a second generation of stars within the decretion discs of the high-mass stars. A requirement of this model, at least in its current form, is that young massive (i.e. proto-globular) clusters are not able to eject the unused gas and dust from the star formation process from the cluster for 20-30 Myr after the formation of the first generation of stars, i.e. the cluster remains embedded within the gas cloud in which it forms. Here, we test this prediction by performing a literature search for young massive clusters in nearby galaxies, which have ages less than 20 Myr that are not embedded. We report that a number of such clusters exist, with masses near or significantly above 106 M⊙, with ages between a few Myr and ˜15 Myr, suggesting that even high-mass clusters are able to clear any natal gas within them within a few Myr after formation. Additionally, one cluster, Cluster 23 in ESO 338-IG04, has a metallicity below that of some Galactic globular clusters that have been found to host multiple stellar populations, mitigating any potential effect of differences in metallicity in the comparison. The clusters reported here are in contradiction to the expectations of the FRMS scenario, at least in its current form.

  6. Star formation in globular clusters and dwarf galaxies and implications for the early evolution of galaxies

    NASA Technical Reports Server (NTRS)

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

    1991-01-01

    Based upon the observed properties of globular clusters and dwarf galaxies in the Local Group, we present important theoretical constraints on star formation in these systems. These constraints indicate that protoglobular cluster clouds had long dormant periods and a brief epoch of violent star formation. Collisions between protocluster clouds triggered fragmentation into individual stars. Most protocluster clouds dispersed into the Galactic halo during the star formation epoch. In contrast, the large spread in stellar metallicity in dwarf galaxies suggests that star formation in their pregenitors was self-regulated: we propose the protocluster clouds formed from thermal instability in the protogalactic clouds and show that a population of massive stars is needed to provide sufficient UV flux to prevent the collapsing protogalactic clouds from fragmenting into individual stars. Based upon these constraints, we propose a unified scenario to describe the early epochs of star formation in the Galactic halo as well as the thick and thin components of the Galactic disk.

  7. Globular Cluster Formation Efficiencies from Black Hole X-Ray Binary Feedback

    NASA Astrophysics Data System (ADS)

    Justham, Stephen; Peng, Eric W.; Schawinski, Kevin

    2015-08-01

    We investigate a scenario in which feedback from black hole X-ray binaries (BHXBs) sometimes begins inside young star clusters before strong supernova (SN) feedback. Those BHXBs could reduce the gas fraction inside embedded young clusters while maintaining virial equilibrium, which may help globular clusters (GCs) to stay bound when SN-driven gas ejection subsequently occurs. Adopting a simple toy model with parameters guided by BHXB population models, we produce GC formation efficiencies consistent with empirically inferred values. The metallicity dependence of BHXB formation could naturally explain why GC formation efficiency is higher at lower metallicity. For reasonable assumptions about that metallicity dependence, our toy model can produce a GC metallicity bimodality in some galaxies without a bimodality in the field-star metallicity distribution.

  8. Globular Clusters, Ultra-Compact Dwarfs, and the Formation of Galaxy Halos

    NASA Astrophysics Data System (ADS)

    Peng, Eric

    2015-08-01

    Globular clusters (GCs) are a distinctive and ubiquitous constituent of galaxy halos. Their existence alludes to an early epoch of galaxy building characterized by the high star formation rates needed to form massive clusters, and a merging process that produced the extended, spheroidal stellar halos in today's galaxies. While studies of stellar halos are generally limited by low surface brightnesses or the faintness of individual halo stars, GCs are bright and compact, making them excellent tracers of stellar halos out to hundreds of megaparsecs. The Next Generation Virgo Cluster Survey (NGVS) is a CFHT Large Program that has acquired imaging of the 104 square degrees within the Virgo Cluster's virial radius. This deep and contiguous imaging of the nearest galaxy cluster provides us a new view of globular clusters across the full range of galaxy morphology and mass, as well as in the regions between galaxies. It also provides the first complete census of ultra-compact dwarfs (UCDs) in Virgo, objects which may be related to massive GCs and galaxy nuclei. In this talk, I will present what we have learned so far about extragalactic GC systems and UCDs from the NGVS, from both photometry and spectroscopy.

  9. Are globular clusters the natural outcome of regular high-redshift star formation?

    NASA Astrophysics Data System (ADS)

    Kruijssen, J. M. Diederik

    2016-02-01

    We summarise the recent progress in understanding the formation and evolution of globular clusters (GCs) in the context of galaxy formation and evolution. It is discussed that an end-to-end model for GC formation and evolution should capture four different phases: (1) star and cluster formation in the high-pressure interstellar medium of high-redshift galaxies, (2) cluster disruption by tidal shocks in the gas-rich host galaxy disc, (3) cluster migration into the galaxy halo, and (4) the final evaporation-dominated evolution of GCs until the present day. Previous models have mainly focussed on phase 4. We present and discuss a simple model that includes each of these four steps - its key difference with respect to previous work is the simultaneous addition of the high-redshift formation and early evolution of young GCs, as well as their migration into galaxy haloes. The new model provides an excellent match to the observed GC mass spectrum and specific frequency, as well as the relations of GCs to the host dark matter halo mass and supermassive black hole mass. These results show (1) that the properties of present-day GCs are reproduced by assuming that they are the natural outcome of regular high-redshift star formation (i.e. they form according to same physical processes that govern massive cluster formation in the local Universe), and (2) that models only including GC evaporation strongly underestimate their integrated mass loss over a Hubble time.

  10. 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

  11. 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.

  12. Protein cluster formation during enzymatic cross-linking of globular proteins.

    PubMed

    Saricay, Yunus; Dhayal, Surender Kumar; Wierenga, Peter Alexander; de Vries, Renko

    2012-01-01

    Work on enzymatic cross-linking of globular food proteins has mainly focused on food functional effects such as improvements of gelation and enhanced stabilization of emulsions and foams, and on the detailed biochemical characterization of the cross-linking chemistry. What is still lacking is a physical characterization of cluster formation and gelation, as has been done for example, for cluster formation and gelation during heat-induced protein aggregation. Here we present preliminary results along these lines. We propose that enzymatic cross-linking of apo-alpha-lactalbumin is a good model system for studying the problem of cluster formation and gelation during enzymatic cross-linking of globular proteins. We present initial results on cluster sizes produced when crosslinking dilute solutions of apo-alpha-lactalbumin with a range of cross-linking enzymes: microbial transglutaminase, horseradish peroxidase, and mushroom tyrosinase. These results are used to highlight similarities and differences between different enzymes, when acting on the same substrate. Next we consider cluster growth and gelation in somewhat more detail for the specific case of cross-linking by horseradish peroxidase, under the periodic addition of H2O2. Upon increasing the initial concentration of apo-alpha-lactalbumin, at a fixed enzyme-to-substrate ratio and fixed reaction time, the size of the clusters at the end of the reaction increases rapidly, and above a critical concentration, gelation occurs. For the conditions that we have used, gelation occurred at very low initial apo-alpha-lactalbumin concentrations of 34% (w/v), indicating a very dilute cross-linked protein network, with a low average number of cross-links per protein. It is found that reactive protein monomers are first rapidly (1-2 h) incorporated into small covalent clusters. This is followed by a much slower phase (up to about 12 h) in which the small clusters are coupled together to form much larger covalent protein

  13. MOCCA code for star cluster simulations - IV. A new scenario for intermediate mass black hole formation in globular clusters

    NASA Astrophysics Data System (ADS)

    Giersz, Mirek; Leigh, Nathan; Hypki, Arkadiusz; Lützgendorf, Nora; Askar, Abbas

    2015-12-01

    We discuss a new scenario for the formation of intermediate mass black holes (IMBHs) in dense star clusters. In this scenario, IMBHs are formed as a result of dynamical interactions of hard binaries containing a stellar-mass black hole (BH), with other stars and binaries. We discuss the necessary conditions to initiate the process of intermediate mass BH formation and the influence of an IMBH on the host global globular cluster (GC) properties. We discuss two scenarios for IMBH formation. The SLOW and FAST scenarios. They occur later or earlier in the cluster evolution and require smaller or extremely large central densities, respectively. In our simulations, the formation of IMBHs is highly stochastic. In general, higher formation probabilities follow from larger cluster concentrations (i.e. central densities). We further discuss possible observational signatures of the presence of IMBHs in GCs that follow from our simulations. These include the spatial and kinematic structure of the host cluster, possible radio, X-ray and gravitational wave emissions due to dynamical collisions or mass transfer and the creation of hypervelocity main-sequence escapers during strong dynamical interactions between binaries and an IMBH. All simulations discussed in this paper were performed with the MOCCA (MOnte Carlo Cluster simulAtor) Monte Carlo code. MOCCA accurately follows most of the important physical processes that occur during the dynamical evolution of star clusters but, as with other dynamical codes, it approximates the dissipative processes connected with stellar collisions and binary mergers.

  14. Extragalactic Globular Clusters: Old Spectroscopic Ages and New Views on Their Formation

    NASA Astrophysics Data System (ADS)

    Strader, Jay; Brodie, Jean P.; Cenarro, A. J.; Beasley, Michael A.; Forbes, Duncan A.

    2005-10-01

    We present the results of a meta-analysis of Keck spectra of extragalactic globular clusters (GCs) in a sample of eight galaxies, ranging from dwarf galaxies to massive elliptical galaxies. We infer ages for the metal-poor and metal-rich GCs in these galaxies through comparisons to Galactic GCs. Both subpopulations appear to be no younger than their Galactic counterparts, with ages >~10 Gyr. This is the largest sample of galaxies for which ages have been constrained spectroscopically. Our results support the formation of most GCs in massive galaxies at high redshift. We propose a scenario for the formation of GC subpopulations that synthesizes aspects of both accretion and in situ approaches in the context of galaxy formation through hierarchical merging.

  15. From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Galaxies

    NASA Astrophysics Data System (ADS)

    Knierman, Karen A.; Gallagher, Sarah C.; Charlton, Jane C.; Hunsberger, Sally D.; Whitmore, Bradley; Kundu, Arunav; Hibbard, J. E.; Zaritsky, Dennis

    2003-09-01

    Using V and I images obtained with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope, we investigate compact stellar structures within tidal tails. Six regions of tidal debris in the four classic ``Toomre sequence'' mergers: NGC 4038/39 (``Antennae''), NGC 3256, NGC 3921, and NGC 7252 (``Atoms for Peace'') have been studied in order to explore how the star formation depends on the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence and tails with and without embedded tidal dwarf galaxies. The six tails are found to contain a variety of stellar structures, with sizes ranging from those of globular clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure the luminosities and colors of the star clusters. NGC 3256 is found to have a large population of blue clusters (0.2<~V-I<~0.9), particularly in its western tail, similar to those found in the inner region of the merger. In contrast, NGC 4038/39 has no clusters in the observed region of the tail, only less luminous point sources likely to be individual stars. NGC 3921 and NGC 7252 have small populations of clusters along their tails. A significant cluster population is clearly associated with the prominent tidal dwarf candidates in the eastern and western tails of NGC 7252. The cluster-rich western tail of NGC 3256 is not distinguished from the others by its dynamical age or by its total H I mass. However, the mergers that have few clusters in the tail all have tidal dwarf galaxies, while NGC 3256 does not have prominent tidal dwarfs. We speculate that star formation in tidal tails may manifest itself either in small structures like clusters along the tail or in large structures such as dwarf galaxies, but not in both. Also, NGC 3256 has the highest star formation rate of the four mergers studied, which may contribute to the high number of star clusters in its tidal tails. Based in part on observations obtained with the

  16. A single model for the variety of multiple-population formation(s) in globular clusters: a temporal sequence

    NASA Astrophysics Data System (ADS)

    D'Antona, F.; Vesperini, E.; D'Ercole, A.; Ventura, P.; Milone, A. P.; Marino, A. F.; Tailo, M.

