We investigate the long-term dynamical evolution of two distinct stellar populations of low-mass stars in globular clusters in order to study whether the energy equipartition process can explain the high number of stars harbouring abundance anomalies seen in globular clusters. We analyse N-body models by artificially dividing the low-mass stars (m<0.9 Msun) into two populations: a small number of stars (second generation) consistent with an invariant IMF and with low specific energies initially concentrated towards the cluster-centre mimic stars with abundance anomalies. These stars form from the slow winds of fast-rotating massive stars. The main part of low-mass (first generation) stars has the pristine composition of the cluster. We study in detail how the two populations evolve under the influence of two-body elaxation and the tidal forces due to the host galaxy.Stars with low specific energy initially concentrated toward the cluster centre need about two relaxation times to achieve a complete homogenisation throughout the cluster. For realistic globular clusters, the number ratio between the two populations increases only by a factor 2.5 due to the preferential evaporation of the population of outlying first generation stars. We also find that the loss of information on the stellar orbital angular momentum occurs on the same timescale as spatial homogenisation.
We show that enhanced extra mixing in low-mass red giants can result in a fluorine abundance that is correlated with abundance variations of other elements participating in H burning, such as C, N, O and Na. This finding is used to explain the fluorine abundance variations recently found in bright red giants of the globular cluster M4.
We investigate the consequences of an increase in the envelope helium abundance of pre-helium flash red giants in globular clusters. Comparing predictions with the CM diagrams of a few crucial GC, one finds no evidence for a substantial increase in the surface helium content of HB members of these clusters, at least for objects in the RR Lyrae region or close to it. The possibility that the most peculiar giants belong to the asymptotic giant branch is discussed. The consequences of a delay in the helium flash are briefly examined.
We explore the relatiohsip between deep mixing on the red-giant branch and the second parameter that gives rise to the extended blue tail seen on the horizontal branch of the globular cluster M 13. We use the [Al/Fe] values of 96 RGB stars and the [Na/Fe] values of 119 RGB stars in M 13 to follow the evolution of mixing in this cluster. The data are compared to an "instantaneous" mixing algorithm to predict the effects of deep mixing on the HB morphology. In addition, we present new data for six giants in the cluster M 3 and three in M 13. Our final results are consistent with deep mixing as a…
We present the results of an observational study of the efficiency of deep mixing in globular cluster red giants as a function of stellar metallicity. We determine [C/Fe] abundances based on low-resolution spectra taken with the Kast spectrograph on the 3m Shane telescope at Lick Observatory. Spectra centered on the 4300 Angstrom CH absorption band were taken for 42 bright red giants in 11 Galactic globular clusters ranging in metallicity from M92 ([Fe/H]=-2.29) to NGC 6712 ([Fe/H]=-1.01). Carbon abundances were derived by comparing values of the CH bandstrength index S2(CH) measured from the …
The observed abundance variations in globular cluster red giants indicate that these stars may be mixing helium from the hydrogen shell outward into the envelope, presumably as a result of internal rotation. We have investigated the implications of such helium mixing for both the red- giant-branch (RGB) and horizontal-branch (HB) phases by computing a number of noncanonical evolutionary sequences for different assumed mixing depths and mass loss rates. We find that the helium-mixed models evolve to higher luminosities during the RGB phase and consequently lose more mass than their canonical co…
Globular cluster ages are more than just lower limits to the age of the universe; the distribution of ages constraints the timescales for galaxy formation and chemical evolution. Globular cluster populations with different metallicities have now been detected in many galaxies, and understanding how these populations formed requires knowing their relative ages. We examined the relative ages of the two M31 globular cluster populations using their color and luminosity distributions and found that the metal-rich clusters could be up to 50% younger than the metal-poor clusters. While a small delay …
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 …
It has been known for a long time that some red giants in globular clusters exhibit large star-to-star variations in the abundances of light elements that are not exhibited by field giants. This fact can be taken as evidence that the extra mixing mechanism(s) that operate in globular cluster giants may be consequences of star-star interactions in the dense stellar environment. In order to constrain the extra mixing mechanism(s), we study the influence of helium enrichment along the red giant branch (RGB) on the evolution of stars through the horizontal branch. Three possible modes of helium en…
One of the largest sources of uncertainty in the computation of globular cluster isochrones and hence in the age determination of globular clusters is the lack of a rigorous description of convection. Therefore, we calibrated the superadiabatic temperature gradient in the envelope of metal-poor low-mass stars according to the results from a new grid of 2D hydrodynamical models, which cover the Main Sequence and the lower Red Giant Branch of globular cluster stars. In practice, we still use for computing the evolutionary stellar models the traditional mixing length formalism, but we fix the mix…
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…
