We have studied the dissolution of initially mass segregated and unsegregated star clusters due to two-body relaxation in external tidal fields, using Aarseth's collisional N-body code NBODY4 on GRAPE6 special-purpose computers. When extrapolating results of initially not mass segregated models to globular clusters, we obtain a correlation between the time until destruction and the slope of the mass function, in the sense that globular clusters which are closer to dissolution are more strongly depleted in low-mass stars. This correlation fits observed mass functions of most globular clusters. The mass functions of several globular clusters are however more strongly depleted in low-mass stars than suggested by these models. Such strongly depleted mass functions can be explained if globular clusters started initially mass segregated. Primordial mass segregation also explains the correlation between the slope of the stellar mass function and the cluster concentration which was recently discovered by De Marchi et al. (2007). In this case, it is possible that all globular clusters started with a mass function similar to that seen in young open clusters in the present-day universe, at least for stars below m=0.8 Msun. This argues for a near universality of the mass function for different star formation environments and metallicities in the range -2 < [Fe/H] < 0. We finally describe a novel algorithm which can initialise stationary mass segregated clusters with arbitrary density profile and amount of mass segregation.
We present the stellar main sequence luminosity function (LF) of the disrupted, low-mass, low-concentration globular cluster Palomar 5 and its well-defined tidal tails, which emanate from the cluster as a result of its tidal interaction with the Milky Way. The results of our deep (B ~ 24.5) wide-field photometry unequivocally indicate that preferentially fainter stars were removed from the cluster so that the LF of the cluster's main body exhibits a significant degree of flattening compared to other globular clusters. There is clear evidence of mass segregation, which is reflected in a radial …
We present the detailed analysis of Hubble Space Telescope observations of the spatial distributions of different stellar species in two young compact star clusters in the Large Magellanic Cloud, NGC 1805 and NGC 1818. Based on a comparison of the characteristic relaxation times in their cores and at their half-mass radii with the observed degree of mass segregation, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805. Both clusters were likely formed with very similar initial mass functions.
Recently de Marchi, Paresce & Pulone (2007) studied a sample of twenty globular clusters and found that all clusters with high concentrations have steep stellar mass-functions while clusters with low concentration have comparatively shallow mass-functions. No globular clusters were found with a flat mass-function and high concentration. This seems curious since more concentrated star clusters are believed to be dynamically more evolved and should have lost more low-mass stars via evaporation, which would result in a shallower mass-function in the low-mass part. We show that this effect can be …
Star clusters - open and globulars - experience dynamical evolution on time scales shorter than their age. Consequently, open and globular clusters provide us with unique dynamical laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, and core collapse. We review briefly, in the framework of star clusters, some elements related to the theoretical expectation of mass segregation, the results from N-body and other computer simulations, as well as the now substantial clear observational evidence.
Observational constraints on the primordial lithium abundance are important for the evaluation of the baryonic density of the universe. Its precise determination however suffers from uncertainties due to the possible depletion of this element inside the stars. Here we present and discuss new results on the lithium abundances in pop II stars obtained with most recent stellar models including the best available physics. We show that it is possible to account for the general behavior of lithium observed in pop II stars without any free parameters. Macroscopic motions inside the stars are needed b…
We present the results of a new study of mass segregation in two-component star clusters, based on a large number of numerical N-body simulations using our recently developed dynamical Monte Carlo code. Specifically, we follow the dynamical evolution of clusters containing stars with individual masses m_1 as well as a tracer population of objects with individual masses m_2=μm_1, using N=10^5 total stars. For heavy tracers, which could represent stellar remnants such as neutron stars or black holes in a globular cluster, we characterize in a variety of ways the tendency for these objects to con…
Globular clusters in galaxies have a mutual feedback with the environment, which is tuned by their dynamical evolution. This feedback may be the explanation of various features of both the globular cluster system and the host galaxy. Relevant examples of these are the radial distribution of globulars and the violent initial activity of galaxies as AGN.