    2016-05-01

    We explain the multiple populations recently found in the `prototype' globular cluster (GC) NGC 2808 in the framework of the asymptotic giant branch (AGB) scenario. The chemistry of the five - or more - populations is approximately consistent with a sequence of star formation events, starting after the Type II supernova epoch, lasting approximately until the time when the third dredge-up affects the AGB evolution (age ˜90-120 Myr), and ending when the Type Ia supernovae begin exploding in the cluster, eventually clearing it from the gas. The formation of the different populations requires episodes of star formation in AGB gas diluted with different amounts of pristine gas. In the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV Legacy Survey of GCs, the nitrogen increase is due to the third dredge-up in the smallest mass AGB ejecta involved in the star formation of this population. The possibly iron-rich small population in NGC 2808 may be a result of contamination by a single Type Ia supernova. The NGC 2808 case is used to build a general framework to understand the variety of `second-generation' stars observed in GCs. Cluster-to-cluster variations are ascribed to differences in the effects of the many processes and gas sources which may be involved in the formation of the second generation. We discuss an evolutionary scheme, based on pollution by delayed Type II supernovae, which accounts for the properties of s-Fe-anomalous clusters.

  17. From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Pairs

    NASA Astrophysics Data System (ADS)

    Knierman, K. A.; Gallagher, S. C.; Charlton, J. C.; Hunsberger, S. D.; Whitmore, B. C.; Kundu, A.; Hibbard, J. E.; Zaritsky, D. F.

    2001-05-01

    Using V and I images obtained with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope, we investigate compact stellar structures within tidal tails. Six regions of tidal debris in the four classic ``Toomre Sequence'' mergers: NGC 4038/9 (``Antennae''), NGC 3256, NGC 3921, and NGC 7252 (``Atoms for Peace'') have been studied in order to explore how the star formation depends upon the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence, and include HI--rich and HI--poor environments. The six tails are found to contain a variety of stellar structures, with sizes ranging from those of globular clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure the luminosities and colors of the star clusters. NGC 3256 is found to have a large population of young clusters lying along both tails, similar to those found in the inner region of the merger. In contrast, NGC 4038/9 has no clusters in the observed region of the tail, only less luminous point sources likely to be individual stars. NGC 3921 and NGC 7252 have small populations of clusters that are concentrated in certain regions of the tail, and particularly in the prominent tidal dwarfs in the eastern and western tails of NGC 7252. The two cluster--rich tails of NGC 3256 are not distinguished from the others by their ages or by their total HI masses. We acknowledge support from NASA through STScI, and from NSF for an REU supplement for Karen Knierman.

  18. Binary Interactions as a Possible Scenario for the Formation of Multiple Stellar Populations in Globular Clusters

    NASA Astrophysics Data System (ADS)

    Jiang, Dengkai; Han, Zhanwen; Li, Lifang

    2014-07-01

    Observations have revealed the presence of multiple stellar populations in globular clusters (GCs) that exhibit wide abundance variations and multiple sequences in the Hertzsprung-Russell (HR) 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 stars with anomalous abundances observed in GCs are merged stars and accretor stars produced by binary interactions—rapidly rotating stars at the moment of their formation—and that these stars are more massive than normal single stars in the same evolutionary stage. We find that, due to their own evolution, these rapidly rotating stars have surface abundances, effective temperatures, and luminosities that are different from normal single stars in the same evolutionary stage. This stellar population of binaries reproduces two important points of observational evidence of multiple stellar populations: a Na-O anticorrelation and multiple sequences in the HR diagram. This evidence suggests that binary interactions may be a possible scenario for the formation of multiple stellar populations in GCs.

  19. Binary interactions as a possible scenario for the formation of multiple stellar populations in globular clusters

    SciTech Connect

    Jiang, Dengkai; Han, Zhanwen; Li, Lifang E-mail: zhanwenhan@ynao.ac.cn

    2014-07-01

    Observations have revealed the presence of multiple stellar populations in globular clusters (GCs) that exhibit wide abundance variations and multiple sequences in the Hertzsprung-Russell (HR) 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 stars with anomalous abundances observed in GCs are merged stars and accretor stars produced by binary interactions—rapidly rotating stars at the moment of their formation—and that these stars are more massive than normal single stars in the same evolutionary stage. We find that, due to their own evolution, these rapidly rotating stars have surface abundances, effective temperatures, and luminosities that are different from normal single stars in the same evolutionary stage. This stellar population of binaries reproduces two important points of observational evidence of multiple stellar populations: a Na-O anticorrelation and multiple sequences in the HR diagram. This evidence suggests that binary interactions may be a possible scenario for the formation of multiple stellar populations in GCs.

  20. Four and one more: The formation history and total mass of globular clusters in the Fornax dSph

    NASA Astrophysics Data System (ADS)

    de Boer, T. J. L.; Fraser, M.

    2016-05-01

    We have determined the detailed star formation history and total mass of the globular clusters in the Fornax dwarf spheroidal using archival HST WFPC2 data. Colour-magnitude diagrams were constructed in the F555W and F814W bands and corrected for the effect of Fornax field star contamination, after which we used the routine Talos to derive the quantitative star formation history as a function of age and metallicity. The star formation history of the Fornax globular clusters shows that Fornax 1, 2, 3, and 5 are all dominated by ancient (>10 Gyr) populations. Clusters Fornax 1, 2, and 3 display metallicities as low as [Fe/H] = -2.5, while Fornax 5 is slightly more metal-rich at [Fe/H] = -1.8, consistent with resolved and unresolved metallicity tracers. Conversely, Fornax 4 is dominated by a more metal-rich ([Fe/H] = -1.2) and younger population at 10 Gyr, inconsistent with the other clusters. A lack of stellar populations overlapping with the main body of Fornax argues against the nucleus cluster scenario for Fornax 4. The combined stellar mass in globular clusters as derived from the SFH is (9.57 ± 0.93) × 105 M⊙, which corresponds to 2.5 ± 0.2 percent of the total stellar mass in Fornax. The mass of the four most metal-poor clusters can also be compared to the metal-poor Fornax field to yield a mass fraction of 19.6 ± 3.1 percent. Therefore, the SFH results provide separate supporting evidence for the unusually high mass fraction of the globular clusters compared to the Fornax field population.

  1. Where Are the Universe's Globular Clusters?

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-04-01

    -rich ones 4 to 1. [Harris 2016]He finds that globular clusters are most commonly found in galaxies within a surprisingly narrow range around the characteristic luminosity of the Schechter function. This means that, at the current time, the collection of galaxies similar in brightness to the Milky Way or Andromeda host the largest total number of globulars in the universe.Metal-Poor DominanceHarris extends these calculations by examining two subpopulations of globulars: blue (metal-poor) and red (metal-rich). Metal-poor globular clusters are found in all galaxies, but metal-rich ones reside preferentially in massive, bright galaxies. Strikingly, Harris finds that this preference results in metal-poor globulars making up almost 80% of all globular clusters in the universe, outnumbering the metal-rich ones by nearly 4 to 1.This result implies that the earliest stages of hierarchical galaxy mergers when most of the available gas was low-metallicity provided the most favorable conditions for the formation of dense, massive star clusters. This early environment birthed the majority of the globular clusters we see today.CitationWilliam E. Harris 2016 AJ 151 102. doi:10.3847/0004-6256/151/4/102

  2. 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.

  3. Globular clusters as the relics of regular star formation in `normal' high-redshift galaxies

    NASA Astrophysics Data System (ADS)

    Kruijssen, J. M. Diederik

    2015-12-01

    We present an end-to-end, two-phase model for the origin of globular clusters (GCs). In the model, populations of stellar clusters form in the high-pressure discs of high-redshift (z > 2) galaxies (a rapid-disruption phase due to tidal perturbations from the dense interstellar medium), after which the galaxy mergers associated with hierarchical galaxy formation redistribute the surviving, massive clusters into the galaxy haloes, where they remain until the present day (a slow-disruption phase due to tidal evaporation). The high galaxy merger rates of z > 2 galaxies allow these clusters to be `liberated' into the galaxy haloes before they are disrupted within the high-density discs. This physically motivated toy model is the first to include the rapid-disruption phase, which is shown to be essential for simultaneously reproducing the wide variety of properties of observed GC systems, such as their universal characteristic mass-scale, the dependence of the specific frequency on metallicity and galaxy mass, the GC system mass-halo mass relation, the constant number of GCs per unit supermassive black hole mass, and the colour bimodality of GC systems. The model predicts that most of these observables were already in place at z = 1-2, although under rare circumstances GCs may still form in present-day galaxies. In addition, the model provides important constraints on models for multiple stellar populations in GCs by putting limits on initial GC masses and the amount of pristine gas accretion. The paper is concluded with a discussion of these and several other predictions and implications, as well as the main open questions in the field.

  4. FORMATION OF COMPACT STELLAR CLUSTERS BY HIGH-REDSHIFT GALAXY OUTFLOWS. III. OBSERVABILITY AND CONNECTION TO HALO GLOBULAR CLUSTERS

    SciTech Connect

    Gray, William J.; Scannapieco, Evan

    2011-12-01

    The early universe hosted a large population of low-mass virialized 'minihalos', that were not massive enough to form stars on their own. While most minihalos were photoevaporated by ionizing photons from star-forming galaxies, these galaxies also drove large outflows, which in some cases would have reached the minihalos in advance of ionization fronts. In the previous papers in this series, we carried out high-resolution, three-dimensional adaptive mesh refinement simulations of outflow-minihalo interactions that included non-equilibrium chemistry, radiative cooling, and turbulent mixing. We found that, for a fiducial set of parameters, minihalos were transformed into dense, chemically homogenous stellar clusters. Here we conduct a suite of simulations that follow these interactions over a wide range of parameters including minihalo mass, minihalo formation redshift, outflow energy, outflow redshift, distance, concentration, and spin. In almost all cases, the shocked minihalos form molecules through non-equilibrium reactions and then cool rapidly to become compact, chemically homogenous stellar clusters. Furthermore, we show that the unique properties of these clusters make them a prime target for direct study with the next generation of telescopes, and that there are many reasons to suspect that their low-redshift counterparts are the observed population of halo globular clusters.

  5. 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.

  6. Two epochs of globular cluster formation from deep field luminosity functions: implications for reionization and the Milky Way satellites

    NASA Astrophysics Data System (ADS)

    Katz, Harley; Ricotti, Massimo

    2013-07-01

    The ages of globular clusters in our own Milky Way are known with precision of about ±1 Gyr, hence their formation history at redshifts z ≳ 3 and their role in hierarchical cosmology and the reionization of the intergalactic medium remain relatively undetermined. Here we analyse the effect of globular cluster formation on the observed rest-frame UV luminosity functions (LFs) and UV continuum slopes of high-redshift galaxies in the Hubble Ultra Deep Fields. We find that the majority of present-day globular clusters have formed during two distinct epochs: at redshifts z ˜ 2-3 and at redshifts z ≳ 6. The birth of proto-GC systems produces the steep, faint-end slopes of the galaxy LFs and, because the brightness of proto-GCs fades 5 Myr after their formation, their blue colours are in excellent agreement with observations. Our results suggest that: (i) the bulk of the old globular cluster population with estimated ages ≳12 Gyr (about 50 per cent of the total population) formed in the relatively massive dwarf galaxies at redshifts z ≳ 6; (ii) proto-GC formation was an important mode of star formation in those dwarf galaxies, and likely dominated the reionization process. Another consequence of this scenario is that some of the most massive Milky Way satellites may be faint and yet undiscovered because tidal stripping of a dominant GC population precedes significant stripping of the dark matter haloes of these satellites. This scenario may alleviate some remaining tensions between cold dark matter simulations and observations.

  7. FIRST EVIDENCE OF GLOBULAR CLUSTER FORMATION FROM THE EJECTA OF PROMPT TYPE Ia SUPERNOVAE

    SciTech Connect

    Tsujimoto, Takuji; Bekki, Kenji

    2012-06-01

    Recent spectroscopic observations of globular clusters (GCs) in the Large Magellanic Cloud (LMC) have discovered that one of the intermediate-age GCs, NGC 1718, with [Fe/H] = -0.7 has an extremely low [Mg/Fe] ratio of {approx}-0.9. We propose that NGC 1718 was formed from the ejecta of Type Ia supernovae mixed with very metal-poor ([Fe/H] <-1.3) gas about {approx}2 Gyr ago. The proposed scenario is shown to be consistent with the observed abundances of Fe-group elements such as Cr, Mn, and Ni. In addition, compelling evidence for asymptotic giant branch stars playing a role in chemical enrichment during this GC formation is found. We suggest that the origin of the metal-poor gas is closely associated with efficient gas transfer from the outer gas disk of the Small Magellanic Cloud to the LMC disk. We anticipate that the outer part of the LMC disk contains field stars exhibiting significantly low [Mg/Fe] ratios, formed through the same process as NGC 1718.