Repeatedly flaring X-ray binaries have recently been discovered in NGC 4697 by Sivakoff and collaborators. We show that these flares can be explained as the result of eccentric binaries in globular clusters which accrete more rapidly at periastron than during the rest of the binary orbit. We show that theoretical timescales for producing eccentricities and circularising the binaries are consistent with what is needed to produce the observed population of flaring sources, although the circularisation timescales are highly uncertain on both observational and theoretical grounds. This model makes…
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…
The unveiled main-sequence splitting in omega Centauri as well as NGC 2808 suggests that matter highly-enriched in He (in terms of its mass fraction Y~0.4) was produced and made the color of some main-sequence stars bluer in these globular clusters (GCs). The potential production site for the He-rich matter is generally considered to be massive AGB stars that experience the second dredge-up. However, it is found that massive AGB stars provide the matter with Y~0.35 at most, while the observed blue-shift requires the presence of Y~0.4 matter. Here, we show that extra mixing, which operates in t…
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…
We present a study of the X-ray spectral properties of the highly variable X-ray emitting black hole in a globular cluster in the elliptical galaxy NGC 4472. The XMM-Newton spectrum of the source in its bright epoch is well described by a multiple blackbody model with a characteristic temperature $kT_{in}\approx$ 0.2 keV. The spectrum of an archival Chandra observation of the source obtained 3.5 years before the XMM data gives similar estimates for the blackbody parameters. We confirm that the fainter interval of the XMM-Newton observation has a spectrum that is consistent with the brighter ep…
Atmospheric parameters (effective temperature Teff, surface gravity log g), masses and helium abundances are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars we derive magnesium and iron abundances as well and find that iron is enriched by a factor of 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster abundance. Radiation pressure may levitate heavy elements like iron to the surface of the star in a diffusive process. Taking into account the enrichment of heavy elements in our spectr…
Evolutionary trends in the surface abundances of heavier elements have recently been identified in the globular cluster NGC 6397 ([Fe/H]=-2), indicating the operation of atomic diffusion in these stars. Such trends constitute important constraints for the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars. We perform an independent check of the reality and size of abundance variations within this metal-poor globular cluster. Observational data covering a large stellar sample, located between the cluster turn-off point and the base o…
The majority of observed mass-to-light ratios of globular clusters are too low to be explained by `canonical' cluster models, in which dynamical effects are not accounted for. Moreover, these models do not reproduce a recently reported trend of increasing M/L with cluster mass, but instead predict mass-to-light ratios that are independent of cluster mass for a fixed age and metallicity. This study aims to explain the M/L of globular clusters in four galaxies by including stellar evolution, stellar remnants, and the preferential loss of low-mass stars due to energy equipartition. Analytical clu…
Red giant stars, both in the field and in globular clusters, present abundance anomalies that can not be explained by standard stellar evolution models. Some of these peculiarities clearly point towards the existence of extra-mixing processes at play inside the stars, the nature of which remains unclear. We present new results of the evolution of a typical globular cluster red giant including a fully consistent treatment of rotation-induced mixing from the ZAMS up to the RGB tip. The uncertainties pertaining to the prescriptions of angular momentum and chemicals transport inside the star are d…
Using the Chandra/ACIS-I detector, we have identified an X-ray source (CXOU 132619.7-472910.8) in the globular cluster NGC 5139 with a thermal spectrum identical to that observed from transiently accreting neutron stars in quiescence. The absence of intensity variability on timescales as short as 4 seconds (< 25% rms variability) and as long as 5 years (<50% variability) supports the identification of this source as a neutron star, most likely maintained at a high effective temperature (approximately 1e6 K) by transient accretion from a binary companion. The ability to spectrally identify quie…
It is now clear that abundance variations from star-to-star among the light elements, particularly C, N, O, Na and Al, are ubiquitous within galactic globular clusters; they appear seen whenever data of high quality is obtained for a sufficiently large sample of stars within such a cluster. The correlations and anti-correlations among these elements and the range of variation of each element appear to be independent of stellar evolutionary state, with the exception that enhanced depletion of C and of O is sometimes seen just at the RGB tip. While the latter behavior is almost certainly due to …
We explore the constraints on globular cluster formation provided by the observed conditions in starbursts where globulars are currently forming, and by the observed properties of young and old globular clusters. We note that the pressure in the ISM of starbursts and mergers implies that molecular clouds in these environments have radii similar to those of globular clusters. Such molecular clouds are therefore viable precursors to globular clusters if the star formation efficiency in the clouds is high. A high star formation efficiency may be a consequence of the high density and associated hi…
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 present the evolution of low mass and low metallicity RGB models computed under the assumption of deep mixing between the convective envelope and the hydrogen burning shell. We discuss the helium enrichment of the envelope which is necessary or allowed to achieve or to keep consistency with the observations, and conclude on the possible connection between chemical anomalies in red giants and the horizontal branch morphology in globular clusters.