We present a wide field CCD photometric study of Galactic globular cluster NGC 7492. The derived VR color-magnitude diagram extends down to about 3.5 mag below the cluster main sequence turn-off. The field covers 42' X 42' about 3 times larger than the known tidal radius of this cluster. The sample of cluster member candidates obtained by CMD-mask process has been used to construct luminosity and mass functions and surface density map. NGC 7492 has a very flat mass function with very little variation in the slope with the distance from the cluster center. However, there is a clear evidence for…
We present a model to explain the mass segregation and shallow mass functions observed in the central parts of dense and young starburst stellar clusters. The model assumes that the initial pre-stellar cores mass function resulting from the turbulent fragmentation of the proto-cluster cloud is significantly altered by the cores coalescence before they collapse to form stars. With appropriate, yet realistic parameters, this model based on the competition between cores coalescence and collapse reproduces the mass spectra of the well studied Arches cluster. Namely, the slopes at the intermediate …
In this second paper in our series, we continue to test primordial scenarios of globular cluster formation which predict that globular clusters formed in the early universe in the potential of dark matter minihalos. In this paper we use high-resolution N-body simulations to model tidal stripping experienced by primordial dark-matter dominated globular clusters in the static gravitational potential of the host dwarf galaxy. We test both cuspy Navarro-Frenk-White (NFW) and flat-core Burkert models of dark matter halos. Our primordial globular cluster with an NFW dark matter halo survives severe …
We begin by presenting the analysis of a set of deep B- and V-band images of the central density cusp of the globular cluster M30 (NGC 7099), taken with the Faint Object Camera aboard the Hubble Space Telescope. These images are the first to resolve lower-mass main-sequence stars in the cluster's central 10''. From the positions of individual stars, we measure an improved position for the cluster center; this new position is 2.6'' from the previously known position. We find no evidence of a ``flat'', constant-surface-density core; however, the data do not rule out the presence of a core of rad…
We have investigated the effect of the number of primordial binaries on the relationship between the total number of detected binaries within globular cluster and its collision rate. We have used simulated populations of binary stars in globular clusters : primordial binaries and binaries formed through gravitational interactions. We show that the initial number of primordial binaries influences the relationship between the number of detected sources and the collision rate, which we find to be a power law. We also show that observing an incomplete sample provides the same results as those obta…
HST imaging of M22 has allowed, for the first time, a detailed and uniform mapping of mass segregation in a globular cluster. Luminosity and mass functions from the turnoff down to the mid to lower main sequence are presented for M22 in annular bins from the centre of the cluster out to five core radii. Within the core, a significant enhancement is seen in the proportion of 0.5-0.8 M(sun) stars compared with their numbers outside the core. Numerical modelling of the spatial mass spectrum of M22 shows that the observed degree of mass segregation can be accounted for by relaxation processes with…
In many theoretical scenarios it is expected that intermediate-mass black holes (IMBHs, with masses M ~ 100-10000 solar masses) reside at the centers of some globular clusters. However, observational evidence for their existence is limited. Several previous numerical investigations have focused on the impact of an IMBH on the cluster dynamics or brightness profile. Here we instead present results from a large set of direct N-body simulations including single and binary stars. These show that there is a potentially more detectable IMBH signature, namely on the variation of the average stellar m…
Star-to-star abundance variations of C, N, O, Na and Al in globular-cluster red giants have been recently supplemented by the finding that [Mg/Fe] is depleted in stars with extremely large [Al/Fe] (Shetrone 1996a). This and other new spectroscopic results allow us to test current models of stellar evolution and nucleosynthesis, as well as those of the formation and chemical enrichment of globular clusters. In an effort to explain self-consistently these observations we have considered two possibilities: (1) a deep mixing scenario which assumes that in red giants some kind of (extra)mixing tran…