  8. 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.

  9. Globular clusters and their contribution to the formation of the Galactic halo

    NASA Astrophysics Data System (ADS)

    Carretta, Eugenio

    2016-08-01

    This is a ``biased'' review because I will show recent evidence on the contribution of globular clusters (GCs) to the halo of our Galaxy seen through the lens of the new paradigm of multiple populations in GCs. I will show a few examples where the chemistry of multiple populations helps to answer hot questions including whether and how much GCs did contribute to the halo population, if we have evidence of the GCs-halo link, what are the strengths and weak points concerning this contribution.

  10. 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.

  11. 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.

  12. 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

  13. 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.

  14. The age of the globular cluster NGC 288, the formation of the Galactic halo, and the second parameter

    SciTech Connect

    Bolte, M. )

    1989-06-01

    A differential comparison of precise CCD photometry in the globular clusters NGC 288, NGC 362, and NGC 1261 shows that differences exist in the positions of the main-sequence turnoff in these clusters that are most naturally explained if NGC 288 is some 3 billion yr older than NGC 362 and about 1 to 2 billion yr older than NGC 1261. This implies that the formation time for the Galactic halo is significantly longer than a freefall time. Consideration of the inferred ages and horizontal-branch morphologies of the clusters Pal 12, NGC 288, NGC 362, and NGC 1261, all with similar metal abundances, suggests that age may be the parameter that, after overall metal abundance, most determines horizontal-branch morphology. 56 refs.

  15. 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.

  16. 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.

  17. 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

  18. 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.

  19. 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.

  20. 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.

  1. 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

  2. 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

  3. Formation of Globular Clusters in Atomic-cooling Halos Via Rapid Gas Condensation and Fragmentation during the Epoch of Reionization

    NASA Astrophysics Data System (ADS)

    Kimm, Taysun; Cen, Renyue; Rosdahl, Joakim; Yi, Sukyoung K.

    2016-05-01

    We investigate the formation of metal-poor globular clusters (GCs) at the center of two dark matter halos with {M}{{halo}}˜ 4× {10}7 {M}ȯ at z\\gt 10 using cosmological radiation-hydrodynamics simulations. We find that very compact (≲1 pc) and massive (˜ 6× {10}5 {M}ȯ ) clusters form rapidly when pristine gas collapses isothermally with the aid of efficient Lyα emission during the transition from molecular-cooling halos to atomic-cooling halos. Because the local free-fall time of dense star-forming gas is very short (\\ll 1 {{Myr}}), a large fraction of the collapsed gas is turned into stars before stellar feedback processes blow out the gas and shut down star formation. Although the early stage of star formation is limited to a small region of the central star-forming disk, we find that the disk quickly fragments due to metal enrichment from supernovae. Sub-clusters formed in the fragmented clouds eventually merge with the main cluster at the center. The simulated clusters closely resemble the local GCs in mass and size but show a metallicity spread that is much wider than found in the local GCs. We discuss a role of pre-enrichment by Pop III and II stars as a potential solution to the latter issue. Although not without shortcomings, it is encouraging that a naive blind (not tuned) cosmological simulation presents a possible channel for the formation of at least some massive GCs.

  4. 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.

  5. 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.

  6. The Formation of First Generation Stars and Globular Clusters in Protogalactic Clouds

    SciTech Connect

    Murray, S

    2003-07-07

    Within collapsing protogalaxies, thermal instability leads to the formation of a population of cool fragments which are confined by the pressure of a residual hot background medium. The critical mass required for the cold clouds to become gravitationally unstable and to form stars is determined by both their internal temperature and external pressure. Massive first generation stars form in primordial clouds with sufficient column density to shield themselves from external UV photons emitted by nearby massive stars or AGNs. Less massive photoionized clouds gain mass due to ram pressure stripping by the residual halo gas. Collisions may also trigger thermal instability and fragmentation into cloudlets. While most cloudlets have substellar masses, the largest become self-gravitating and collapse to form protostellar cores without further fragmentation. The initial stellar mass function is established as these cores capture additional residual cloudlets. Energy dissipation from the mergers ensures that the cluster remains bound in the limit of low star formation efficiency. Dissipation also promotes the formation and retention of the most massive stars in the cluster center. On the scale of the protogalactic clouds, the formation of massive stars generates intense UV radiation which photoionizes gas and quenches star formation in nearby regions. As gas density accumulates in the center of the galactic potential, the self-regulated star formation rate increases. At the location where most of the residual gas can be converted into stars on its internal dynamical timescale, a galaxy attains its asymptotic kinematic structure such as exponential profiles, Tully-Fisher, and Faber-Jackson laws.

  7. 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.

  8. FORMATION OF METAL-POOR GLOBULAR CLUSTERS IN Ly{alpha} EMITTING GALAXIES IN THE EARLY UNIVERSE

    SciTech Connect

    Elmegreen, Bruce G.; Malhotra, Sangeeta; Rhoads, James

    2012-09-20

    The size, mass, luminosity, and space density of Ly{alpha} emitting (LAE) galaxies observed at intermediate to high redshift agree with expectations for the properties of galaxies that formed metal-poor halo globular clusters (GCs). The low metallicity of these clusters is the result of their formation in low-mass galaxies. Metal-poor GCs could enter spiral galaxies along with their dwarf galaxy hosts, unlike metal-rich GCs, which form in the spirals themselves. Considering an initial GC mass larger than the current mass to account for multiple stellar populations, and considering the additional clusters that are likely to form with massive clusters, we estimate that each GC with a mass today greater than 2 Multiplication-Sign 10{sup 5} M{sub Sun} was likely to have formed among a total stellar mass {approx}> 3 Multiplication-Sign 10{sup 7} M{sub Sun }, a molecular mass {approx}> 10{sup 9} M{sub Sun }, and 10{sup 7} to 10{sup 9} M{sub Sun} of older stars, depending on the relative gas fraction. The star formation rate would have been several M{sub Sun} yr{sup -1} lasting for {approx}10{sup 7} yr, and the Ly{alpha} luminosity would have been {approx}> 10{sup 42} erg s{sup -1}. Integrating the LAE galaxy luminosity function above this minimum, considering the average escape probability for Ly{alpha} photons (25%), and then dividing by the probability that a dwarf galaxy is observed in the LAE phase (0.4%), we find agreement between the comoving space density of LAEs and the average space density of metal-poor GCs today. The local galaxy WLM, with its early starburst and old GC, could be an LAE remnant that did not get into a galaxy halo because of its remote location.

  9. 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

  10. 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

  11. CANDIDATE TIDAL DWARF GALAXIES IN Arp 305: LESSONS ON DWARF DETACHMENT AND GLOBULAR CLUSTER FORMATION

    SciTech Connect

    Hancock, Mark; Smith, Beverly J.; Giroux, Mark L.; Hurlock, Sabrina; Struck, Curtis E-mail: smithbj@etsu.edu E-mail: zshh7@goldmail.etsu.edu

    2009-06-15

    To search for Tidal Dwarf Galaxies (TDGs) and to study star formation (SF) in tidal features, we are conducting a large UV imaging survey of interacting galaxies selected from the Arp (1996) Atlas using the Galaxy Evolution Explorer (GALEX) telescope. As part of that study, we present a GALEX UV and Sloan Digital Sky Survey and SARA optical study of the gas-rich interacting galaxy pair Arp 305 (NGC 4016/7). The GALEX UV data reveal much extended diffuse UV emission and SF outside the disks. This includes a luminous star-forming region between the two galaxies, and a number of such regions in tidal tails. We have identified 45 young star-forming clumps in Arp 305, including several TDG candidates. By comparing the UV and optical colors to population synthesis models, we determined that the clumps are very young, with several having ages {approx}6 Myr. We do not find many intermediate age clumps in spite of the fact that the last closest encounter was about 300 Myr ago. We have used a smooth particle hydrodynamics code to model the interaction and determine the fate of the star clusters and candidate TDGs.

  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. The Hubble Space TelescopeUV Legacy Survey of Galactic Globular Clusters - V. Constraints on formation scenarios

    NASA Astrophysics Data System (ADS)

    Renzini, A.; D'Antona, F.; Cassisi, S.; King, I. R.; Milone, A. P.; Ventura, P.; Anderson, J.; Bedin, L. R.; Bellini, A.; Brown, T. M.; Piotto, G.; van der Marel, R. P.; Barbuy, B.; Dalessandro, E.; Hidalgo, S.; Marino, A. F.; Ortolani, S.; Salaris, M.; Sarajedini, A.

    2015-12-01

    We build on the evidence provided by our Legacy Survey of Galactic globular clusters (GC) to submit to a crucial test four scenarios currently entertained for the formation of multiple stellar generations in GCs. The observational constraints on multiple generations to be fulfilled are manifold, including GC specificity, ubiquity, variety, predominance, discreteness, supernova avoidance, p-capture processing, helium enrichment and mass budget. We argue that scenarios appealing to supermassive stars, fast rotating massive stars and massive interactive binaries violate in an irreparable fashion two or more among such constraints. Also the scenario appealing to asymptotic giant branch (AGB) stars as producers of the material for next generation stars encounters severe difficulties, specifically concerning the mass budget problem and the detailed chemical composition of second-generation stars. We qualitatively explore ways possibly allowing one to save the AGB scenario, specifically appealing to a possible revision of the cross-section of a critical reaction rate destroying sodium, or alternatively by a more extensive exploration of the vast parameter space controlling the evolutionary behaviour of AGB stellar models. Still, we cannot ensure success for these efforts and totally new scenarios may have to be invented to understand how GCs formed in the early Universe.

  15. 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.

  16. 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.

  17. Globular Cluster Orbits from HST Proper Motions: Constraining the Formation and Mass of the Milky Way Halo

    NASA Astrophysics Data System (ADS)

    Sohn, S. Tony; Van Der Marel, Roeland P.; Deason, Alis J.; Bellini, Andrea; Besla, Gurtina; Watkins, Laura

    2016-06-01

    The globular cluster (GC) system of the Milky Way (MW) provides important information on the MW's present structure and past evolution. GCs in the halo are particularly useful tracers; because of their long dynamical timescales, their orbits retain imprints of their origin or accretion history. Full 3D motions are required to calculate past orbits. While most GCs have known line of sight velocities, accurate proper motion (PM) measurements are currently available for only a few halo GCs. Our goal is to create the first high-quality PM database for halo GCs. We have identified suitable 1st-epoch data in the HST Archive for 20 halo GCs at 10-100 kpc from the Galactic Center. We are in the process of obtaining the necessary 2nd-epoch data to determine absolute PMs of the target GCs through our HST program GO-14235. We will use the same advanced astrometric techniques that allowed us to measure the PMs of M31 and Leo I. Previous studies of the halo GC system based on e.g., stellar populations, metallicities, RR Lyrae properties, and structural properties have revealed a dichotomy between old and young halo GCs. This may reflect distinct formation scenarios (in situ vs. accreted). Orbit calculations based on our PMs will directly test this. The PMs will also yield the best handle yet on the velocity anisotropy profile of any tracer population in the halo. This will resolve the mass-anisotropy degeneracy to provide an improved estimate of the MW mass, which is at present poorly known. In summary, our project will deliver the first accurate PMs for halo GCs, and will significantly increase our understanding of the formation, evolution, and mass of the MW.

  18. 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.

  19. 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.

  20. 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.