A new grid of theoretical isochrones based on the Yale stellar evolution code using the OPAL and Kurucz opacities has been constructed. The grid of isochrones spans a wide range of metallicities, helium abundances and masses. The construction of the isochrones is described and the isochrones are compared to galactic globular cluster observations. A solar calibrated mixing length ($α= 1.7$) yields a good fit to globular cluster colour-magnitude diagrams. Ages for 40 globular clusters are determined using the \dv method and the formation of the halo is discussed.
Blue horizontal branch and UV bright stars in several globular clusters are analysed spectroscopically and the results are compared with predictions of stellar evolutionary theory. We find that the distribution of temperatures and surface gravities of the blue HB stars may be explained by the effects of deep mixing. The masses derived for these stars are too low unless one uses the long distance scale for globular clusters. First results on blue HB stars in metal rich clusters are presented. Analyses of hot UV bright stars in globular clusters uncovered a lack of genuine post-asymptotic giant …
In spite of the efforts made in the latest years, still there is no comprehensive explanation for the chemical anomalies of globular cluster stars. Among these, the most striking is oxygen depletion, which reaches values down to [O/Fe]~-0.4 in most clusters, but in M13 it goes down to less than [O/Fe]~ - 1. In this work we suggest that the anomalies are due to the super position of two different events: 1) PRIMORDIAL SELF-ENRICHMENT: this is asked to explain the oxygen depletion down to a minimum value [O/Fe]~ -0.4; 2) EXTRA MIXING IN A FRACTION OF THE STARS ALREADY BORN WITH ANOMALOUS COMPOSI…
Time-dependent nuclear network calculations at constant temperature show that for zero-metal stars >= 20 Msun (i) beta-decay reactions and (ii) the 13N(p,gamma)14O reaction must be included. It is also shown that the nuclear timescale in these zero-metal stars is shorter than the mixing timescale and therefore the assumption of instantaneous mixing across convective regions is not fulfilled. We conclude that proper modeling of these processes may alter the evolution of massive zero-metal stars.
We report the detection, with HST, of an optical counterpart to the transient supersoft X-ray source 1E 1339.8+2837, in the globular cluster M3. The counterpart is found near the faint end of the subgiant branch in the V vs V-I color magnitude diagram, but is extremely bright in U. Variability is detected over a range of timescales suggesting the presence of an accretion disk and perhaps also ellipsoidal variations of the subgiant secondary. The optical colors of the binary are similar to those of cataclysmic variables recently discovered in 47 Tucanae and NGC 6397. We suggest that magneticall…
We have serendipitously discovered an extremely lithium-rich star on the red giant branch of the globular cluster M3 (NGC 5272). An echelle spectrum obtained with the Keck I HIRES reveals a Li I 6707 Angstrom resonance doublet of 520 milli-Angstrom equivalent width, and our analysis places the star among the most Li-rich giants known: log[epsilon(Li)] ~= +3.0. We determine the elemental abundances of this star, IV-101, and three other cluster members of similar luminosity and color, and conclude that IV-101 has abundance ratios typical of giants in M3 and M13 that have undergone significant mi…
In the most prominent current scenario of galaxy formation, galaxies form hierarchically through the merger of smaller systems. Such mergers could leave behind dynamical signatures which may linger long after the event. In particular, the globular cluster system (GCS) of a merging satellite galaxy may remain as a distinct sub-population within the GCS of a massive galaxy. Using the latest available globular cluster velocities and metallicities, we present the results of a search for grouping in the GCS of our nearest large spiral galaxy neighbor, M31. A modified friends-of-friends algorithm is…
We present a review of the present state of knowledge regarding the relative ages of Galactic globular clusters. First, we discuss the relevant galaxy formation models and describe the detailed predictions they make with respect to the formation timescale and chemical evolution of the globular clusters. Next, the techniques used to estimate globular cluster ages are described and evaluated with particular emphasis on the advantages and disadvantages of each method. With these techniques as a foundation, we present arguments in favor of the following assertions: 1) The age of a globular cluster…
