Once fixed the age and metallicity; the colour distribution of horizontal branch stars in a globular cluster depends on few parameters: the helium abundance of the population and the mass lost during the pre-HB stages. These parameters are usually derived from the HB itself, hence they are degenerate. Breaking this degeneracy and understanding their role is a tricky and challenging problem that no study has tried to solve yet. Combining the information obtained from the chromosome maps and the analysis of multi-band photometry with state of the art stellar evolution models, we can obtain a solid estimate of Y for the various stellar populations in a GC. We will then have, for the first time, the possibility to break the parameters degeneracy on the HB, understand the role of the mass loss, and lay the foundation to build another piece of the multiple populations mosaic.
Multiple stellar populations in the Milky Way globular clusters manifest themselves with a large variety. Although chemical abundance variations in light elements, including He, are ubiquitous, the amount of these variations is different in different globulars. Stellar populations with distinct Fe, C+N+O and slow-neutron capture elements have been now detected in some globular clusters, whose number will likely increase. All these chemical features correspond to specific photometric patterns. I review the chemical+photometric features of the multiple stellar populations in globular clusters an…
This paper presents a review of the characteristics of the multiple stellar populations observed in globular clusters, and of their possible origin. The current theoretical issues and the many open questions are discussed.
Aims. We present a new technique to estimate the relative ages of multiple stellar populations hosted by five globular clusters: NGC 104 (47 Tuc), NGC 6121 (M4), NGC 6352, NGC 6362 and NGC 6723. Methods. We used the catalogs of the database "HST UV Globular cluster Survey (HUGS)" to create color-magnitude and two-color diagrams of the Globular Clusters. We identified the multiple populations within each globular cluster, and we divided them into two main stellar populations: POPa or first generation (1G) and POPb, composed of all the successive generations of stars. The new technique allows us…
Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high spe…
The membership of Matsunaga's variable 1, a carbon-rich, mass-losing, Mira variable, in the globular cluster Lynga 7 is discussed on the basis of radial velocities. We conclude that it is a member, the first known C-Mira in a globular cluster. Since such a variable is expected to have an age of $\sim 1-2$ Gyr and an initial mass of $\sim 1.5$ solar masses, we conclude that this star must be the product of a stellar merger.
We use Hubble Space Telescope (HST) imaging from the ACS Treasury Survey to determine fits for single population isochrones of 69 Galactic globular clusters. Using robust Bayesian analysis techniques, we simultaneously determine ages, distances, absorptions, and helium values for each cluster under the scenario of a "single" stellar population on model grids with solar ratio heavy element abundances. The set of cluster parameters is determined in a consistent and reproducible manner for all clusters using the Bayesian analysis suite BASE-9. Our results are used to re-visit the age-metallicity …
Synthetic RGBB magnitudes are generated with the most recent theoretical stellar evolution models computed with the Dartmouth Stellar Evolution Program (DSEP) code. They are compared to the observational work of Nataf et al., who present RGBB magnitudes for 72 globular clusters. A DSEP model using a chemical composition with enhanced $α$ capture [$α$/Fe] $ =+0.4$ and an age of 13 Gyr shows agreement with observations over metallicities ranging from [Fe/H] = $0$ to [Fe/H] $\approx-1.5$, with discrepancy emerging at lower metallicities.