  1. Multiple Metal-rich Sub-populations in Galaxies: the Star Formation/Assembly History of Galaxies Probed by Globular Clusters

    NASA Astrophysics Data System (ADS)

    Kissler-Patig, Markus

    Extragalactic globular clusters are used since a decade to probe the star formation and assembly history of nearby galaxies. In the early 90's, Zepf & Ashman (1993, MNRAS 264, 611) showed that several galaxies host at least two sub-populations of globular clusters. This fact was reinforced by subsequent studies (e. g. Gebhardt & Kissler-Patig 1999, AJ 118, 1526; Kundu 1999, PhD thesis), and many following studies dealt with the interpretation of the two main sub-populations (e. g. Kissler-Patig 2000, Reviews in Modern Astronomy, Vol. 13, p. 13 for a recent review) and their implications for galaxy formation scenarios. The nature of the metal-poor sub-populations is slowly understood as being a very uniform population in the nearby universe and most probably having formed in small fragments at the very beginning of galaxy formation and assembly. The nature of the metal-rich sub-populations is still hotly debated. It remains unclear whether its origin it mostly due to major mergers, whether it formed in situ during a monolithic bulge collapse, or whether it was accreted or cannibalized. The key to the answer lies in a more detailed analysis of the metal-rich sub-population. Is it itself composed by several sub-components? And can these sub-components tell us which mechanism dominated? First studies of extragalactic globular clusters in the optical and near-infrared show that the metal-rich sub-population might indeed be composed of several components. If confirmed, it would point to a significant contribution of hierarchical-like processes (accretion or mergers) to the metal-rich cluster and stellar component of galaxies.

  2. 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

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  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. 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.

  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. 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

  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. 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.

  2. 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

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  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. 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.

  10. A possible formation channel for blue hook stars in globular cluster II - Effects of metallicity, mass ratio, tidal enhancement efficiency and helium abundance

    NASA Astrophysics Data System (ADS)

    Lei, Zhenxin; Zhao, Gang; Zeng, Aihua; Shen, Lihua; Lan, Zhongjian; Jiang, Dengkai; Han, Zhanwen

    2016-09-01

    Employing tidally enhanced stellar wind, we studied in binaries the effects of metallicity, mass ratio of primary to secondary, tidal enhancement efficiency and helium abundance on the formation of blue hook (BHk) stars in globular clusters (GCs). Totally, 28 sets of binary models combined with different input parameters are studied. For each set of binary model, we presented the range of initial orbital periods which is needed to produce BHk stars in binaries. All the binary models could produce BHk stars within different range of initial orbital periods. We also compared our results with the observation in the Teff-logg diagram of GC NGC 2808 and ω Cen. Most of the BHk stars in these two GCs locate well in the region predicted by our theoretical models, especially when C/N enhanced model atmospheres is considered. We found that mass ratio of primary to secondary and tidal enhancement efficiency have little effects on the formation of BHk stars in binaries, while metallicity and helium abundance would play important roles, especially for helium abundance. Specifically, with helium abundance increasing in binary models, the space range of initial orbital periods needed to produce BHk stars becomes obviously wider, regardless of other input parameters adopted. Our results were discussed with recent observations and other theoretical models.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. New insights into the star formation histories of candidate intermediate-age early-type galaxies from K'-band imaging of globular clusters

    NASA Astrophysics Data System (ADS)

    Georgiev, Iskren Y.; Goudfrooij, Paul; Puzia, Thomas H.

    2012-02-01

    We investigate the age and metallicity distributions of bright globular clusters (GCs) in the candidate intermediate-age early-type galaxies NGC 3610, 584 and 3377 using a combination of new Gemini's Near InfraRed Imager and Spectrometer K'-band imaging and existing optical VI photometry from Hubble Space Telescope data. The V-I versus I-K' colour-colour diagram is found to break the age-metallicity degeneracy present in optical colours and spectroscopy, as I-K' primarily measures a population's metallicity. In addition, it is relatively insensitive to the effect of hot horizontal branch (HB) stars that are known to be present in massive old GCs. By interpolation between simple stellar population model tracks, we derive photometric cluster ages, metallicities and masses. In general, we find that 'metal-poor' GCs with [Z/H] ≲-0.7 dex are older than more metal rich GCs. For the most massive GCs (?) in NGC 3610 with available spectroscopic data, the photometric ages are older by ˜2 Gyr, and this difference is more pronounced for the metal-poor GCs. However, the photometric and spectroscopic metallicities are in good agreement. We suggest that this indicates the presence of a hot HB in these massive clusters, which renders spectroscopic ages from Balmer line strengths to be underestimated. To support this suggestion, we show that all Galactic GCs with ? feature hot HBs, except 47 Tuc. Using a recent observational relation between the luminosity of the most massive GC and the galaxies' star formation rate (SFR) at a given age, we find that the galaxies' peak SFR was attained at the epoch of the formation of the oldest (metal-poor) GCs. The age and [Z/H] distributions of the metal-rich GCs are broad, indicating prolonged galaxy star formation histories. The peak ages of the metal-rich GCs in the sample galaxies NGC 3610, 584 and 3377 are 3.7, 5.9 and 8.9 Gyr, respectively. The peak value of the age and metallicity distributions of the GCs is correlated with the host

  16. 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.

  17. 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

  18. 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

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. 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

  5. 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

  6. 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.

  7. 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.

  8. 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

  9. 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.

  10. 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.

  11. 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.

  12. CHEMICAL ABUNDANCE EVIDENCE OF ENDURING HIGH STAR FORMATION RATES IN AN EARLY-TYPE GALAXY: HIGH [Ca/Fe] IN NGC 5128 GLOBULAR CLUSTERS

    SciTech Connect

    Colucci, Janet E.; Duran, Maria Fernanda; Bernstein, Rebecca A.

    2013-08-20

    We present [Fe/H], ages, and Ca abundances for an initial sample of 10 globular clusters in NGC 5128 obtained from high-resolution, high signal-to-noise ratio echelle spectra of their integrated light. All abundances and ages are obtained using our original technique for high-resolution integrated light abundance analysis of globular clusters. The clusters have a range in [Fe/H] between -1.6 and -0.2. In this sample, the average [Ca/Fe] for clusters with [Fe/H] <-0.4 is +0.37 {+-} 0.07, while the average [Ca/Fe] in our Milky Way (MW) and M31 GC samples is +0.29 {+-} 0.09 and +0.24 {+-} 0.10, respectively. This may imply a more rapid chemical enrichment history for NGC 5128 than for either the MW or M31. This sample provides the first quantitative picture of the chemical history of NGC 5128 that is directly comparable to what is available for the MW. Data presented here were obtained with the MIKE echelle spectrograph on the Magellan Clay Telescope.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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).

  1. 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.

  2. 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

  3. 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.

  4. 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.

  5. 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

  6. 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.

  7. 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.

  8. Binary Black Hole Mergers from Globular Clusters: Implications for Advanced LIGO.

    PubMed

    Rodriguez, Carl L; Morscher, Meagan; Pattabiraman, Bharath; Chatterjee, Sourav; Haster, Carl-Johan; Rasio, Frederic A

    2015-07-31

    The predicted rate of binary black hole mergers from galactic fields can vary over several orders of magnitude and is extremely sensitive to the assumptions of stellar evolution. But in dense stellar environments such as globular clusters, binary black holes form by well-understood gravitational interactions. In this Letter, we study the formation of black hole binaries in an extensive collection of realistic globular cluster models. By comparing these models to observed Milky Way and extragalactic globular clusters, we find that the mergers of dynamically formed binaries could be detected at a rate of ∼100 per year, potentially dominating the binary black hole merger rate. We also find that a majority of cluster-formed binaries are more massive than their field-formed counterparts, suggesting that Advanced LIGO could identify certain binaries as originating from dense stellar environments. PMID:26274407

  9. The evolution of and starburst-agn connection in luminous and ultraluminous infrared galaxies and their link to globular cluster formation

    NASA Astrophysics Data System (ADS)

    Fiorenza, Stephanie Lynn

    increased AGN activity if they are older. However, I also find that a young U/LIRG can show a relatively large amount of AGN activity if a very large starburst is present. Then, I quantify the timescales at which the starburst activity in my U/LIRGs evolves with the use of the Tukey-Kramer method of statistical analysis, and fit an exponential curve to the data to describe the expected amount of decrease in SFR seen for a U/LIRG in my sample over a given change in starburst age. Finally, I find evidence that the stellar mass and starburst mass fractions influence whether a U/LIRG in my sample will have a strong AGN and SFR, respectively. I compare the SFR-Mstar relationship seen in my sample with those predicted by models and found from previous observations. I find that the U/LIRGs with older starbursts (>125 Myr) agree with previous results, while those with younger starbursts show a large dispersion in Mstar. I conclude that this is supporting evidence that the star formation histories and timescales at which the IR power sources in U/LIRGs evolve are responsible for the scatter found for the SFR-Mstar relationship. U/LIRGs that form from merging gas-rich disk galaxies could also represent a stage of galaxy evolution involving heavy formation of globular clusters (GCs). It has been suggested that a large number of stellar clusters form during the merging of two gas-rich disk galaxies, leading to open and young massive clusters with the latter likely evolving into GCs. Furthering our understanding of GC formation can uncover the connection between GCs and their host galaxies, which could, at some point during their formation or evolution, be U/LIRGs. To understand GC formation in the context of hierarchical galaxy formation, it is necessary to understand the origin of their abundance patterns. To this effort, I use SDSS spectra from Data Release 8 and 9 to estimate carbon (C) abundances for five GCs by matching synthetic spectra, created with TURBOSPECTRUM using

  10. 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.

  11. 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

  12. Signatures of multiple stellar populations in unresolved extragalactic globular/young massive star clusters

    SciTech Connect

    Peacock, Mark B.; Zepf, Stephen E.; Finzell, Thomas

    2013-06-01

    We present an investigation of potential signatures of the formation of multiple stellar populations in recently formed extragalactic star clusters. All of the Galactic globular clusters for which good samples of individual stellar abundances are available show evidence for multiple populations. This appears to require that multiple episodes of star formation and light element enrichment are the norm in the history of a globular cluster. We show that there are detectable observational signatures of multiple formation events in the unresolved spectra of massive, young extragalactic star clusters. We present the results of a pilot program to search for one of the cleanest signatures that we identify—the combined presence of emission lines from a very recently formed population and absorption lines from a somewhat older population. A possible example of such a system is identified in the Antennae galaxies. This source's spectrum shows evidence of two stellar populations with ages of 8 Myr and 80 Myr. Further investigation shows that these populations are in fact physically separated, but only by a projected distance of 59 pc. We show that the clusters are consistent with being bound and discuss the possibility that their coalescence could result in a single globular cluster hosting multiple stellar populations. While not the prototypical system proposed by most theories of the formation of multiple populations in clusters, the detection of this system in a small sample is both encouraging and interesting. Our investigation suggests that expanded surveys of massive young star clusters should detect more clusters with such signatures.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. Full spectral fitting of Milky Way and M 31 globular clusters: ages and metallicities

    NASA Astrophysics Data System (ADS)

    Cezario, E.; Coelho, P. R. T.; Alves-Brito, A.; Forbes, D. A.; Brodie, J. P.

    2013-01-01

    Context. The formation and evolution of disk galaxies are long standing questions in astronomy. Understanding the properties of globular cluster systems can lead to important insights on the evolution of its host galaxy. Aims: We aim to obtain the stellar population parameters - age and metallicity - of a sample of M 31 and Galactic globular clusters. Studying their globular cluster systems is an important step towards understanding their formation and evolution in a complete way. Methods: Our analysis employs a modern pixel-to-pixel spectral fitting technique to fit observed integrated spectra to updated stellar population models. By comparing observations to models we obtain the ages and metallicities of their stellar populations. We apply this technique to a sample of 38 globular clusters in M 31 and to 41 Galactic globular clusters, used as a control sample. Results: Our sample of M 31 globular clusters spans ages from 150 Myr to the age of the Universe. Metallicities [Fe/H] range from -2.2 dex to the solar value. The age-metallicity relation obtained can be described as having two components: an old population with a flat age-[Fe/H] relation, possibly associated with the halo and/or bulge, and a second one with a roughly linear relation between age and metallicity, higher metallicities corresponding to younger ages, possibly associated with the M 31 disk. While we recover the very well known Galactic GC metallicity bimodality, our own analysis of M 31's metallicity distribution function (MDF) suggests that both GC systems cover basically the same [Fe/H] range yet M 31's MDF is not clearly bimodal. These results suggest that both galaxies experienced different star formation and accretion histories. Table 4 is available in electronic form at http://www.aanda.org

  1. THE SIZE DIFFERENCE BETWEEN RED AND BLUE GLOBULAR CLUSTERS IS NOT DUE TO PROJECTION EFFECTS

    SciTech Connect

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

    2012-11-10

    Metal-rich (red) globular clusters in massive galaxies are, on average, smaller than metal-poor (blue) globular clusters. One of the possible explanations for this phenomenon is that the two populations of clusters have different spatial distributions. We test this idea by comparing clusters observed in unusually deep, high signal-to-noise images of M87 with a simulated globular cluster population in which the red and blue clusters have different spatial distributions, matching the observations. We compare the overall distribution of cluster effective radii as well as the relationship between effective radius and galactocentric distance for both the observed and simulated red and blue sub-populations. We find that the different spatial distributions does not produce a significant size difference between the red and blue sub-populations as a whole or at a given galactocentric distance. These results suggest that the size difference between red and blue globular clusters is likely due to differences during formation or later evolution.