We present the first study of the dynamical evolution of a star cluster that combines a significant population of primordial binaries with the presence of a central black hole. We use direct N-body simulations, with a black hole mass of about one percent of the total mass of the cluster. The evolution of the binary population is strongly influenced by the presence of the black hole, which gives the cluster a large core with a central density cusp. Starting from a Plummer profile, we first encounter a phase, that last approximately 10 half-mass relaxation times, in which binaries are disrupted …
We present a study of mass segregation of the young (20-35 Myr isochronal age), open cluster NGC 2547. We find good evidence that mass segregation exists in NGC 2547 down to 3 Msun, and weak evidence for mass segregation down to 1 Msun. Theoretical models of an initially unsegregated model of NGC 2547 using the NBODY2 code show weaker mass segregation, implying that at least some of the observed mass segregation has a primordial origin. We also report the discovery of three possible escaped cluster members, which share the proper motion and colours of the cluster, but lie nearly a degree from …
Self-enrichment processes occurring in the early stages of a globular cluster lifetime are generally invoked to explain the observed CNONaMgAl abundance anticorrelations within individual Galactic globulars.We have tested, with fully consistent stellar evolution calculations, if theoretical isochrones for stars born with the observed abundance anticorrelations satisfy the observational evidence that objects with different degrees of these anomalies lie on essentially identical sequences in the Color-Magnitude-Diagram (CMD). To this purpose, we have computed for the first time low-mass stellar …
Observations and theoretical work suggest that globular clusters may be born with initially very large binary fractions. We present first results from our newly modified Monte-Carlo cluster evolution code, which treats binary interactions exactly via direct N-body integration. It is shown that binary scattering interactions generate significantly less energy than predicted by the recipes that have been used in the past to model them in approximate cluster evolution methods. The new result that the cores of globular clusters in the long-lived binary-burning phase are smaller than previously pre…
Most observations of the projected half-light radii of metal-rich globular clusters in a variety of galaxies have shown them to be ~20% smaller than those of their metal-poor counterparts. We show using multi-mass isotropic Michie-King models that the combined effects of mass segregation and the dependence of main sequence lifetimes on metallicity can account for this difference, under the assumption that clusters with similar central potentials have the same distribution of half mass radii. If confirmed, this would represent a new constraint on theories of globular cluster formation and evolu…
Evolution with mass segregation and the evolution of the rotation of cores are both discussed for self-similar core collapse. Evolution with angular velocity proportional to the square root of the density is predicted. On the Dynamical Main Sequence of globular clusters the energy emission from binaries balances the energy expended in expanding the halo. Newton's exactly solved N-body problem is then given, along with recent generalisations, all of which have no violent relaxation, but a new type of statistical equilibrium is discussed. Finally, we set the creation of streams in the Galaxy's h…
We study the dynamical evolution of globular clusters using our 2D Monte Carlo code with the inclusion of primordial binary interactions for equal-mass stars. We use approximate analytical cross sections for energy generation from binary-binary and binary-single interactions. After a brief period of slight contraction or expansion of the core over the first few relaxation times, all clusters enter a much longer phase of stable "binary burning" lasting many tens of relaxation times. The structural parameters of our models during this phase match well those of most observed globular clusters. At…
We present a chemical composition analysis of three dozen giant stars in the nearby ``CN-bimodal'' mildly metal-poor (<[Fe/H]> = -1.18) globular cluster M4. The analysis combined traditional spectroscopic abundance methods with modifications to the line-depth ratio technique pioneered by Gray (1994). Silicon and aluminum are found to be primordially overabundant by factors exceeding the mild overabundances usually seen in alpha- and light odd elements among halo field and globular cluster giants of comparable metallicity. In addition, barium is found to be overabundant by a factor of about fou…