We are coming to believe that stellar populations in globular clusters are not as simple as they were once thought to be. A growing amount of photometric and spectroscopic evidence shows that globular clusters host at least two different stellar populations. In our contribution to these proceedings we present the first results of a survey we are conducting to look for the presence of multiple populations in a significant number of Galactic globular clusters, using the Strömgren system. We intend to photometrically separate these populations and characterize their radial distributions and exten…
Since the discovery of chemically peculiar stars in globular clusters in the last century, the study of multiple populations has become increasingly important, given that chemical inhomogeneity is found in almost all globular clusters. Despite various proposed theories attempting to explain this phenomenon, fitting all the observational evidence in globular clusters with one single theory remains notoriously difficult and currently unsuccessful. In order to improve existing models and motivate new ones, we are observing globular clusters at critical conditions, e.g., metal-rich end, metal-poor…
An increasing amount of spectroscopic and photometric evidence is showing that the stellar populations of globular clusters are not as simple as they have been considered for many years. The presence of at least two different populations of stars is being discovered in a growing number of globular clusters, both in our Galaxy and in others. We have started a series of observations of Galactic globular clusters using the Strömgren photometric system in order to find the signatures of these multiple populations and establish their presence in a more complete sample of globular clusters in the Mi…
We review spectroscopic results concerning multiple stellar populations in globularclusters. The cluster initial mass is the most important parameter determining the fraction of second generation stars. The threshold for the onset of the multiple population phenomenon is 1-3x10^5 MSun. Nucleosynthesis is influenced by metallicity: Na/O and Mg/Al anti-correlations are more extended in metal-poor than in metal-rich clusters. Massive clusters are more complex systems than the smaller ones, with several populations characterized by different chemical compositions. The high Li abundance observed in…
I present the latest results from our group about the multiple stellar populations in the old Milky Way globular clusters (GCs) and in the young systems both in the Magellanic Clouds and in the Milky Way. For the ancient GCs in our Galaxy I summarize the chemical properties of the stellar populations as observed on the chromosome map. Both Type I and Type II GCs are discussed. For the youngest clusters I will briefly report our latest spectroscopic analysis on the Large Magellanic Cloud cluster NGC1818 and the Galactic open cluster NGC6705 (M11), which supports the co-existence of stellar popu…
In this paper I present an overview of the main observational properties of a special class of exotic objects (the so-called Blue Straggler Stars, BSSs) in Galactic Globular Clusters (GCs). The BSS specific frequency and their radial distribution are discussed in the framework of using this stellar population as probe of GC internal dynamics. In particular, the shape of the BSS radial distribution has been found to be a powerful tracer of the dynamical evolution of stellar systems, thus allowing the definition of an empirical "clock" able to measure the dynamical age of stellar aggregates from…
I review the methods, mostly developed in the last decade, that are commonly used to identify and characterize multiple populations (MPs) in Globular Clusters based on photometry. I summarize the results from the recent surveys of MPs with the Hubble Space Telescope and ground-based facilities and provide a list of the main properties of MPs as inferred from these studies.
Multiple stellar populations are a widespread phenomenon among Galactic globular clusters. Even though the origin of the enriched material from which new generations of stars are produced remains unclear, it is likely that self-enrichment will be feasible only in clusters massive enough to retain this enriched material. We searched for multiple populations in the low mass (M~1.4 x 10^4 M_sun) globular cluster E 3, analyzing SOAR/Goodman multi-object spectroscopy centered on the blue CN absorption features of 23 red giant branch stars. We find that the CN abundance does not present the typical …
Evidence of Supernova Between Formation of Stellar Populations in a Globular Cluster
University of Notre Dame（圣母大学）；Northwestern University（西北大学）；Center for Interdisciplinary Exploration and Research in Astronomy, Northwestern University（西北大学跨学科天文探索与研究中心）；Fordham University（福特汉姆大学）；City University of New York（纽约市立大学）
We present an analysis of the multiple stellar populations of the globular cluster NGC 1851. We used lower resolution spectra of giant stars to measure CN, CH, and calcium H & K spectral indices, and determine elemental abundances for carbon and nitrogen. The CN and CH indices were used to confirm that there are four populations of stars in the cluster. The primordial population of stars, with the lowest CN, was found to be generally chemically distinct in elemental abundances from the second generation populations. As expected, [N/Fe] increases with increasing CN strength, but the only other …
We present early results from a detailed analysis of the BSS population in Galactic GCs based on HST data.Using proper motion cleaning of the color-magnitude diagrams we construct a large catalog of BSSs and study some population properties.Stellar evolutionary models are used to find stellar mass and age estimates for the BSS populations in order to establish constraints related to the dynamical interactions in which they may have formed.