  2. 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

  3. 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.

  4. 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.

  5. Implications of intermediate mass black hole in globular cluster G1 on dark matter detection.

    SciTech Connect

    Zaharijas, G.; High Energy Physics

    2008-07-01

    Recently there has been growing evidence in favor of the presence of an intermediate mass black hole in the globular cluster G1, in Andromeda Galaxy. Under the assumption that formation of this globular cluster occurred within a dark matter halo, we explore whether the presence of a black hole could result in an observable gamma ray signal due to dark matter annihilation in this globular cluster. Starting from an initial Navarro-Frenk-White matter profile, with density parameters consistent with G1 observations, we find that indeed, if the spike in the density has been formed and has survived until the present, the signal could be observed by GLAST and current atmospheric Cerenkov telescope detectors.

  6. Implications of the intermediate mass black hole in globular cluster G1 on dark matter detection

    SciTech Connect

    Zaharijas, Gabrijela

    2008-07-15

    Recently there has been growing evidence in favor of the presence of an intermediate mass black hole in the globular cluster G1, in Andromeda Galaxy. Under the assumption that formation of this globular cluster occurred within a dark matter halo, we explore whether the presence of a black hole could result in an observable gamma ray signal due to dark matter annihilation in this globular cluster. Starting from an initial Navarro-Frenk-White matter profile, with density parameters consistent with G1 observations, we find that indeed, if the spike in the density has been formed and has survived until the present, the signal could be observed by GLAST and current atmospheric Cerenkov telescope detectors.

  7. 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.

  8. 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

  9. 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.

  10. 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.

  11. 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

  12. Chemical Tagging in the SDSS-III/APOGEE Survey: New Identifications of Halo Stars with Globular Cluster Origins

    NASA Astrophysics Data System (ADS)

    Martell, Sarah L.; Shetrone, Matthew D.; Lucatello, Sara; Schiavon, Ricardo P.; Mészáros, Szabolcs; Allende Prieto, Carlos; García Hernández, D. A.; Beers, Timothy C.; Nidever, David L.

    2016-07-01

    We present new identifications of five red giant stars in the Galactic halo with chemical abundance patterns that indicate they originally formed in globular clusters. Using data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Survey available through Sloan Digital Sky Survey (SDSS) Data Release 12, we first identify likely halo giants, and then search those for the well-known chemical tags associated with globular clusters, specifically enrichment in nitrogen and aluminum. We find that 2% of the halo giants in our sample have this chemical signature, in agreement with previous results. Following the interpretation in our previous work on this topic, this would imply that at least 13% of halo stars originally formed in globular clusters. Recent developments in the theoretical understanding of globular cluster formation raise questions about that interpretation, and we concede the possibility that these migrants represent a small fraction of the halo field. There are roughly as many stars with the chemical tags of globular clusters in the halo field as there are in globular clusters, whether or not they are accompanied by a much larger chemically untaggable population of former globular cluster stars.

  13. 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.

  14. 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.

  15. Tidal stripping stellar substructures around four metal-poor globular clusters in the galactic bulge

    SciTech Connect

    Chun, Sang-Hyun; Kang, Minhee; Jung, DooSeok; Sohn, Young-Jong

    2015-01-01

    We investigate the spatial density configuration of stars around four metal-poor globular clusters (NGC 6266, NGC 6626, NGC 6642, and NGC 6723) in the Galactic bulge region using wide-field deep J, H, and K imaging data obtained with the Wide Field Camera near-infrared array on the United Kingdom Infrared Telescope. A statistical weighted filtering algorithm for the stars on the color–magnitude diagram is applied in order to sort cluster member candidates from the field star contamination. In two-dimensional isodensity contour maps of the clusters, we find that all four of the globular clusters exhibit strong evidence of tidally stripped stellar features beyond the tidal radius in the form of tidal tails or small density lobes/chunks. The orientations of the extended stellar substructures are likely to be associated with the effect of dynamic interaction with the Galaxy and the cluster's space motion. The observed radial density profiles of the four globular clusters also describe the extended substructures; they depart from theoretical King and Wilson models and have an overdensity feature with a break in the slope of the profile at the outer region of clusters. The observed results could imply that four globular clusters in the Galactic bulge region have experienced strong environmental effects such as tidal forces or bulge/disk shocks of the Galaxy during the dynamical evolution of globular clusters. These observational results provide further details which add to our understanding of the evolution of clusters in the Galactic bulge region as well as the formation of the Galaxy.

  16. 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.

  17. 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.

  18. Hubble Space Telescope observations of the globular cluster systems of early type galaxies

    NASA Astrophysics Data System (ADS)

    Kundu, Arunav

    1999-11-01

    We have studied the globular cluster systems of 60 early type galaxies using archival Hubble Space Telescope images. We discovered that the color distributions of the cluster systems of 30 to 60 percent of ellipticals is bimodal. The bimodality is most likely a signature of two major epochs of globular cluster formation in the history of these galaxies. At least 10% of the S0 galaxies in our sample show similar evidence of bimodality. However, the mechanism for the formation of the redder or metal-rich set of clusters appears to be different in the two galaxies that we studied in most detail, M87 the giant elliptical at the center of the Virgo cluster, and the S0 galaxy NGC 3115. While the red clusters in M87 appear to have formed during a major merger, the metal-rich clusters in NGC 3115 are associated with the thick disk of the galaxy. It seems likely that the red clusters in NGC 3145 were formed during a minor merger that did not destroy the disk. We measured the turnover luminosity of the globular cluster luminosity function in both the V and I-band. Comparing the turnover luminosities with other distance estimates we find that the absolute magnitude of the turnover luminosity in both bands is constant to within ~ 0.1 to ~ 0.2 mag. We conclude that the peak of the globular cluster luminosity function is an excellent distance indicator, with an accuracy comparable to other competing methods. We have also measured the sizes of individual clusters in our sample. The median size of the clusters appears to be the same in all galaxies. We suggest that this can be exploited to directly measure the distance to galaxies by simple geometrical considerations.

  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. 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.

  2. 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

  3. Uncovered: Progenitors of globular clusters showing off their multiple stellar populations

    NASA Astrophysics Data System (ADS)

    de Grijs, Richard; Li, Chengyuan; Deng, Licai; Geller, Aaron M.; Xin, Yu; Hu, Yi; Faucher-Giguere, Claude-Andre

    2016-01-01

    Stars in star clusters are thought to form in a single burst from a common progenitor cloud of molecular gas, resulting in so-called simple stellar populations. However, old, massive globular clusters—with ages greater than 10 billion years—often host multiple stellar populations, indicating that more than one star-forming event may have occurred during their lifetimes. The most popular scenario for their formation invokes colliding stellar winds from late-stage, asymptotic-giant-branch stars. If this were correct, the initial globular cluster masses should be at least 10 times more massive than their current masses of typically a few x 105 Msun. However, large populations of clusters with masses greater than a few x 106 Msun are not found in the local Universe. Here we present Hubble Space Telescope observations of three 1-2 billion-year-old, massive star clusters in the Magellanic Clouds which show unequivocal evidence of burst-like star-formation activity that occurred a few x 108 years after their initial formation era. The spatial distributions of the younger stellar generations suggest that they may have originated from ambient gas clouds accreted by the clusters while orbiting in the disks of their host galaxies rather than from colliding stellar winds. Simple models imply that such clusters could indeed accrete sufficient gas reservoirs to form these stars. This may eventually give rise to the appearance of multiple stellar populations in globular clusters.

  4. 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).

  5. 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.

  6. 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.

  7. 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.

  8. Are the globular clusters with significant internal [Fe/H] spreads all former dwarf galaxy nuclei?

    NASA Astrophysics Data System (ADS)

    Da Costa, Gary Stewart

    2015-08-01

    In this presentation I will advance the idea that the 'globular clusters' with significant internal [Fe/H] dispersions are in fact the former nuclei or nuclear star clusters of dwarf galaxies that have been disrupted during the formation of the Galactic halo. I'll discuss the characteristics of these clusters, particularly their common properties, and indicate that at present the number of such clusters known is broadly consistent with the disruption hypothesis. Identification of significantly more clusters with large internal [Fe/H] abundance ranges may, however, cast doubt on the hypothesis, if the stars of the disrupted dwarfs remain in the Galactic halo.

  9. FORS2/VLT survey of Milky Way globular clusters. II. Fe and Mg abundances of 51 Milky Way globular clusters on a homogeneous scale

    NASA Astrophysics Data System (ADS)

    Dias, B.; Barbuy, B.; Saviane, I.; Held, E. V.; Da Costa, G. S.; Ortolani, S.; Gullieuszik, M.; Vásquez, S.

    2016-05-01

    Context. Globular clusters trace the formation and evolution of the Milky Way and surrounding galaxies, and outline their chemical enrichment history. To accomplish these tasks it is important to have large samples of clusters with homogeneous data and analysis to derive kinematics, chemical abundances, ages and locations. Aims: We obtain homogeneous metallicities and α-element enhancement for 51 Galactic bulge, disc, and halo globular clusters that are among the most distant and/or highly reddened in the Galaxy's globular cluster system. We also provide membership selection based on stellar radial velocities and atmospheric parameters. The implications of our results are discussed. Methods: We observed R ~ 2000 spectra in the wavelength interval 456-586 nm for over 800 red giant stars in 51 Galactic globular clusters. We applied full spectrum fitting with the code ETOILE together with libraries of observed and synthetic spectra. We compared the mean abundances of all clusters with previous work and with field stars. We used the relation between mean metallicity and horizontal branch morphology defined by all clusters to select outliers for discussion. Results: [Fe/H], [Mg/Fe], and [α/Fe] were derived in a consistent way for almost one-third of all Galactic globular clusters. We find our metallicities are comparable to those derived from high-resolution data to within σ = 0.08 dex over the interval -2.5< [Fe/H] < 0.0. Furthermore, a comparison of previous metallicity scales with our values yields σ< 0.16 dex. We also find that the distribution of [Mg/Fe] and [α/Fe] with [Fe/H] for the 51 clusters follows the general trend exhibited by field stars. It is the first time that the following clusters have been included in a large sample of homogeneous stellar spectroscopic observations and metallicity derivation: BH 176, Djorg 2, Pal 10, NGC 6426, Lynga 7, and Terzan 8. In particular, only photometric metallicities were available previously for the first three

  10. FORS2/VLT survey of Milky Way globular clusters. II. Fe and Mg abundances of 51 Milky Way globular clusters on a homogeneous scale

    NASA Astrophysics Data System (ADS)

    Dias, B.; Barbuy, B.; Saviane, I.; Held, E. V.; Da Costa, G. S.; Ortolani, S.; Gullieuszik, M.; Vásquez, S.