We investigate the in-spiraling timescales of globular clusters in dwarf spheroidal (dSph) and dwarf elliptical (dE) galaxies, due to dynamical friction. We address the problem of these timescales having been variously estimated in the literature as much shorter than a Hubble time. Using self-consistent two-component (dark matter and stars) models, we explore mechanisms which may yield extended dynamical friction timescales in such systems in order to explain why dwarf galaxies often show globular cluster systems. As a general rule, dark matter and stars both give a comparable contribution to …
By means of analytical calculations, we explore the self-enrichment scenario for Globular Cluster formation. According to this scenario, an initial burst of star formation occurs inside the core radius of the initial gaseous distribution. The outward-propagating shock wave sweeps up a shell in which gravitational instabilities may arise, leading to the formation of a second, metal-enriched, population of stars. We find a minimum mass of the proto-globular cluster of the order of 10^6 solar masses. We also find that the observed spread in the Magnitude-Metallicity relation can be explained assu…
Recent Hubble Space Telescope observations have found that the horizontal branches (HB's) in the metal-rich globular clusters NGC 6388 and NGC 6441 slope upward with decreasing B-V. Such a slope is not predicted by canonical HB models and cannot be produced by either a greater cluster age or enhanced mass loss along the red-giant branch (RGB). The peculiar HB morphology in these clusters may provide an important clue for understanding the second-parameter effect. We have carried out extensive evolutionary calculations and numerical simulations in order to explore three non-canonical scenarios …
Deep images in the 10 micron spectral region have been obtained for five massive Galactic globular clusters, NGC 104 (=47 Tuc), NGC 362, NGC 5139 (omega Cen), NGC 6388, NGC 7078 (=M15) and NGC 6715 (=M54) in the Sagittarius Dwarf Spheroidal using ISOCAM in 1997. A significant sample of bright giants have an ISOCAM counterpart but only < 20% of these have a strong mid-IR excess indicative of dusty circumstellar envelopes. From a combined physical and statistical analysis we derive mass loss rates and frequency. We find that i) significant mass loss occurs only at the very end of the Red Giant B…
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…
We extend our previous method to determine globular cluster ages using the luminosity function (Jimenez \& Padoan 1996). We show that the luminosity function depends on both age and distance modulus and that it is possible to distinguish between the two. This method provides at the same time independent determinations of distance and age of a GC by simply counting the number of stars found inside specified luminosity bins. The main uncertainties in other traditional methods for determining GCs ages are absent (e.g. mixing length, color-$T_{\rm eff}$ calibration, morphology of the color-magnitu…
We present a new method to compute stellar ages in Globular Clusters (GC) that is ten times more precise than the traditional isochrone fitting procedure. The method relies on accurate stellar evolutionary tracks and on photometry for GCs complete down to the main sequence, and it is based on counting number of stars in two different regions of the CMD: the red giant branch and the main-sequence. We have applied this method to the globular cluster M68 and found an age of 16.4$\pm$0.2 Gyr for $(m-M)_V=15.3$. This new method reduces the error associated to the uncertainty in the distance modulus…
If the winds of an asymptotic-giant-branch stars are sufficiently strong are slightly asymmetric, they can alter the star's trajectory through a globular cluster; therefore, if these winds are asymmetric, one would expect young white dwarfs to be less radially concentrated than either their progenitors or older white dwarfs in globular clusters. This latter effect has recently been observed. Additionally the young white dwarfs should have larger typical velocities than their progenitors. After phase mixing this latter effect is vastly diminished relative to the changes in the spatial distribut…
We compare [alpha/Fe], metallicity, and age distributions of globular clusters in elliptical, lenticular, and spiral galaxies, which we derive from Lick line index measurements. We find a large number of globular clusters in elliptical galaxies that reach significantly higher [alpha/Fe] values (>0.5) than any clusters in lenticular and spiral galaxies. Most of these extremely alpha-enriched globular clusters are old (t > 8 Gyr) and cover the metallicity range -1 <~ [Z/H] <~ 0. A comparison with supernova yield models suggests that the progenitor gas clouds of these globular clusters must have …
We investigate the timescales for stochasticity and chaotic mixing in a family of triaxial potentials that mimic the distribution of light in elliptical galaxies. Some of the models include central point masses designed to represent nuclear black holes. Most of the boxlike orbits are found to be stochastic, with mean Liapunov times that are 3-6 times the period of the long-axis orbit. In models with large cores or small black holes, the stochastic orbits mimic regular box orbits for hundreds of oscillations at least. However a small core radius or significant black hole mass causes most of the…
