Properties of stellar generations in globular clusters and relations with global parameters (original) (raw)
2010, Arxiv preprint arXiv: …
We revise the scenario of the formation of Galactic globular clusters (GCs) by adding the observed detailed chemical composition of their different stellar generations to the set of their global parameters. We exploit the unprecedented set of homogeneous abundances of more than 1200 red giants in 19 clusters, as well as additional data from literature, to give a new definition of bona fide GCs, as the stellar aggregates showing the Na-O anticorrelation. We propose a classification of GCs according to their kinematics and location in the Galaxy in three populations: disk/bulge, inner halo, and outer halo. We find that the luminosity function of GCs is fairly independent of their population, suggesting that it is imprinted by the formation mechanism only marginally affected by the ensuing evolution. We show that a large fraction of the primordial population should have been lost by the proto-GCs. The extremely low Al abundances found for the primordial population of massive GCs indicate a very fast enrichment process before the formation of the primordial population. We suggest a scenario for the formation of GCs that includes at least three main phases: i) the formation of a precursor population (likely due to the interaction of cosmological structures similar to those that led to the formation of dwarf spheroidals, but residing at smaller Galactocentric distances, with the early Galaxy or with other structures); ii) the triggering of a long episode of star formation (the primordial population) from the precursor population; and iii) the formation of the current GC, mainly within a cooling flow formed by the slow winds of a fraction of the primordial population. The precursor population is very effective in raising the metal content in massive and/or metal-poor (mainly halo) clusters, while its rôle is minor in small and/or metal-rich (mainly disk) ones. Finally, we use principal component analysis and multivariate relations to study the phase of metal enrichment from first to second generation. We conclude that most of the chemical signatures of GCs may be ascribed to a few parameters, the most important being metallicity, mass, and cluster age. Location within the Galaxy (as described by the kinematics) also plays some rôle, while additional parameters are required to describe their dynamical status.
Sign up for access to the world's latest research.
checkGet notified about relevant papers
checkSave papers to use in your research
checkJoin the discussion with peers
checkTrack your impact
Related papers
Chemical Evolution of Clusters of Galaxies
The high metallicity of the intra-cluster medium (ICM) is generally interpreted on the base of the galactic wind scenario for elliptical galaxies. In this framework, we develop a toy-model to follow the chemical evolution of the ICM, formulated in analogy to chemical models for individual galaxies. Just as the ingredients for usual models are (a) the stellar yields, amount of metals newly synthesized and re-ejected by stars; (b) the Star Formation Rate and (c) the stellar Initial Mass Function (IMF), our model for clusters involves: (a') "galactic yields" derived from galactic wind models of ellipticals; (b') a parametric Galactic Formation Rate; (c') a Press-Schechter-like Galactic Initial Mass Function. The model is used to test the response of the predicted metal content and abundance evolution of the ICM to varying input galactic models. The resulting luminosity function of cluster galaxies is also calculated, in order to constrain model parameters.
The ages of Galactic globular clusters in the context of self-enrichment
Proceedings of the International Astronomical Union, 2008
A significant fraction of stars in globular clusters (about 70%-85%) exhibit peculiar chemical patterns with strong abundance variations in light elements along with constant abundances in heavy elements. These abundance anomalies can be created in the H-burning core of a first generation of fast rotating massive stars and the corresponding elements are convoyed to the stellar surface thanks to rotational induced mixing. If the rotation of the stars is fast enough this matter is ejected at low velocity through a mechanical wind at the equator. It then pollutes the ISM from which a second generation of chemically anomalous stars can be formed. The proportion of anomalous to normal star observed today depends on at least two quantities : (1) the number of polluter stars; (2) the dynamical history of the cluster which may lose during its lifetime first and second generation stars in different proportions. Here we estimate these proportions based on dynamical models for globular clusters. When internal dynamical evolution and dissolution due to tidal forces are accounted for, starting from an initial fraction of anomalous stars of 10% produces a present day fraction of about 25%, still too small with respect to the observed 70-85%. In case gas expulsion by supernovae is accounted for, much higher fraction is expected to be produced. In this paper we also address the question of the evolution of the second generation stars that are He-rich, and deduce consequences for the age determination of globular clusters.
Metal abundances in extremely distant Galactic old open clusters. I. Berkeley 29 and Saurer 1
The Astronomical …, 2005
We report on high resolution spectroscopy of four giant stars in the Galactic old open clusters Berkeley 29 and Saurer 1 obtained with HIRES at the Keck telescope. These two clusters possess the largest galactocentric distances insofar known for open star clusters, and therefore are crucial objects to probe the chemical pattern and evolution of the outskirts of the Galactic disk. We find that [F e/H] = −0.38 ± 0.14 and [F e/H] = −0.44 ± 0.18 for Saurer 1 and Berkeley 29, respectively. Based on these data, we first revise the fundamental parameters of the clusters, and then discuss them in the context of the Galactic disk radial abundance gradients. Both clusters seem to significantly deviate from the general trend, suggesting that the outer part of the Galactic disk underwent a completely different evolution compared to the inner disk. In particular Berkeley 29 is clearly associated with the Monoceros stream, while Saurer 1 exhibits very different properties. The abundance ratios suggest that the chemical evolution of the outer disk was dominated by the Galactic halo.