Multi-band Hubble Space Telescope photometry reveals that the main sequence, sub-giant, and the red giant branch of the globular cluster NGC6752 splits into three main components in close analogy with the three distinct segments along its horizontal branch stars. These triple sequences are consistent with three stellar groups: a stellar population with a chemical composition similar to field halo stars (population a), a population (c) with enhanced sodium and nitrogen, depleted carbon and oxygen and enhanced helium abundance (Delta Y ~0.03), and a population (b) with an intermediate (between p…
We present Ca-CN-CH-NH photometry for the well-known globular cluster (GC) M3 (NGC 5272). We show new evidence for two M3 populations with distinctly different carbon and nitrogen abundances, seen in a sharp division between CN-weak and CN-strong red-giant branches (RGBs) in M3. The CN-strong population shows a C-N anticorrelation that is a natural consequence of the CN cycle, while the CN-weak population shows no or a weak C-N anticorrelation. Additionally, the CN-weak population exhibits an elongated spatial distribution that is likely linked to its fast rotation. Our derived metallicity rev…
We present an investigation of synthetic spectroscopic indices that can plausibly help in identifying the presence of multiple stellar populations in globular clusters. The study is based on a new grid of stellar model atmospheres and high-resolution (R=500,000) synthetic spectra, that consider chemical partitions that have been singled out in Galactic globular clusters. The database is composed of 3472 model atmospheres and theoretical spectra calculated with the collection of Fortran codes DFSYNTHE, ATLAS9 and SYNTHE, developed by Robert L. Kurucz. They cover a range of effective temperature…
In the multiple population framework, a number of studies have been accomplished in order to explore the behaviour of lithium with proton-capture element abundances (e.g., Na, O, Al) in globular cluster stars. Lithium offers perhaps one of the most severe constraints on the stellar source of internal pollution in these complex systems. Given its vulnerability, we expect that material processed via the hot H-burning, re-cycled in the formation of the subsequent generation(s), is free of Li. However, Nature breaks our expectations. In this contribution, we will review the current status of this …
We combine MUSE spectroscopy and Hubble Space Telescope ultraviolet (UV) photometry to perform a study of the chemistry and dynamics of the Galactic globular cluster Messier 80 (M80, NGC 6093). Previous studies have revealed three stellar populations that not only vary in their light-element abundances, but also in their radial distributions, with concentration decreasing with increasing nitrogen enrichment. This remarkable trend, which sets M80 apart from the other Galactic globular clusters, points towards a complex formation and evolutionary history. To better understand how M80 formed and …
We started a photometric survey using the WFC3/UVIS instrument onboard the Hubble Space Telescope to search for multiple populations within Magellanic Cloud star clusters at various ages. In this paper, we introduce this survey. As first results of this programme, we also present multi-band photometric observations of NGC 121 in different filters taken with the WFC3/UVIS and ACS/WFC instruments. We analyze the colour-magnitude diagram (CMD) of NGC 121, which is the only "classical" globular cluster within the Small Magellanic Cloud. Thereby, we use the pseudo-colour C_(F336W,F438W,F343N)=(F336…
The internal dynamics of multiple stellar populations in Globular Clusters (GCs) provides unique constraints on the physical processes responsible for their formation. Specifically, the present-day kinematics of cluster stars, such as rotation and velocity dispersion, seems to be related to the initial configuration of the system. In recent work (Milone et al. 2018), we analyzed for the first time the kinematics of the different stellar populations in NGC0104 (47Tucanae) over a large field of view, exploiting the Gaia Data Release 2 proper motions combined with multi-band ground-based photomet…
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 varyi…
Many possible scenarios for the formation of multiple stellar populations (MSP) in globular clusters (GCs) have been discussed so far, including the involvement of asymptotic giant branch stars, fast rotating main sequence stars, very massive main sequence stars and mass-transferring massive binaries based on stellar evolution modelling. But self-consistent, dynamical simulations of very young GCs are usually not considered. In this work, we perform direct $N$-body modelling such systems with total masses up to $3.2\times10^5$ M$_\odot$, taking into account the observationally constrained prim…
We present new large field-of-view ($\sim$1°$\times$1°) Ca-CN photometry of the prototypical metal-rich globular cluster 47 Tucanae (NGC 104). Our results are the following. (1) The populational number ratios of the red giant branch (RGB) and red horizontal branch (RHB) are in excellent agreement: n(CN-w):n(CN-s) = 30:70 ($\pm$1--2), where the CN-w and CN-s stand for the CN-weak and CN-strong populations, respectively. Both the CN-s RGB and RHB populations are more centrally concentrated than those of CN-w populations are. (2) Our photometric metallicities of individual RGB stars in each popul…
Globular clusters are among the most congested stellar systems in the Universe. Internal dynamical evolution drives them toward states of high central density, while simultaneously concentrating the most massive stars and binary systems in their cores. As a result, these clusters are expected to be sites of frequent close encounters and physical collisions between stars and binaries, making them efficient factories for the production of interesting and observable astrophysical exotica. I describe some elements of the competition among stellar dynamics, stellar evolution, and other processes th…