    2016-04-01

    Context. Globular clusters trace the formation and evolution of the Milky Way and surrounding galaxies, and outline their chemical enrichment history. To accomplish these tasks it is important to have large samples of clusters with homogeneous data and analysis to derive kinematics, chemical abundances, ages and locations. Aims: We obtain homogeneous metallicities and α-element enhancement for 51 Galactic bulge, disc, and halo globular clusters that are among the most distant and/or highly reddened in the Galaxy's globular cluster system. We also provide membership selection based on stellar radial velocities and atmospheric parameters. The implications of our results are discussed. Methods: We observed R ~ 2000 spectra in the wavelength interval 456-586 nm for over 800 red giant stars in 51 Galactic globular clusters. We applied full spectrum fitting with the code ETOILE together with libraries of observed and synthetic spectra. We compared the mean abundances of all clusters with previous work and with field stars. We used the relation between mean metallicity and horizontal branch morphology defined by all clusters to select outliers for discussion. Results: [Fe/H], [Mg/Fe], and [α/Fe] were derived in a consistent way for almost one-third of all Galactic globular clusters. We find our metallicities are comparable to those derived from high-resolution data to within σ = 0.08 dex over the interval -2.5< [Fe/H] < 0.0. Furthermore, a comparison of previous metallicity scales with our values yields σ< 0.16 dex. We also find that the distribution of [Mg/Fe] and [α/Fe] with [Fe/H] for the 51 clusters follows the general trend exhibited by field stars. It is the first time that the following clusters have been included in a large sample of homogeneous stellar spectroscopic observations and metallicity derivation: BH 176, Djorg 2, Pal 10, NGC 6426, Lynga 7, and Terzan 8. In particular, only photometric metallicities were available previously for the first three

  11. THE GLOBULAR CLUSTER MASS FUNCTION AS A REMNANT OF VIOLENT BIRTH

    SciTech Connect

    Elmegreen, Bruce G.

    2010-04-01

    The log-normal shape of the mass function for metal-poor halo globular clusters is proposed to result from an initial M {sup -2} power law modified rapidly by evaporation, collisions with clouds, and mutual cluster interactions in the dense environment of a redshift z {approx} 5-15 disk galaxy. Galaxy interactions subsequently spray these clusters into the galaxy group environment, where they fall into other growing galaxies and populate their halos. Clusters forming later in z {approx} 2-5 galaxies, and those formed during major mergers, produce metal-rich globulars. Monte Carlo models of evolving cluster populations demonstrate the early formation of a log-normal mass function for typical conditions in high-redshift galaxies.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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

  17. 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.

  18. 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.

  19. Single & Multiple Stellar Populations in Globular Clusters: Chemical Tagging, Photometric Sequences, and Dynamics

    NASA Astrophysics Data System (ADS)

    Piotto, Giampaolo

    2015-08-01

    The discovery of multiple stellar populations in globular clusters has revolutionized our view of these objects one thought to be simple, single population stellar systems. Different star formation scenarios have been proposed in order to account for the photometric and spectroscopic properties of the different populations hosted by the single cluster, and some of them imply that the original cluster should have been much more massive than it is now, with a significant fraction of the original stars lost into the environment (Galaxy halo or bulge). Because of this, globular clusters become relevant not only as tracers of the general process of galaxy halo formation, but also possible incubators of most (all?) halo stars.In my talk I will briefly summarize the basic observational facts that made the community at large to accept the idea of population multiplicity.I will also present the newest results coming from an extensive, multi-wavelength astrometric and photometric survey, which includes UV data from ACS and WFC3/HST of close to half of the Milky Way globular clusters. The increasing number of spectroscopic surveys of stars in globular clusters, coupled with the capability of (UV) photometry to distinguish different populations has largely increased our capability to trace the basic chemical properties of the many populations within a single cluster.I will present a census of the presence of multiple populations in GCs, their chemical tagging, radial distribution, and kinematics.Possible correlations of multiple populations characterizing quantities with the main cluster parameters will also be presented. Implications on multiple stellar populations formation will be discussed as well as the still open issues.

  20. 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

  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

    2015-08-01

    We investigated stellar spatial density distribution around four metal-poor globular clusters (NGC 6266, NGC 6626, NGC 6642 and NGC 6723) in order to find extended stellar substructures. Wide-field deep J, H, and K imaging data were taken using the WFCAM near-infrared array on United Kingdom Infrared Telescope (UKIRT). The contamination of field stars around clusters was minimised by applying a statistical weighted filtering algorithm for the stars on the color-magnitude diagram. In two-dimensional isodensity contour map, we find that all four of the globular clusters shows tidal stripping stellar features in the form of tidal tails (NGC 6266 and NGC 6723) or small density lobes/chunk (NGC 6642 and NGC 6723). The stellar substructures extend toward the Galactic centre or anticancer, and the proper motion direction of the clusters. The radial density profiles of the clusters also depart from theoretical King and Wilson models and show overdensity feature with a break in a slope of profile at the outer region of clusters. The observed results indicate that four globular clusters in the Galactic bulge have experienced strong tidal force or bulge/disk shock effect of the Galaxy. These observational results provide us further constraints to understand the evolution of clusters in the Galactic bulge region as well as the formation of the Galaxy.

  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. Multiple populations in globular clusters and the origin of the Oosterhoff dichotomy

    NASA Astrophysics Data System (ADS)

    Jang, S.; Lee, Y.-W.

    2016-05-01

    The globular cluster community is now facing a new paradigm of multiple stellar populations. In light of this, we have recently proposed a new model to explain the origin of the difference in mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In our model, the instability strip in the metal-poor group II clusters, such as M15, is populated by second-generation stars (G2) with mildly enhanced helium and CNO abundances, while the RR Lyraes in the relatively metal-rich group I clusters such as M3 are produced mostly by first-generation stars (G1) without these enhancements. When these models are extended to all metallicity regimes, the observed dichotomies in the inner and outer halo globular clusters can be naturally reproduced. We found that specific star formation histories are required for the inner and outer halos, which is consistent with the dual origin of the Milky Way halo.

  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. FURTHER DEFINITION OF THE MASS-METALLICITY RELATION IN GLOBULAR CLUSTER SYSTEMS AROUND BRIGHTEST CLUSTER GALAXIES

    SciTech Connect

    Cockcroft, Robert; Harris, William E.; Wehner, Elizabeth M. H.; Whitmore, Bradley C.; Rothberg, Barry E-mail: harris@physics.mcmaster.ca E-mail: whitmore@stsci.edu

    2009-09-15

    We combine the globular cluster (GC) data for 15 brightest cluster galaxies and use this material to trace the mass-metallicity relations (MMRs) in their globular cluster systems (GCSs). This work extends previous studies which correlate the properties of the MMR with those of the host galaxy. Our combined data sets show a mean trend for the metal-poor subpopulation that corresponds to a scaling of heavy-element abundance with cluster mass Z {approx} M {sup 0.30{+-}}{sup 0.05}. No trend is seen for the metal-rich subpopulation which has a scaling relation that is consistent with zero. We also find that the scaling exponent is independent of the GCS specific frequency and host galaxy luminosity, except perhaps for dwarf galaxies. We present new photometry in (g',i') obtained with Gemini/GMOS for the GC populations around the southern giant ellipticals NGC 5193 and IC 4329. Both galaxies have rich cluster populations which show up as normal, bimodal sequences in the color-magnitude diagram. We test the observed MMRs and argue that they are statistically real, and not an artifact caused by the method we used. We also argue against asymmetric contamination causing the observed MMR as our mean results are no different from other contamination-free studies. Finally, we compare our method to the standard bimodal fitting method (KMM or RMIX) and find our results are consistent. Interpretation of these results is consistent with recent models for GC formation in which the MMR is determined by GC self-enrichment during their brief formation period.

  7. A Deep X-ray Study of the Globular Cluster M4

    NASA Astrophysics Data System (ADS)

    Pooley, David

    2015-08-01

    We know from observations that globular clusters are very efficient catalysts in forming unusual binary systems, such as low-mass X-ray binaries (LMXBs), cataclysmic variables (CVs), and millisecond pulsars (MSPs), with formation rates per unit mass exceeding those in the Galactic disk by orders of magnitude. The high stellar densities in globular clusters trigger various dynamical interactions: exchange encounters, direct collisions, destruction of binaries, and tidal capture. This binary population is, in turn, critical to the stabilization of globular clusters against gravitational collapse; the long-term stability of a cluster is thought to depend on tapping into the gravitational binding energy of such close binaries.We have also learned that not all X-ray sources in a globular cluster are dynamically formed. Chandra X-ray Observatory observations of low-density clusters have shown that the magnetically active main-sequence binaries in those clusters are largely primordial, but few clusters have been observed deeply enough in X-rays to uncover a substantial fraction of these binaries.We report on the results of deep Chandra observations of M4 that were motivated, in part, to uncover a nearly complete census of its active binaries. These observations reach X-ray luminosities below 1029 erg/s, a sensitivity that should detect ~90% of the active main-sequence binary population. We detect ~100 X-ray sources within the half-light radius of M4 and characterize their nature by investigating their optical counterparts (or lack thereof) in deep Hubble Space Telescope observations. We compare the populations of X-ray sources in M4 to other well-studied clusters.

  8. 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

  9. 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.

  10. The Scale Sizes of Globular Clusters: Tidal Limits, Evolution, and the Outer Halo

    NASA Astrophysics Data System (ADS)

    Harris, William

    2011-10-01

    The physical factors that determine the linear sizes of massive star clusters are not well understood. Their scale sizes were long thought to be governed by the tidal field of the parent galaxy, but major questions are now emerging. Globular clusters, for example, have mean sizes nearly independent of location in the halo. Paradoxically, the recently discovered "anomalous extended clusters" in M31 and elsewhere have scale sizes that fit much better with tidal theory, but they are puzzlingly rare. Lastly, the persistent size difference between metal-poor and metal-rich clusters still lacks a quantitative explanation. Many aspects of these observations call for better modelling of dynamical evolution in the outskirts of clusters, and also their conditions of formation including the early rapid mass loss phase of protoclusters. A new set of accurate measurements of scale sizes and structural parameters, for a large and homogeneous set of globular clusters, would represent a major advance in this subject. We propose to carry out a {WFC3+ACS} imaging survey of the globular clusters in the supergiant Virgo elliptical M87 to cover the complete run of the halo. M87 is an optimum target system because of its huge numbers of clusters and HST's ability to resolve the cluster profiles accurately. We will derive cluster effective radii, central concentrations, luminosities, and colors for more than 4000 clusters using PSF-convolved King-model profile fitting. In parallel, we are developing theoretical tools to model the expected distribution of cluster sizes versus galactocentric distance as functions of cluster mass, concentration, and orbital anisotropy.

  11. A Search for X-ray Emitting Binary Stars in the Globular Cluster Omega Centauri

    NASA Astrophysics Data System (ADS)

    Deveny, Sarah; Gallien, Michael; Rickards Vaught, Ryan; Waters, Miranda; Cool, Adrienne; Bellini, Andrea; Anderson, Jay; Henleywillis, Simon; Haggard, Daryl; Heinke, Craig O.

    2016-06-01

    Omega Centauri is one of the most widely studied globular clusters, and is expected to harbor a significant population of binary stars. Binaries play a crucial role in determining the progression of stellar dynamics within globular clusters, and as such are relevant to questions concerning the possible formation of intermediate black holes at their centers. One effective way to identify certain classes of binary systems is to first locate X-ray sources in the cluster and then to search for their optical counterparts. Using Chandra X-ray Observatory's ACIS-I instrument we have identified 275 X-ray sources in and toward Omega Cen, more than 50 of which lie within the cluster's core radius. Here we present a search for the optical counterparts of these core sources using an extensive database of archival Hubble Space Telescope images. Using WFC3/UVIS data from 11 different filters, we construct color-magnitude diagrams that reveal a diverse array of objects, including (in addition to background and foreground objects) cataclysmic variables, coronally active binaries, and, interestingly, stars that lie on Omega Cen's anomalous giant branch. We discuss the significance of these results in the context of studies of the formation and evolution of binary stars in globular clusters.

  12. Evolution of Compact Binary Populations in Globular Clusters: A Boltzmann Study. II. Introducing Stochasticity

    NASA Astrophysics Data System (ADS)

    Banerjee, Sambaran; Ghosh, Pranab

    2008-06-01

    We continue the exploration that we began in Paper I of using the Boltzmann scheme to study the evolution of compact binary populations of globular clusters, introducing in this paper our method of handling the stochasticity inherent in the dynamical processes of binary formation, destruction, and hardening in globular clusters. We describe these stochastic processes as "Wiener processes," whereupon the Boltzmann equation becomes a stochastic partial differential equation, the solution of which involves the use of "Itō calculus" (this use being the first, to our knowledge, in this subject), in addition to ordinary calculus. As in Paper I, we focus on the evolution of (1) the number of X-ray binaries NXB in globular clusters and (2) the orbital period distribution of these binaries. We show that, although the details of the fluctuations in the above quantities differ from one "realization" to another of the stochastic processes, the general trends follow those found in the continuous-limit study of Paper I, and the average result over many such realizations is very close to the continuous-limit result. We investigate the dependence of NXB found by these calculations on two essential globular cluster properties, namely, the star-star and star-binary encounter rate parameters Γ and γ, for which we coined the name "Verbunt parameters" in Paper I. We compare our computed results with those from Chandra observations of Galactic globular clusters, showing that the expected scalings of NXB with the Verbunt parameters are in good agreement with those observed. We indicate additional features that can be incorporated into the scheme in the future, as well as how more elaborate problems can be tackled.