Globular clusters cores harbour many low-mass X-ray binaries, cataclysmic variables, and millisecond pulsar binaries which are likely to have been formed via the process of tidal capture. Tidal capture binaries were originally thought to be responsible for halting core collapse and for subsequent re-expansion via the process of ``binary heating''. The standard model, now suggests that these binaries are no longer viable as a direct energy source. We present the results of a study which suggests that tidal capture binaries may indeed be a significant direct source of energy for the cores of glo…
We simulate black hole binary interactions to examine the probability of mergers and black hole growth and gravitational radiation signals using a specific initial distribution of masses for black holes in globular clusters and a simple semi-analytic formalism for dynamical interactions. We include 3-body recoil and the latest results in numerical relativity for gravitational radiation recoil. It is found that while 99% of binaries are ejected from low metallicity, low mass clusters; metal rich massive clusters retain 5% of their binaries. An interesting fraction of the ejected binaries, espec…
Spectra with a high signal-to-noise ratio of 50 stars which are just below the main sequence turnoff and are members of M13 are presented. They yield indices for the strength of the CH and the ultraviolet CN band. There is no evidence for a variation in the strength of either feature from star to star in this intermediate-metallicity galactic globular cluster, and thus no evidence for primordial variations in the abundance of C and N in M13. This supports the hypothesis that abundance variations found among the light elements in the evolved stars of M13 by Suntzeff (1981), and commonly seen on…
The dynamical friction timescale for globular clusters to sink to the center of a dwarf elliptical galaxy (dE) is significantly less than a Hubble time if the halos have King-model or isothermal profiles and the globular clusters formed with the same radial density profile as the underlying stellar population. We examine the summed radial distribution of the entire globular cluster systems and the bright globular cluster candidates in 51 Virgo and Fornax Cluster dEs for evidence of dynamical friction processes. We find that the summed distribution of the entire globular cluster population clos…
Using the NIRSPEC spectrograph at Keck II, we have obtained infrared echelle spectra covering the range 1.5-1.8 micron for the moderately reddened bulge globular clusters NGC 6342 and NGC 6528, finding [Fe/H]= -0.60 and -0.17 dex, respectively. We measure an average alpha-enhancement of approx +0.33 dex in both clusters, consistent with previous measurements on other metal rich bulge clusters, and favoring the scenario of a rapid bulge formation and chemical enrichment. We also measure very low 12C/13C isotopic ratios (approx 5 in NGC 6342 and approx 8 in NGC 6528), suggesting that extra-mixin…
We construct a speculative scenario for rotation-induced extra helium mixing to the envelope of horizontal branch (HB) stars. This scenario differs from previous ones in that the mixing occurs after the star has left the red giant branch (RGB). We follow the evolution of a low metallicity star from the RGB to the HB, and examine the density profile and radius in the core-envelope boundary region. In the transition from the RGB to the HB the envelope shrinks by two orders of magnitude in size and the core swells, such that any non-negligible rotation on the RGB will result in a strong rotationa…
Luminous X-ray binaries (>1E34 erg/s, LMXBs) have a neutron star or black hole primary, and in globular clusters, most of these close binaries are expected to be have evolved from wider binaries through dynamical interactions with other stars. We sought to find a predictor of this formation rate that is representative of the initial properties of globular clusters rather than of the highly evolved core quantities. Models indicate the half-light quantities best reflect the initial conditions, so we examine whether the associated dynamical interaction rate, proportional to L^1.5 r^-2.5, is usefu…
Spectra with a high signal-to-noise ratio of 79 stars which are just below the main sequence turnoff of M71 are presented. They yield indices for the strength of the G band of CH and the ultraviolet CN band at 3885 Å. These indices are each to first order bimodal and they are anti-correlated. There are approximately equal numbers of CN weak/CH strong and CN strong/CH weak main sequence stars in M71. It is not yet clear whether these star-to-star variations arise from primordial variations or from mixing within a fraction of individual stars as they evolve.