According to the classical view of globular clusters, stars inside globular clusters are evolved from the same giant molecular cloud. Then their stars' chemical compositions must be the same. But recent photometric and spectroscopic studies of globular clusters reveal the presence of more-than-one stellar populations inside globular clusters. This finding challenges our classical view of globular clusters. In this work, we investigated the possibility of solving multiple stellar populations problem in globular clusters using dark matter assumptions. We showed that the presence of dark matter i…
We investigate the early evolution of two distinct populations of low-mass stars in globular clusters under the influence of primordial gas expulsion driven by supernovae to study if this process can increase the fraction of second generation stars at the level required by observations. We analyse N-body models that take into account the effect of primordial gas expulsion. We divide the stars into two populations which mimic the chemical and dynamical properties of stars in globular clusters so that second generation stars start with a more centrally concentrated distribution. The main effect …
We report the discovery of a large timescale candidate microlensing event of a bulge stellar source based on near-infrared observations with the VISTA Variables in the Via Lactea Survey (VVV). The new microlensing event is projected only 3.5 arcmin away from the center of the globular cluster NGC 6553. The source appears to be a bulge giant star with magnitude Ks = 13.52, based on the position in the color-magnitude diagram. The foreground lens may be located in the globular cluster, which has well-known parameters such as distance and proper motions. If the lens is a cluster member, we can di…
Context. Globular clusters host multiple populations of long-lived low-mass stars whose origin remains an open question. Several scenarios have been proposed to explain the associated photometric and spectroscopic peculiarities. They differ, for instance, in the maximum helium enrichment they predict for stars of the second population, which these stars can inherit at birth as the result of the internal pollution of the cluster by different types of stars of the first population. Aims. We present the distribution of helium-rich stars in present-day globular clusters as it is expected in the or…
In the present work we analyzed seven globular clusters selected from their location in the Galactic bulge and with metallicity values in the range $-1.30\lesssim\rm{[Fe/H]}\lesssim-0.50$. The aim of this work is first to derive cluster ages assuming single stellar populations, and secondly, to identify the stars from first (1G) and second generations (2G) from the main sequence, subgiant and red giant branches, and to derive their age differences. Based on a combination of UV and optical filters used in this project, we apply the Gaussian mixture models to distinguish the multiple stellar pop…
I simulate the evolution of a stellar system hosting two stellar populations whose initial set up is defined according to the two main scenarios proposed for the origin of multiple populations in Galactic globular clusters: (i) formation of a second generation from a cooling flow of pristine+polluted gas and (ii) accretion of polluted gas onto the proto-stellar disks of a fraction of low-mass stars. For this purpose, Monte Carlo simulations containing from $10^{5}$ up to $3\cdot 10^{6}$ particles have been run including the effect of stellar evolution, binary interactions, external tidal field…
Our current understanding of the stellar initial mass function and massive star evolution suggests that young globular clusters may have formed hundreds to thousands of stellar-mass black holes, the remnants of stars with initial masses from $\sim 20 - 100\, M_\odot$. Birth kicks from supernova explosions may eject some black holes from their birth clusters, but most should be retained. Using a Monte Carlo method we investigate the long-term dynamical evolution of globular clusters containing large numbers of stellar black holes. We describe numerical results for 42 models, covering a range of…
We study the dynamical evolution of globular clusters containing primordial binaries, including full single and binary stellar evolution using our Monte Carlo cluster evolution code updated with an adaptation of the single and binary stellar evolution codes SSE/BSE from Hurley et. al (2000, 2002). We describe the modifications we have made to the code. We present several test calculations and comparisons with existing studies to illustrate the validity of the code. We show that our code finds very good agreement with direct N-body simulations including primordial binaries and stellar evolution…
As part of our massive spectroscopic survey of 25 Galactic globular clusters with MUSE, we performed multiple epoch observations of NGC 3201 with the aim of constraining the binary fraction. In this cluster, we found one curious star at the main-sequence turn-off with radial velocity variations of the order of 100 km/s, indicating the membership to a binary system with an unseen component since no other variations appear in the spectra. Using an adapted variant of the generalized Lomb-Scargle periodogram, we could calculate the orbital parameters and found the companion to be a detached stella…
We investigate the multiple stellar populations of the globular clusters M3, M5, M13, and M71 using $g^\prime$ and intermediate-band CN-$λ3883$ photometry obtained with the WIYN 0.9-m telescope on Kitt Peak. We find a strong correlation between red giant stars' CN$-g^\prime$ colors and their spectroscopic sodium abundances, thus demonstrating the efficacy of the two-filter system for stellar population studies. In all four clusters, the observed spread in red giant branch CN$-g^\prime$ colors is wider than that expected from photometric uncertainty, confirming the well-known chemical inhomogen…
An increasing number of both photometric and spectroscopic observations over the last years have shown the existence of distinct sub-populations in many Galactic globular clusters and shattered the paradigm of globulars hosting single, simple stellar populations. These multiple populations manifest themselves in a split of different evolutionary sequences in the cluster color-magnitude diagrams and in star-to-star abundance variations. In this paper we will summarize the observational scenario.