  13. Star Clusters in M31. VII. Global Kinematics and Metallicity Subpopulations of the Globular Clusters

    NASA Astrophysics Data System (ADS)

    Caldwell, Nelson; Romanowsky, Aaron J.

    2016-06-01

    We carry out a joint spatial–kinematical–metallicity analysis of globular clusters (GCs) around the Andromeda Galaxy (M31), using a homogeneous, high-quality spectroscopic data set. In particular, we remove the contaminating young clusters that have plagued many previous analyses. We find that the clusters can be divided into three major metallicity groups based on their radial distributions: (1) an inner metal-rich group ([Fe/H] > -0.4); (2) a group with intermediate metallicity (with median [Fe/H] = ‑1) and (3) a metal-poor group, with [Fe/H] < -1.5. The metal-rich group has kinematics and spatial properties like those of the disk of M31, while the two more metal-poor groups show mild prograde rotation overall, with larger dispersions—in contrast to previous claims of stronger rotation. The metal-poor GCs are the least concentrated group; such clusters occur five times less frequently in the central bulge than do clusters of higher metallicity. Despite some well-known differences between the M31 and Milky Way GC systems, our revised analysis points to remarkable similarities in their chemodynamical properties, which could help elucidate the different formation stages of galaxies and their GCs. In particular, the M31 results motivate further exploration of a metal-rich GC formation mode in situ, within high-redshift, clumpy galactic disks.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. A dynamical study of Galactic globular clusters under different relaxation conditions

    NASA Astrophysics Data System (ADS)

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

    2012-03-01

    Aims: We perform a systematic combined photometric and kinematic analysis of a sample of globular clusters under different relaxation conditions, based on their core relaxation time (as listed in available catalogs), by means of two well-known families of spherical stellar dynamical models. Systems characterized by shorter relaxation time scales are expected to be better described by isotropic King models, while less relaxed systems might be interpreted by means of non-truncated, radially-biased anisotropic f(ν) models, originally designed to represent stellar systems produced by a violent relaxation formation process and applied here for the first time to the study of globular clusters. Methods: The comparison between dynamical models and observations is performed by fitting simultaneously surface brightness and velocity dispersion profiles. For each globular cluster, the best-fit model in each family is identified, along with a full error analysis on the relevant parameters. Detailed structural properties and mass-to-light ratios are also explicitly derived. Results: We find that King models usually offer a good representation of the observed photometric profiles, but often lead to less satisfactory fits to the kinematic profiles, independently of the relaxation condition of the systems. For some less relaxed clusters, f(ν) models provide a good description of both observed profiles. Some derived structural characteristics, such as the total mass or the half-mass radius, turn out to be significantly model-dependent. The analysis confirms that, to answer some important dynamical questions that bear on the formation and evolution of globular clusters, it would be highly desirable to acquire larger numbers of accurate kinematic data-points, well distributed over the cluster field. Appendices are available in electronic form at http://www.aanda.org

  19. 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.

  20. Photometric and kinematic studies of extragalactic globular cluster systems

    NASA Astrophysics Data System (ADS)

    Dowell, Jessica

    Globular clusters (GCs) are old, luminous, compact collections of stars found in galaxy halos that formed during the early stages of galaxy formation. Because of this, GCs serve as excellent tracers of the formation, structure, and merger history of their host galaxies. My dissertation will examine both the photometric and kinematic properties of GC systems and their relationship to their host galaxies. In the first section, I will present the analysis of the GC systems of two spiral galaxies, NGC 891 and NGC 1055. I will discuss the photometric methods used to detect GCs using wide-field BVR imaging and to quantify the global properties of the system such as the total number of GCs and their radial distribution. My results for these two GC systems were compared to those of other galaxies. I will also present the results of spectroscopic follow-up for two giant galaxies: the S0 galaxy NGC 4594 (M104), and the elliptical galaxy NGC 3379 (M105). I measured the radial velocities of GCs in these two galaxies, and combined them with published results to determine the mass distribution and mass-to-light (M/L) ratio profile for each galaxy out to large effective radius (7-9 Re). For both galaxies, I found that the M/L profiles increase with radius and do not flatten, which suggests that the dark matter halos in these galaxies extend to the edge of my data. I also looked for evidence of rotation in the GC systems, and found that neither system exhibits significant rotation around the host galaxy. I examined the velocity dispersion profile of each GC system and found kinematic differences between the red and blue GC subpopulations. Finally, I compared my results to mass estimates for these galaxies from other kinematic tracers and considered them in the context of galaxy formation models.

  1. 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

  2. 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

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. Binary black hole mergers from globular clusters: Masses, merger rates, and the impact of stellar evolution

    NASA Astrophysics Data System (ADS)

    Rodriguez, Carl L.; Chatterjee, Sourav; Rasio, Frederic A.

    2016-04-01

    The recent discovery of GW150914, the binary black hole merger detected by Advanced LIGO, has the potential to revolutionize observational astrophysics. But to fully utilize this new window into the Universe, we must compare these new observations to detailed models of binary black hole formation throughout cosmic time. Expanding upon our previous work [C. L. Rodriguez, M. Morscher, B. Pattabiraman, S. Chatterjee, C.-J. Haster, and F. A. Rasio, Phys. Rev. Lett. 115, 051101 (2015).], we study merging binary black holes formed in globular clusters using our Monte Carlo approach to stellar dynamics. We have created a new set of 52 cluster models with different masses, metallicities, and radii to fully characterize the binary black hole merger rate. These models include all the relevant dynamical processes (such as two-body relaxation, strong encounters, and three-body binary formation) and agree well with detailed direct N -body simulations. In addition, we have enhanced our stellar evolution algorithms with updated metallicity-dependent stellar wind and supernova prescriptions, allowing us to compare our results directly to the most recent population synthesis predictions for merger rates from isolated binary evolution. We explore the relationship between a cluster's global properties and the population of binary black holes that it produces. In particular, we derive a numerically calibrated relationship between the merger times of ejected black hole binaries and a cluster's mass and radius. With our improved treatment of stellar evolution, we find that globular clusters can produce a significant population of massive black hole binaries that merge in the local Universe. We explore the masses and mass ratios of these binaries as a function of redshift, and find a merger rate of ˜5 Gpc-3yr-1 in the local Universe, with 80% of sources having total masses from 32 M⊙ to 64 M⊙. Under standard assumptions, approximately one out of every seven binary black hole mergers

  8. Characterization and Modeling of Mass Segregation and Intermediate-Mass Black Holes in Globular Clusters

    NASA Astrophysics Data System (ADS)

    van der Marel, Roeland

    2011-10-01

    Studies of the dynamics and stellar populations of globular clusters are at the forefront of HST research. These two topics are deeply intertwined. Clusters experience an interplay of collisional processes that drive stars toward energy equipartition, thus segregating more massive stars to the core. In young clusters, this can even lead to the formation of intermediate-mass black holes {IMBHs}, which are of great astrophysical interest, although evidence for them continues to be disputed. Our recent HST{-supported} observational and theoretical studies indicate that equipartition in a cluster is not generally attained. Measurement of the actual mass segregation in clusters can yield significant insight into some of the most important cluster parameters, including the mass of any IMBH {which tends to quench mass segregation}. We have demonstrated this explictly on archival data of NGC2298 and M10. HST imaging {including parallel fields} exists in fact over large radial ranges for many globular clusters. This trove of information remains largely untapped, as studies generally focus on the cluster core. We propose here to rigorously quantify and model the mass segregation in all 66 Galactic globular clusters with suitable HST data. We will create CMDs, LFs, and MFs as function of radius, and will release the resulting Legacy data products to the community to enable a host of ancillary investigations. We will run N-body models to interpret the observed mass segregation in the sample clusters. Data-model comparisons will constrain the mass of any IMBHs, will identify IMBH-candidates for more targeted follow-up, and will shed new light on cluster structure and evolution.

  9. M31 GLOBULAR CLUSTER STRUCTURES AND THE PRESENCE OF X-RAY BINARIES

    SciTech Connect

    Agar, J. R. R.; Barmby, P.

    2013-11-01

    The Andromeda galaxy, M31, has several times the number of globular clusters found in the Milky Way. It contains a correspondingly larger number of low-mass X-ray binaries (LMXBs) associated with globular clusters, and as such can be used to investigate the cluster properties that lead to X-ray binary formation. The best tracer of the spatial structure of M31 globulars is the high-resolution imaging available from the Hubble Space Telescope (HST), and we have used HST data to derive structural parameters for 29 LMXB-hosting M31 globular clusters. These measurements are combined with structural parameters from the literature for a total of 41 (of 50 known) LMXB clusters and a comparison sample of 65 non-LMXB clusters. Structural parameters measured in blue bandpasses are found to be slightly different (smaller core radii and higher concentrations) than those measured in red bandpasses; this difference is enhanced in LMXB clusters and could be related to stellar population differences. Clusters with LMXBs show higher collision rates for their mass compared to clusters without LMXBs, and collision rates estimated at the core radius show larger offsets than rates estimated at the half-light radius. These results are consistent with the dynamical formation scenario for LMXBs. A logistic regression analysis finds that, as expected, the probability of a cluster hosting an LMXB increases with increasing collision rate and proximity to the galaxy center. The same analysis finds that probability of a cluster hosting an LMXB decreases with increasing cluster mass at a fixed collision rate, although we caution that this could be due to sample selection effects. Metallicity is found to be a less important predictor of LMXB probability than collision rate, mass, or distance, even though LMXB clusters have a higher metallicity on average. This may be due to the interaction of location and metallicity: a sample of M31 LMXBs with a greater range in galactocentric distance would

  10. The expected very-high-energy flux from a population of globular clusters

    NASA Astrophysics Data System (ADS)

    Venter, C.; Kopp, A.

    Given their old ages, globular clusters are expected to harbour many evolved stellar objects. Their high core densities enhance stellar encounter rates, also facilitating the formation of stellar end products. In particular, many millisecond pulsars are found in these clusters. Such a population of millisecond pulsars is expected to radiate several spectral components in the radio through gamma -ray waveband. We present ongoing work involving a refined spectral model that assumes millisecond pulsars as sources of relativistic particles to model the multi-wavelength emission properties of globular clusters. We apply the model to a population of globular clusters that have been observed by H.E.S.S. and use upper limits derived from stacking analyses to test the viability of this ``millisecond pulsar scenario''. We derive general expressions for the ensemble-averaged flux and its error stemming from the uncertainty in free model parameters. The errors exceed this calculated average flux value so that there are regions in parameter space for which the model predictions satisfy the H.E.S.S. upper limits. Improved constraints on single-cluster parameters are therefore needed to aid in discriminating between competing spectral models.

  11. Gravitational microlensing by low-mass objects in the globular cluster M22.

    PubMed

    Sahu, K C; Casertano, S; Livio, M; Gilliland, R L; Panagia, N; Albrow, M D; Potter, M

    2001-06-28

    Gravitational microlensing offers a means of determining directly the masses of objects ranging from planets to stars, provided that the distances and motions of the lenses and sources can be determined. A globular cluster observed against the dense stellar field of the Galactic bulge presents ideal conditions for such observations because the probability of lensing is high and the distances and kinematics of the lenses and sources are well constrained. The abundance of low-mass objects in a globular cluster is of particular interest, because it may be representative of the very early stages of star formation in the Universe, and therefore indicative of the amount of dark baryonic matter in such clusters. Here we report a microlensing event associated with the globular cluster M22. We determine the mass of the lens to be 0.13(+0.03)(-0.02) solar masses. We have also detected six events that are unresolved in time. If these are also microlensing events, they imply that a non-negligible fraction of the cluster mass resides in the form of free-floating planetary-mass objects. PMID:11429596

  12. 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.