The size of the core is one of the main diagnostics of the evolutionary state of a globular cluster. Much has been learned over the last few years about the behavior of the core radius during and after core collapse, under a variety of different conditions related to the presence or absence of large numbers of binaries. An overview is presented of the basic physical principles that can be used to estimate the core radius. Four different situations are discussed, and expressions are presented for the ratio $r_c/r_h$ of core radius to half mass radius. The regimes are: deep collapse in the absen…
We present the detailed analysis of Hubble Space Telescope observations of the spatial distributions of different stellar species in two young compact star clusters in the Large Magellanic Cloud (LMC), NGC 1805 and NGC 1818. Based on a comparison of the characteristic relaxation times in their cores and at their half-mass radii with the observed degree of mass segregation, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805. Both clusters were likely formed with very similar initial mass functions (IMFs). In fact, we provide str…
We have used extensive V, I photometry (down to V=20.9) of 33615 stars in the direction of the globular cluster M55 to study the dynamical interaction of this cluster with the tidal fields of the Galaxy. An entire quadrant of the cluster has been covered, out to ~2.0 times the tidal radius. A CMD down to about 4 magnitudes below the turn-off is presented and analysed. A large population of BS has been identified. The BS are significantly more concentrated in the inner 300 arcsec, while they become less concentrated in the cluster envelope. We have obtained luminosity functions at various radia…
I-band CCD images of two large regions of the Galactic globular cluster NGC 1261 have been used to construct stellar luminosity functions (LF) for 14000 stars in three annuli from 1.4' from the cluster center to the tidal radius. The LFs extend to M_I~8 and tend to steepen from the inner to the outer annulus, in agreement with the predictions of the multimass King-Michie model that we have calculated for this cluster. The LFs have been transformed into mass functions. Once corrected for mass segregation the global mass function of NGC 1261 has a slope x_0=0.8+/-0.5
Galactic globular clusters are ancient building blocks of our Galaxy. They represent a very interesting family of stellar systems in which some fundamental dynamical processes have been taking place for more than 10 Gyr, but on time scales shorter than the age of the universe. In contrast with galaxies, these star clusters represent unique laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, stellar collisions, stellar mergers, core collapse, and tidal disruption. This review briefly summarizes some of the tremendous developments that have taken p…
We present N-body simulations of globular clusters, in orbits around the Galaxy, in order to study quantitatively and geometrically the tidal effects they encounter. The clusters are modelised with multi-mass King-Michie models (Michie 1963), including mass segregation at initial conditions. The Galaxy is modelled as realistic as possible, with three components: bulge, disk and dark halo. The main finding is that there exist two giant tidal tails around the globuler cluster in permanence along its orbit, whatever this orbit. The length of these tails is of the order of 5 tidal radii, or greate…
Recent observational studies have revealed star-to-star abundance inhomogeneity among light elements (e.g., C, N, O, Na, and Al) of stars on the main sequence in the Galactic globular clusters (GCs). One of promising interpretations for this result is that the observed abundance inhomogeneity is due to the second generation of stars formed from ejecta of the first generation of evolved stars (e.g., AGB stars) within GCs. However it remains unclear whether and how this primordial pollution can occur within GCs. We here propose a new scenario in which primordial pollution of GCs is highly likely…
Galactic globular clusters, which are ancient building blocks of our Galaxy, represent a very interesting family of stellar systems in which some fundamental dynamical processes have taken place on time scales shorter than the age of the universe. In contrast with galaxies, these clusters represent unique laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, stellar collisions, stellar mergers, and core collapse. In the present review, we summarize the tremendous developments, as much theoretical as observational, that have taken place during the l…
We answer the long-standing question of which production mechanism is responsible for the cataclysmic variables (CVs) in globular clusters. Arguments have been given that range from mostly primordial presence to a significant contribution of later dynamical formation in close stellar encounters. We conclude, based on a thorough analysis of a homogeneous {\it Chandra} data set, that the majority of CVs in globulars has a dynamical origin.