We investigate archival Hubble Space Telescope ACS/SBC F140LP observations of NGC~1399 to search for evidence of multiple stellar populations in extragalactic globular clusters. Enhanced FUV populations are thought to be indicators of He-enhanced second generation populations in globular clusters, specifically extreme/blue horizontal branch stars. Out of 149 globular clusters in the field of view, 58 have far ultraviolet (FUV) counterparts with magnitudes brighter than 28.5. Six of these FUV-deteced globular clusters are also detected in X-rays, including one ultraluminous X-ray source ($L_X >…
We briefly summarize the impact of the chemical peculiarities associated to the multiple population phenomenon in Galactic Globular Clusters, on the evolutionary properties and spectral energy distribution of second generation stars, in comparison with the primordial stellar component.
Although numerous photometric and spectroscopic studies of stars in Galactic globular clusters suggest the existence of several stellar generations in the majority of clusters studied so far, kinematical properties of these stellar generations are still relatively poorly know. In this contribution we present first results of our study of kinematical properties of chemically-tagged stellar populations in the Galactic globular cluster 47 Tuc. Our analysis reveals the existence of three stellar generations in 47 Tuc that are different in their [Li/Na] and [Na/O] ratios. Additionally, we find that…
We present a comparison of high-resolution, integrated-light, detailed chemical abundances for Galactic and extragalactic globular clusters in both massive galaxies and dwarf galaxies. We include measurements of Fe, Ca, Si, Na, and Al for globular cluster samples in the Milky Way, M31, Large Magellanic Cloud, and NGC 5128. These and other recent results from our group on M31 and NGC 5128 are the first chemical abundances derived from discrete absorption features in old stars beyond the Milky Way and its nearest neighbors. These abundances can provide both galaxy enrichment histories and constr…
Globular clusters have long been considered the closest approximation to a physicist's laboratory in astrophysics, and as such a near-ideal laboratory for (low-mass) stellar evolution. However, recent observations have cast a shadow on this long-standing paradigm, suggesting the presence of multiple populations with widely different abundance patterns, and -- crucially -- with widely different helium abundances as well. In this review we discuss which features of the Hertzsprung-Russel diagram may be used as helium abundance indicators, and present an overview of available constraints on the h…
The Age of the Universe with Globular Clusters IV: Multiple Stellar Populations
Berkeley Center for Cosmological Physics and Department of Physics, University of California, Berkeley（加州大学伯克利分校）；ICC, University of Barcelona（巴塞罗那大学）；ICREA（加泰罗尼亚高级研究学院）；Dipartimento di Fisica e Astronomia “Augusto Righi Università di Bologna（博洛尼亚大学）；INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna（意大利国家天体物理研究所波洛尼亚天文与空间科学观测站）；Lawrence Berkeley National Lab（劳伦斯伯克利国家实验室）
Hundreds of stellar-mass black holes likely form in a typical globular star cluster, with all but one predicted to be ejected through dynamical interactions. Some observational support for this idea is provided by the lack of X-ray-emitting binary stars comprising one black hole and one other star ("black-hole/X-ray binaries") in Milky Way globular clusters, even though many neutron-star/X-ray binaries are known. Although a few black holes have been seen in globular clusters around other galaxies, the masses of these cannot be determined, and some may be intermediate-mass black holes that form…
This chapter presents an overview of the main observational results obtained to date about Blue Straggler Stars (BSSs) in Galactic Globular Clusters (GCs). The BSS specific frequency, radial distribution, chemical composition and rotational properties are presented and discussed in the framework of using this stellar population as probe of GC internal dynamics. In particular, the shape of the BSS radial distribution has been found to be a powerful tracer of the dynamical age of stellar systems, thus allowing the definition of the first empirical "dynamical clock".