  13. Globular cluster clustering and tidal features around ultra-compact dwarf galaxies in the halo of NGC 1399

    NASA Astrophysics Data System (ADS)

    Voggel, Karina; Hilker, Michael; Richtler, Tom

    2016-02-01

    We present a novel approach to constrain the formation channels of ultra-compact dwarf galaxies (UCDs). They most probably are an inhomogeneous class of objects, composed of remnants of tidally stripped dwarf elliptical galaxies and star clusters that occupy the high mass end of the globular cluster luminosity function. We use three methods to unravel their nature: 1) we analyzed their surface brightness profiles; 2) we carried out a direct search for tidal features around UCDs; and 3) we compared the spatial distribution of GCs and UCDs in the halo of their host galaxy. Based on FORS2 observations under excellent seeing conditions, we studied the detailed structural composition of a large sample of 97 UCDs in the halo of NGC 1399, the central galaxy of the Fornax cluster, by analyzing their surface brightness profiles. We found that 13 of the UCDs were resolved above the resolution limit of 23 pc and we derived their structural parameters fitting a single Sérsic function. When decomposing their profiles into composite King and Sérsic profiles, we find evidence for faint stellar envelopes at μ = ~ 26 mag arcsec-2, surrounding the UCDs up to an extension of 90 pc in radius. We also show new evidence for faint asymmetric structures and tidal tail-like features surrounding several of these UCDs, a possible tracer of their origin and assembly history within their host galaxy halos. In particular, we present evidence for the first discovery of a significant tidal tail with an extension of ~350 pc around UCD-FORS 2. Finally, we studied the local overdensities in the spatial distribution of globular clusters within the halo of NGC 1399 out to 110 kpc to see if they are related to the positions of the UCDs. We found a local overabundance of globular clusters on a scale of ≤1 kpc around UCDs, when we compared it to the distribution of globulars from the host galaxy. This effect is strongest for the metal-poor blue GCs. We discuss how likely it is that these clustered

  14. The Globular Cluster System of the Spiral Galaxy NGC 7814

    NASA Astrophysics Data System (ADS)

    Rhode, Katherine L.; Zepf, Stephen E.

    2003-11-01

    We present the results of a wide-field photometric study of the globular cluster (GC) system of the edge-on Sab spiral NGC 7814. This is the first spiral to be fully analyzed from our survey of the GC systems of a large sample of galaxies beyond the Local Group. NGC 7814 is of particular interest because a previous study estimated that it has 500-1000 GCs, giving it the largest specific frequency (SN) known for a spiral. Understanding this galaxy's GC system is important in terms of our understanding of the GC populations of spirals in general and has implications for the formation of massive galaxies. We observed the galaxy in BVR filters with the WIYN 3.5 m telescope and used image classification and three-color photometry to select GC candidates. We also analyzed archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images of NGC 7814, both to help quantify the contamination level of the WIYN GC candidate list and to detect GCs in the inner part of the galaxy halo. Combining HST data with high-quality ground-based images allows us to trace the entire radial extent of this galaxy's GC system and determine the total number of GCs directly through observation. We find that rather than being an especially high-SN spiral, NGC 7814 has <~200 GCs and SN~1, making it comparable to the two most well-studied spiral galaxies, the Milky Way and M31. We explore the implications of these results for models of the formation of galaxies and their GC systems. The initial results from our survey suggest that the GC systems of typical elliptical galaxies can be accounted for by the merger of two or more spirals, but that for highly luminous elliptical galaxies, additional physical processes may be needed.

  15. 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.

  16. 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

  17. 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).

  18. 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.

  19. Evidence for the Disky Origin of Luminous Virgo Dwarf Ellipticals from the Kinematics of Their Globular Cluster Systems

    NASA Astrophysics Data System (ADS)

    Beasley, Michael A.; Cenarro, A. Javier; Strader, Jay; Brodie, Jean P.

    2009-06-01

    We report evidence for dynamically significant rotation in the globular cluster systems of two luminous Virgo dwarf ellipticals, VCC1261 and VCC1528. Including previous results for VCC1087, the globular cluster systems of all three Virgo dwarf ellipticals studied in detail to date exhibit v rot/σlos>1. Taking the rotation seen in the globular clusters as a maximal disk rotation, and accounting for the possible fading of any hypothetical progenitor galaxy, we find all three dwarf elliptical galaxies (dEs) lie on the r-band Tully-Fisher relation. We argue that these data support the hypothesis that luminous dEs are the remnants of transformed disk galaxies. We also obtained deep, longslit data for the stars in VCC1261 and VCC1528. Both these galaxies show rapid rotation in their inner regions, with spatial scales of ~0.5 kpc. These rotation velocities are surprisingly similar to those seen in the globular cluster systems. At larger radii, we see little rotation in the dEs themselves. Since our longslit data for Virgo dEs extend out to 1-2 effective radii (typical of deep observations), whereas the globular clusters extend out to 4-7 effective radii, we conclude that nondetections of rotation in many luminous dEs may simply be due to a lack of radial coverage in the stellar data, and that globular clusters represent singularly sensitive probes of the dynamics of dEs. Based on these data, we suggest that gas disks are significant sites of globular cluster formation in the early universe. Some of 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.

  20. 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.

  1. 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.

  2. Age estimates of globular clusters in the Milky Way: constraints on cosmology.

    PubMed

    Krauss, Lawrence M; Chaboyer, Brian

    2003-01-01

    Recent observations of stellar globular clusters in the Milky Way Galaxy, combined with revised ranges of parameters in stellar evolution codes and new estimates of the earliest epoch of globular cluster formation, result in a 95% confidence level lower limit on the age of the Universe of 11.2 billion years. This age is inconsistent with the expansion age for a flat Universe for the currently allowed range of the Hubble constant, unless the cosmic equation of state is dominated by a component that violates the strong energy condition. This means that the three fundamental observables in cosmology-the age of the Universe, the distance-redshift relation, and the geometry of the Universe-now independently support the case for a dark energy-dominated Universe. PMID:12511641

  3. Discovery of near-ultraviolet counterparts to millisecond pulsars in the globular cluster 47 Tucanae

    NASA Astrophysics Data System (ADS)

    Rivera Sandoval, Liliana E.

    2016-07-01

    Up to date 144 radio millisecond pulsars have been found in Galactic globular clusters, of which about two-thirds are in a binary. However, until recently only for 10 of those binary millisecond pulsars the companion has been firmly identified at optical wavelengths. We present the discovery of 2 likely He white dwarf companions to millisecond pulsars in the globular cluster 47 Tucanae, as well as the confirmation of 2 tentative identifications in the same cluster, using near-ultraviolet images obtained with the Hubble Space Telescope. This represents an important contribution to the total number of optical counterparts known in Galactic globular clusters so far. We have also analyzed optical observations taken with Hubble. From these images, we obtained H_α results for some of the counterparts. Based on our UV photometry and He WD cooling models we derived the ages, the masses and the bolometric luminosities for all the He WD companions. I will discuss our results and their implications in the context of the standard millisecond pulsar formation scenario.

  4. 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

  5. Photometric and Kinematic Studies of Extragalactic Globular Cluster Systems

    NASA Astrophysics Data System (ADS)

    Windschitl-Dowell, Jessica L.

    2015-01-01

    Globular clusters (GCs) are compact, luminous collections of stars created during the early stages of galaxy formation. As a result, the properties of GC systems provide important clues about the formation, merger history, and structure of their host galaxies. In particular, kinematic studies of GCs can be used to investigate the dark matter distribution in galaxy halos and provide observational evidence that can be used to constrain models of galaxy formation. I will present our study of the GC systems of two spiral galaxies, NGC 891 and NGC 1055, and show how we used wide-field BVR imaging from the WIYN 3.5-m telescope to detect the GC population and measure the global properties of the system. We quantified the radial distribution of the GC system and total number of GCs in these galaxies and compared the results to those of other galaxies.I will also present the results of spectroscopic follow-up for two giant galaxies: the S0 galaxy NGC 4594 (M104), and the elliptical galaxy NGC 3379 (M105). Using spectra taken with AAT/AAOmega, WIYN/HYDRA, and MMT/Hectospec, I measured the radial velocities of GCs, and combined them with published results to determine the mass distribution and V-band mass-to-light (M/LV) ratio profile for each galaxy out to large effective radius (7-9 Re). I compared our results to mass estimates from other kinematic tracers and also considered them in the context of galaxy formation models. For both galaxies, I found that the M/LV profiles increase with radius and do not flatten, which suggests that the dark matter halos in these galaxies extend to the edge of our data. I also looked for evidence of rotation within the GC systems, and found that neither system exhibits significant rotation around the host galaxy. Finally, I examined the velocity dispersion of each GC system as a function of radius and found kinematic differences between the red, metal-rich and blue, metal-poor GC subpopulations.

  6. The Realm of the Galactic Globular Clusters and the Mass of Their Primordial Clouds

    NASA Astrophysics Data System (ADS)

    Tenorio-Tagle, Guillermo; Muñoz-Tuñón, Casiana; Cassisi, Santi; Silich, Sergiy

    2016-07-01

    By adopting the empirical constraints related to the estimates of helium enhancement ({{Δ }}Y), the present mass ratio between first and second stellar generations ({M}1{{G}}/{M}2{{G}}), and the actual mass of Galactic globular clusters (M GC), we envisage a possible scenario for the formation of these stellar systems. Our approach allows for the possible loss of stars through evaporation or tidal interactions and different star-formation efficiencies. In our approach, the star-formation efficiency of the first generation (ɛ 1G) is the central factor that links the stellar generations because it not only defines both the mass in stars of the first generation and the remaining mass available for further star formation, but it also fixes the amount of matter required to contaminate the second stellar generation. In this way, ɛ 1G is fully defined by the He enhancement between successive generations in a GC. We also show that globular clusters fit well within a ΔY versus {M}1{{G}}/{M}2{{G}} diagram that indicates three different evolutionary paths. The central one is for clusters that have not lost stars through tidal interactions from either of their stellar generations, and thus their present M GC value is identical to the amount of low-mass stars (M * ≤ 1 M ⊙) that resulted from both stellar generations. Other possible evolutions imply either the loss of first-generation stars or the combination of a low star-formation efficiency in the second stellar generation and a loss of stars from the second generation. From these considerations, we derive a lower limit to the mass (M tot) of the individual primordial clouds that gave origin to globular clusters.

  7. 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.

  8. 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.

  9. 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.

  10. Multiple stellar populations in the globular cluster NGC 1851

    NASA Astrophysics Data System (ADS)

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

    2011-09-01

    We present a detailed chemical tagging of individual stellar populations in the Galactic globular cluster (GC) NGC 1851. Abundances are derived from FLAMES spectra for the largest sample of giants (124) and the most extensive number of elements ever analysed in this peculiar GC. The chemistry is characterised using homogeneous abundances of proton-capture (O, Na, Mg, Al, Si), α-capture (Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr, Ba, La, Ce, Nd, Eu, Dy). We confirm the presence of an [Fe/H] spread larger than the observational errors in this cluster, but too small to clearly separate different sub-populations. We instead propose a classification scheme using a combination of Fe and Ba (which is much more abundant in the more metal-rich group) by means of a cluster analysis. With this approach, we separated stars into two components of a metal-rich (MR) and a metal-poor (MP) population. Each component displays a Na-O anticorrelation, which is a signature of a genuine GC, but has different ratios of primordial (FG) to polluted (SG) stars. Moreover, clear (anti)correlations of Mg and Si with Na and O are found for each component. The level of [α/H] tracks iron and is higher in the MR population, which might therefore have received an additional contribution from core-collapse supernovae. When considering all s-process elements, the MR population shows a larger enrichment than the MP one. This is probably due to the contribution of intermediate-low mass stars, because we find that the level of heavy s-process elements is higher than that of light s-process nuclei in the MR stars; however, a large contribution from low mass stars is unlikely, because it would likely cancel the O-Na anticorrelation. Finally, we confirm the presence of correlations between the amount of proton-capture elements and the level of s-process elements previously found by other investigations, at least for the MR population. This finding apparently requires a