We consider the effect of binary stars on the main-sequence luminosity functions observed in the core of globular clusters, with specific reference to NGC 6752. We find that mass segregation results in an increased binary fraction at fainter magnitudes along the main-sequence. If this effect is not taken into account when analyzing luminosity functions, erroneous conclusions can be drawn regarding the distribution of single stars, and the dynamical state of the cluster. In the core of NGC 6752, our HST data reveal a flat luminosity function, in agreement with previous results. However, when we…
The previous two companion papers demonstrate that slowly varying perturbations do not result in adiabatic cutoffs and provide a formalism for computing the long-term effects of time-dependent perturbations on stellar systems. Here, the theory is implemented in a Fokker-Planck code and a suite of runs illustrating the effects of shock heating on globular cluster evolution are described. Shock heating alone results in considerable mass loss for clusters with $R_g\lta8\kpc$: a concentration $c=1.5$ cluster with $R_g=8\kpc$ loses up to $95\%$ of its initial mass in $15\Gyr$. Only those with conce…
We present details of the database from a large Cycle 7 HST project to study the formation and evolution of rich star clusters in the Large Magellanic Cloud (see Elson et al., this volume). Our data set, which includes NICMOS, WFPC2 and STIS images of the cores and outer regions of 8 clusters, will enable us to derive deep luminosity functions for the clusters and to investigate the universality of the stellar initial mass function. We will look for age spreads in the youngest clusters, quantify the population of binary stars in the cores of the clusters and at the half-mass radii, and follow …
The interpretation of the difference observed between the radial distribution of globular clusters and that of halo-bulge stars in elliptical galaxies is discussed in terms of evolution of their globular cluster systems. I present a short summary of the evidence that dynamical evolution of globular cluster systems is not only able to explain the flattening of their distribution toward the galactic center, but also it may have played an important role on the primordial activity of the parent galaxy.
We have completely mapped the globular cluster NGC 1851 with VI CCD photometry, obtaining a CMD for ca. 20500 stars. From the apparent luminosity of the HB we derive a distance modulus (m-M)o=15.44+/-0.20. The presence of 7 "supra-HB" stars is confirmed. Six of them are identified as evolved descendants from HB progenitors. Synthetic HR diagrams demonstrate that the problem of the HB morphology could be solved by assuming a bimodal efficiency of the mass loss along the RGB. We find evidence that the radial distribution of the blue HB stars is different from that of the red HB and SGB stars. Th…
We make use of high resolution, high signal-to-noise ratio spectra of 12 turn-off stars in the metal-poor globular cluster NGC 6397 to measure its lithium content. We conclude that they all have the same lithium abundance A(Li) = 2.34 with a standard deviation of 0.056 dex. We use this result, together with Monte Carlo simulations, to estimate that the maximum allowed intrinsic scatter is of the order of 0.035 dex. This is a new stringent constraint to be fulfilled by stellar models which predict Li depletion. We argue that although a mild depletion of 0.1 -- 0.2 dex, as predicted by recent mo…
We present the first results from the analysis of GIRAFFE spectra of more than 1200 red giants stars in 19 Galactic Globular Clusters (GCs), to study the chemical composition of second generation stars and their link with global cluster parameters. We confirm that the extension of the Na-O anticorrelation (the most striking signature of polluted, second generation populations) is strictly related to the very blue (and hot) extreme of the Horizontal Branch (HB). Long anticorrelations seem to require large mass and large-sized, eccentric orbits, taking the GCs far away from the central regions o…
We investigate the interactions of high-redshift galaxy outflows with low-mass virialized (Tvir < 10,000K) clouds of primordial composition. While atomic cooling allows star formation in larger primordial objects, such "minihalos" are generally unable to form stars by themselves. However, the large population of high-redshift starburst galaxies may have induced widespread star formation in these objects, via shocks that caused intense cooling both through nonequilibrium H2 formation and metal-line emission. Using a simple analytic model, we show that the resulting star clusters naturally repro…
We outline the steps needed in to calibrate the Monte Carlo code in order to perform large scale simulations of real globular clusters. We calibrate the results against $N$-body simulations for $N = 2500$, 10000 and for the old open cluster M67. The calibration is done by choosing appropriate free code parameters.
Close to 70 radio pulsars have now been discovered in 23 globular clusters, with a record 22 pulsars observed in 47 Tuc alone. Accurate timing solutions, including positions in the cluster, are known for many of the pulsars. These recent observations provide a unique opportunity to re-examine theoretically the formation and evolution of recycled pulsars in globular clusters. This brief review focuses on dynamical exchange interactions between neutron stars and primordial binaries, through which neutron stars can acquire intermediate-mass binary companions. The later evolution of these intermed…
We describe the status of a project whose main goal is to detect variability along the extreme horizontal branch of the globular cluster NGC 6752. Based on Magellan 6.5m data, preliminary light curves are presented for some candidate variables. By combining our time-series data, we also produce a deep CMD of unprecedented quality for the cluster which reveals a remarkable lack of main sequence binaries, possibly pointing to a low primordial binary fraction.