We develop a Bayesian model for globular clusters composed of multiple stellar populations, extending earlier statistical models for open clusters composed of simple (single) stellar populations (vanDyk et al. 2009, Stein et al. 2013). Specifically, we model globular clusters with two populations that differ in helium abundance. Our model assumes a hierarchical structuring of the parameters in which physical properties---age, metallicity, helium abundance, distance, absorption, and initial mass---are common to (i) the cluster as a whole or to (ii) individual populations within a cluster, or ar…
Observations revealed the presence of multiple stellar populations in globular clusters (GCs) that exhibit wide abundance variations and multiple sequences in 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 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 and are more massive than normal single stars in…
We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival ACS Treasury observations of Galactic Globular Clusters to find and characterize two stellar populations in NGC 5024 (M53), NGC 5272 (M3), and NGC 6352. For these three clusters, both single and double-population analyses are used to determine a best fit isochrone(s). We employ a sophisticated Bayesian analysis technique to simultaneously fit the cluster parameters (age, distance, absorption, and metallicity) that characterize each cluster. For the two-population analysis, unique population level helium values are also …
We investigate the relationship between the mass of a globular cluster core and the sizes of its various stellar populations in a sample of 56 globular clusters. The number of core red giant branch stars is found to scale sub-linearly with core mass at the 3-$σ$ confidence level, whereas the relation is linear to within one standard deviation for main-sequence turn-off and sub-giant branch stars. We interpret our results as evidence for a surplus of red giant branch stars in the least massive cluster cores which is not seen for main-sequence turn-off and sub-giant branch stars. We explore vari…
Globular clusters were thought to be simple stellar populations, but recent photometric and spectroscopic evidence suggests that the clusters' early formation history was more complicated. In particular, clusters show star-to-star abundance variations, and multiple sequences in their colour-magnitude diagrams. These effects seem to be restricted to globular clusters, and are not found in open clusters or the field. In this paper, we combine the two competing models for these multiple populations and include a consideration of the effects of stellar collisions. Collisions are one of the few phe…
The observed distribution of globular cluster binary radio pulsars in the eccentricity versus orbital period plane can be explained as a result of binary-single star interactions. Our numerical and analytical study hints that the highest eccentricity binaries in clusters are likely to be from exchange and/or merger of a single star with a binary component, while the intermediate eccentricity systems are probably results of fly-by interactions.
Our understanding of the formation and early evolution of globular clusters (GCs) has been totally overthrown with the discovery of the peculiar chemical properties of their long-lived host stars. As a consequence, the interpretation of the observed color-magnitude diagrams and of the properties of the GC stellar populations requires the use of stellar models computed with relevant chemical compositions. We present a grid of 224 stellar evolution for low-mass stars with initial masses between 0.3 and 1.0 Msun and initial helium mass fraction between 0.248 and 0.8 computed for [Fe/H]=-1.75 with…
(Abridged) Globular clusters host multiple stellar populations showing different sodium enrichments. These various populations can be observed along the main sequence, red giant and horizontal branch phases. Recently it was shown, however, that at least in the globular cluster NGC 6752, no sodium-rich stars are observed along the early asymptotic giant branch, posing an apparent problem for stellar evolution. We present an explanation for this lack of sodium-rich stars in this region of the colour-magnitude diagram.
We have computed low-mass stellar models and synthetic spectra for an initial chemical composition that includes the full C-N, O-Na, and Mg-Al abundance anticorrelations observed in second generation stars belonging to a number of massive Galactic globular clusters. This investigation extends a previous study that has addressed the effect of only the C-N and O-Na anticorrelations, seen in all globulars observed to date. We find that the impact of Mg-Al abundance variations at fixed [Fe/H] and Helium abundance is negligible on stellar models and isochrones (from the main sequence to the tip of …
Metallicity is the first parameter to influence the horizontal branch (HB) morphology of globular clusters (GCs). It has been found, however, that some other parameters may also play an important role in affecting the morphology. While the nature of these important parameters remains unclear, they are believed to be likely correlated with wind mass-loss of red giants, since this mass loss determines their subsequent locations on the HB. Unfortunately, the mass loss during the red giant stages of the stellar evolution is poorly understood at present. The stellar winds of red giants may be tidal…