The discovery of the Sagittarius dwarf galaxy, which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars, and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is omega Centauri, and here we report that omega Centauri also has several distinct stellar populations, as traced by red-giant-branch stars.…
Hydrogen-burning is the root cause of the star-to-star abundance variations of light nuclei in Galactic globular clusters (GC). In the present work we constrain the physical conditions that gave rise to the observed abundance patterns of Li, C, N, O, Na, Mg, Al, as well as Mg isotopes in the typical case of NGC6752. We perform nucleosynthesis calculations at constant temperature, adopting realistic initial abundances for the proto-cluster gas. We use a detailed nuclear reaction network and state-of-the-art nuclear reaction rates. Although simplistic, our analysis provides original results and …
We performed high-resolution simulations of two stellar collisions relevant for stars in globular clusters. We considered one head-on collision and one off-axis collision between two 0.6 M_sun main sequence stars. We show that a resolution of about 100 000 particles is sufficient for most studies of the structure and evolution of blue stragglers. We demonstrate conclusively that collision products between main-sequence stars in globular clusters do not have surface convection zones larger than 0.004 M_sun after the collision, nor do they develop convection zones during the `pre-main-sequence' …
We report the discovery of two extremely low luminosity globular clusters in the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory. The clusters, Koposov 1 and Koposov 2, are located at $\sim 40-50$ kpc and appear to have old stellar populations and luminosities of only $M_V \sim -1$ mag. Their observed sizes of $\sim 3$ pc are well within the expected tidal limit of $\sim$10 pc at that distance. Together with Palomar 1, AM 4 and Whiting 1, these new clusters are the lowest luminosity globu…
We report the results of new SPH calculations of parabolic collisions between two main-sequence stars in a globular cluster. Such collisions are directly relevant to the formation of blue stragglers. In particular, we consider parent stars of mass $M/M_{TO}=0.2,0.5,0.75$, and $1$, where $M_{TO}$ is the cluster turnoff mass (typically about $0.8\,M_\odot$). Low-mass stars (with $M=0.2 M_{TO}$ or $0.5 M_{TO}$) are assumed to be fully convective and are therefore modeled as $n=1.5$ polytropes. Stars at the turnoff (with $M= M_{TO}$) are assumed to be mostly radiative and are modeled as $n=3$ poly…
We present a chemical abundance study of three inner old halo clusters NGC 6287, NGC 6293, and NGC 6541, finding [Fe/H] = -2.01 +/- 0.05, -1.99 +/- 0.02, and -1.76 +/- 0.02, respectively, and our metallicity measurements are in good agreement with previous estimates. The mean alpha-element abundances of our program clusters are in good agreement with other globular clusters, confirming previous results. However, the individual alpha-elements appear to follow different trends. The silicon abundances of the inner halo clusters appear to be enhanced and the titanium abundances appear to be deplet…
The main sequence turnoff luminosity is the best stellar `clock' which can be used to determine the absolute ages of globular clusters. This is due to the fact that it is generally assumed that the luminosity and lifetimes of main sequence globular cluster stars are independent of the properties of stellar convection and atmospheres, two areas of stellar evolution which are poorly understood. Several possible sources of error in this stellar clock are discussed, and isochrones are constructed using a variety of different physical assumptions. The mean age of the oldest globular clusters are de…
The behaviour of the Delta nu =2 CO bands around 2.3 micron was examined by comparing observed and synthetic spectra in stars in globular clusters of different metallicity. Changes in the 12C/13C isotopic ratio and the carbon abundances were investigated in stars from 3500--4900 K in the galactic globular clusters M71, M5, M3 and M13, covering the metallicity range from --0.7 to --1.6. We found relatively low carbon abundances that are not affected by the value of oxygen abundance. For most giants the 12C/13C ratios determined are consistent with the equilibrium value for the CN cycle. This su…
We suggest to use the shape of the Red Giant Branch (RGB) Bump in metal-rich globular clusters as a diagnostic of partial mixing processes between the base of the convective envelope and the H-burning shell. The Bump located along the differential luminosity function of cluster RGB stars is a key observable to constrain the H-profile inside these structures. In fact, standard evolutionary models that account for complete mixing in the convective unstable layers and radiative equilibrium in the innermost regions do predict that the first dredge-up lefts over a very sharp H-discontinuity at the …
We study the production of Na and Al around the hydrogen shell of two red-giant sequences of different metallicity in order to explain the abundance variations seen in globular cluster stars in a mixing scenario. Using detailed stellar models together with an extensive nuclear reaction network, we have calculated the distribution of the various isotopic abundances around the hydrogen shell at numerous points along the red-giant branch. These calculations allow for the variation in both temperature and density in the shell region as well as the timescale of the nuclear processing, as governed b…