We have developed a new parallel supercomputer code based on Henon's Monte Carlo method for simulating the dynamical evolution of globular clusters. This new code allows us to calculate the evolution of a cluster containing a realistic number of stars (N ~ 10^5 - 10^6) in about a day of computing time. The discrete, star-by-star representation of the cluster in the simulation allows us to treat naturally a number of important processes, including single and binary star evolution, all dynamical interactions of single stars and primordial binaries, and tidal interactions with the Galaxy.
We derive the proper motions, membership probabilities, and velocity dispersions of stars in the regions of the young (about 2-4 Myr-old) open clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC 6530 (the dominant cluster in the Sgr OB1 association) from photographic plate material obtained at Shanghai Astronomical Observatory, with time baselines of 34 and 87 years, respectively. Both clusters show clear evidence of mass segregation, but they do not exhibit any significant velocity-mass (or, equivalently, a velocity-luminosity) dependence. This provides strong supp…
We modify our combined ("deep mixing" plus "primordial") scenario explaining the star-to-star abundance variations in globular cluster red giants in such a way that it confirms with new experimental data: (i) the new and better constrained (NACRE) thermonuclear reaction rates and (ii) a discovery of the O-Na anticorrelation in stars below the main-sequence turn-off in the cluster NGC6752. For the latter we propose that some main-sequence dwarfs in globular clusters accrete material lost by red giant primary components of close binaries during a common envelope event. As a consequence of the ne…
We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y from 0.28 to 0.40) compared to the primordial value. We assume that a previous generation of massive Asymptotic Giant Branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant.
I use literature data and a new temperature calibration to determine the Li abundances in the globular cluster M 92. Based on the same data, Boesgaard et al. have claimed that there is a dispersion in Li abundances in excess of observational errors. This result has been brought as evidence for Li depletion in metal-poor dwarfs. In the present note I argue that there is no strong evidence for intrinsic dispersion in Li abundances, although a dispersion as large as 0.18 dex is possible. The mean Li abundance, A(Li)=2.36, is in good agreement with recent results for field stars and TO stars in th…
We study the dynamical evolution of globular clusters containing populations of primordial binaries, using our newly updated Monte Carlo cluster evolution code with the inclusion of direct integration of binary scattering interactions. We describe the modifications we have made to the code, as well as improvements we have made to the core Monte Carlo method. We present several test calculations to verify the validity of the new code, and perform many comparisons with previous analytical and numerical work in the literature. We simulate the evolution of a large grid of models, with a wide range…
We present low resolution UV-blue spectroscopic observations of red giant stars in the globular cluster M53 ([Fe/H]=-1.84), obtained to study primordial abundance variations and deep mixing via the CN and CH absorption bands. The metallicity of M53 makes it an attractive target: a bimodal distribution of 3883 angstrom CN bandstrength is common in moderate- and high-metallicity globular clusters ([Fe/H] > -1.6) but unusual in those of lower metallicity ([Fe/H] < -2.0). We find that M53 is an intermediate case, and has a broad but not strongly bimodal distribution of CN bandstrength, with CN and…
Low-mass X-ray binaries, recycled pulsars, cataclysmic variables and magnetically active binaries are observed as X-ray sources in globular clusters. We discuss the classification of these systems, and find that some presumed active binaries are brighter than expected. We discuss a new statistical method to determine from observations how the formation of X-ray sources depends on the number of stellar encounters and/or on the cluster mass. We show that cluster mass is not a proxy for the encounter number, and that optical identifications are essential in proving the presence of primordial bina…
Model surface brightness profiles based on Fokker-Planck simulations have been used to assess the high resolution surface brightness profile of the globular cluster NGC 6397 of Lauzeral et al. (1992). The profile is well fitted by the model in the maximum-expansion phase of a gravothermal oscillation with a core radius of 0.06 pc (6''), but an unresolved core cannot be ruled out. A core this size is also expected in highly evolved clusters including significant numbers of primordial binaries. A more massive cluster will provide better statistics, as would star counts. The greater distance of t…