Monthly Notices of the Royal Astronomical Society, 2012
Residual-gas expulsion after cluster formation has recently been shown to leave an imprint in the low-mass present-day stellar mass function (PDMF) which allowed the estimation of birth conditions of some Galactic globular clusters (GCs) such as mass, radius and star formation efficiency. We show that in order to explain their characteristics (masses, radii, metallicity, PDMF) their stellar initial mass function (IMF) must have been top-heavy. It is found that the IMF is required to become more top-heavy the lower the cluster metallicity and the larger the pre-GC cloudcore density are. The deduced trends are in qualitative agreement with theoretical expectation. The results are consistent with estimates of the shape of the high-mass end of the IMF in the Arches cluster, Westerlund 1, R136 and NGC 3603, as well as with the IMF independently constrained for ultra-compact dwarf galaxies (UCDs). The latter suggests that GCs and UCDs might have formed along the same channel or that UCDs formed via mergers of GCs. A fundamental plane is found which describes the variation of the IMF with density and metallicity of the pre-GC cloud-cores. The implications for the evolution of galaxies and chemical enrichment over cosmological times are expected to be major.
Monthly Notices of the Royal Astronomical Society, 2010
By comparing the outcome of N -body calculations that include primordial residualgas expulsion with the observed properties of 20 Galactic globular clusters (GCs) for which the stellar mass function (MF) has been measured, we constrain the time-scale over which the gas of their embedded cluster counterparts must have been removed, the star formation efficiency the progenitor cloud must have had and the strength of the tidal-field the clusters must have formed in. The three parameters determine the expansion and mass-loss during residual-gas expulsion. After applying corrections for stellar and dynamical evolution we find birth cluster masses, sizes and densities for the GC sample and the same quantities for the progenitor gas clouds. The pre-cluster cloud core masses were between 10 5 − 10 7 M ⊙ and half-mass radii were typically below 1 pc and reach down to 0.2 pc. We show that the low-mass present day MF (PDMF) slope, initial half-mass radius and initial density of clusters correlates with cluster metallicity, unmasking metallicity as an important parameter driving cluster formation and the gas expulsion process. This work predicts that PD low-concentration clusters should have a higher binary fraction than PD high-concentration clusters.
The Chemical Evolution of Globular Clusters I. Reactive Elements and NonMetals
Monthly Notices of The Royal Astronomical Society, 2009
We propose a new chemical evolution model aimed at explaining the chemical properties of globular clusters (GC) stars. Our model depends upon the existence of (i) a peculiar pre-enrichment phase in the GC's parent galaxy associated with very lowmetallicity Type II supernovae (SNe II), and (ii) localized inhomogeneous enrichment from a single Type Ia supernova (SNe Ia) and intermediate-mass (4-7 M ⊙ ) asymptotic giant branch (AGB) field stars. GC formation is then assumed to take place within this chemically-peculiar region. Thus, in our model the first low-mass GC stars to form are those with peculiar abundances (i.e., O-depleted and Na-enhanced) while "normal" stars (i.e., O-rich and Na-depleted) are formed in a second stage when self-pollution from SNe II occurs and the peculiar pollution from the previous phase is dispersed. In this study, we focus on three different GCs: NGCs 6752, NGC 6205 (M 13) and NGC 2808. We demonstrate that, within this framework, a model can be constructed which is consistent with (i) the elemental abundance anti-correlations, (ii) isotopic abundance patterns, and the extreme [O/Fe] values observed in NGC 2808 and M 13, without violating the global constraints of approximately unimodal [Fe/H] and C+N+O.
Arxiv preprint arXiv: …, 2010
We investigate the origin of the shape of the globular cluster (GC) system scaling parameters as a function of galaxy mass, i.e. specific frequency (S N ), specific luminosity (S L ), specific mass (S M ), and specific number (T ) of GCs. In the low-mass galaxy regime (M V −16 mag) our analysis is based on HST/ACS observations of GC populations of faint, mainly late-type dwarf galaxies in low-density environments. In order to sample the entire range in galaxy mass (M V = −11 to −23 mag = 10 6 −10 11 L ), environment, and morphology we augment our sample with data of spiral and elliptical galaxies from the literature, in which old GCs are reliably detected. This large dataset confirms (irrespective of galaxy type) the increase of the specific frequencies of GCs above and below a galaxy magnitude of M V −20 mag. Over the full mass range, the S L −value of early-type galaxies is, on average, twice that of late-types. To investigate the observed trends we derive theoretical predictions of GC system scaling parameters as a function of host galaxy mass based on the models of in which star-formation processes are regulated by stellar and supernova feedback below a stellar mass of 3×10 10 M , and by virial shocks above it. We find that the analytical model describes remarkably well the shape of the GC system scaling parameter distributions with a universal specific GC formation efficiency, η, which relates the total mass in GCs to the total galaxy halo mass. Early-type and late-type galaxies show a similar mean value of η = 5.5 × 10 −5 , with an increasing scatter towards lower galaxy masses. This can be due to the enhanced stochastic nature of the star and star-cluster formation processes for such systems. Some massive galaxies have excess η values compared to what is expected from the mean model prediction for galaxies more luminous than M V −20 mag (L V 10 10 L ). This may be attributed to a very efficient early GC formation, less efficient production of field stars or accretion of predominantly low-mass/luminosity high−η galaxies, or a mixture of all these effects. with similar chemical composition. As fossil records of the first star formation episodes of their host galaxies, the distribution of their integrated properties (age, mass, metallicity, structural parameters) as well as the general properties of all GCs in a galaxy (total numbers, spatial and dynamical distributions) hold important clues to the initial physical conditions at which they have formed and evolved. For that reason, the global properties of globular cluster systems (GCSs) have long been recognized as promising tools to study the major galaxy star formation episodes and to serve as observational constraints to differentiate between various models of galaxy formation (Kissler-Patig 2000; van arXiv:1004.2039v1 [astro-ph.CO]
Loading Preview
Sorry, preview is currently unavailable. You can download the paper by clicking the button above.