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Globular clusters, often regarded as the most boring denizens of our galaxy, have just become a lot more interesting. Rogue, hot stars have been discovered inside them.
The tens of thousands of stars that make up each globular cluster were all thought to have formed at the same time, with exactly the same composition, from a single spherical cloud of gas. Then, two years ago, a hint of a rogue population of hot, helium-rich stars was detected in the largest known globular cluster, Omega Centauri. This cluster, however, is known to be exceptional.
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Title: The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario
Authors: A. I. Karakas, Y. Fenner, Alison Sills, S. W. Campbell and J. C. Lattanzio
We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y ~ 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.
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Title: Helium enhancements in globular Cluster stars from Asymptotic Giant Branch star pollution
Authors: A. Karakas1, Y. Fenner, Alison Sills, S. W. Campbell and J. C. Lattanzio
Using a chemical evolution model we investigate the intriguing suggestion that there are populations of stars in some globular clusters (e.g. NGC 2808, Omega Centauri) with enhanced levels of helium (Y ~ 0.28 to 0.40) compared to the majority of the population that presumably have a primordial helium abundance. We assume that a previous generation of massive low-metallicity Asymptotic Giant Branch (AGB) stars has polluted the cluster gas via a slow stellar wind. We use two independent sets of AGB yields computed from detailed models to follow the evolution of helium, carbon, nitrogen and oxygen in the cluster gas using a Salpeter initial mass function (IMF) and a number of top-heavy IMFs. In no case were we able to fit the observational constraints, Y > 0.30 and C+N+O ≈ constant. Depending on the shape of the IMF and the yields, we either obtained Y & 0.30 and large increases in C+N+O or Y < 0.30 and C+N+O ≈ constant. These results suggest that either AGB stars alone are not responsible for the large helium enrichment or that any dredge-up from this generation of stars was less than predicted by standard models.
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06-September-2006, 12:19 PM
Helium enhancements in globular Cluster stars
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