We review the complications involved in the conversion of stellar luminosities into masses and apply a range of mass-to-luminosity relations to our Hubble Space Telescope observations of the young LMC star clusters NGC 1805 and NGC 1818. Both the radial dependence of the mass function (MF) and the dependence of the cluster core radii on mass indicate clear mass segregation in both clusters at radii r <= 20-30'', for masses in excess of ~1.6-2.5 Msun. This result does not depend on the mass range used to fit the slopes or the metallicity assumed. It is clear that the cluster MFs, at any radius,…
We have used ESO telescopes at La Silla and the Hubble Space Telescope (HST) in order to obtain accurate B,V,I CCD photometry for the stars located within 200" (~= 2 half-mass radii, r_h = 1.71') from the center of the cluster NGC 6101. Color-Magnitude Diagrams extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO (V = 20.05 +- 0.05) have been constructed. The following results have been obtained from the analysis of the CMDs: a) The overall morphology of the main branches confirms previous results from the literature, in particular the existence of a size…
Stars in globular clusters (GCs) exhibit a peculiar chemical pattern with strong abundance variations in light elements along with a constant abundance in heavy elements. These abundance anomalies can be explained by a primordial pollution due to a first generation of fast rotating massive stars which released slow winds into the ISM from which a second generation of chemically anomalous stars can be formed. In particular the observed ratio of anomalous and standard stars in clusters can be used to constrain the dynamical evolution of GCs as around 95% of the standard stars need to be lost by …
Astronomers have constructed models of globular clusters for over 100 years. These models mainly fall into two categories: (i) static models, such as King's model and its variants, and (ii) evolutionary models. Most attention has been given to static models, which are used to estimate mass-to-light ratios and mass segregation, and to combine data from proper motions and radial velocities. Evolutionary models have been developed for a few objects using the gaseous model, the Fokker-Planck model, Monte Carlo models and N-body models. These models have had a significant role in the search for mas…
We have studied the dynamical evolution of rotating globular clusters with direct $N$-body models. Our initial models are rotating King models; we obtained results for both equal-mass systems and systems composed out of two mass components. Previous investigations using a Fokker-Planck solver have revealed that rotation has a noticeable influence on stellar systems like globular clusters, which evolve by two-body relaxation. In particular, it accelerates their dynamical evolution through the gravogyro instability. We have validated the occurence of the gravogyro instability with direct $N$-bod…
Abundance variations in moderately metal-rich globular clusters can give clues about the formation and chemical enrichment of globular clusters. CN, CH, Na, Mg and Al indices in spectra of 89 stars of the template metal-rich globular cluster M71 are measured and implications on internal mixing are discussed. Stars from the turn-off up to the Red Giant Branch (0.87 $<$ log g $<$ 4.65) observed with the GMOS multi-object spectrograph at the Gemini-North telescope are analyzed. Radial velocities, colours, effective temperatures, gravities and spectral indices are determined for the sample. Previo…
I present a scenario for the production of low mass, degenerate dwarfs of mass $>0.1 M_{\odot}$ via the mechanism of Lenzuni, Chernoff & Salpeter (1992). Such objects meet the mass limit requirements for halo dark matter from microlensing surveys while circumventing the chemical evolution constraints on normal white dwarf stars. I describe methods to observationally constrain this scenario and suggest that such objects may originate in small clusters formed from the thermal instability of shocked, heated gas in dark matter haloes, such as suggested by Fall & Rees (1985) for globular clusters.
In a series of two papers, we test the primordial scenario of globular cluster formation using results of high-resolutions N-body simulations. In this first paper we study the initial relaxation of a stellar core inside a live dark matter minihalo in the early universe. Our dark-matter dominated globular clusters show features which are usually attributed to the action of the tidal field of the host galaxy. Among them are the presence of an apparent cutoff ("tidal radius") or of a "break" in the outer parts of the radial surface brightness profile, and a flat line-of-sight velocity dispersion …
