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Multiple Stellar Populations in Globular Clusters: Collection of Information from the Horizontal Branch

Published online by Cambridge University Press:  01 September 2007

Francesca D'Antona  and
Vittoria Caloi
Francesca D'Antona
Affiliation:
INAF - Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monte Porzio, Italy email: [email protected]
Vittoria Caloi
Affiliation:
INAF - IASF, via Fosso del Cavaliere, I-00133 Roma, Italy email: [email protected]
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Abstract

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The majority of the inhomogeneities in the chemical composition of Globular Cluster (GC) stars appear due to primordial enrichment by hot-CNO cycled material processed in stars belonging to a first stellar generation. Either massive AGB envelopes subject to hot bottom burning, or the envelopes of massive fastly rotating stars could be the progenitors. In both cases, the stars showing chemical anomalies must have also enhanced helium abundance, and we have proposed that this higher helium could be at the basis of the many different morphologies of GC horizontal branches (HB) for similar ages and metallicities. The helium variations have been beautifully confirmed by the splitting of the main sequence in the clusters ω Cen and NGC 2808, but this effect can show up only for somewhat extreme helium abundances. Therefore it is important to go on using the HB morphology to infer the number ratio of the first to the second generation in as many clusters as possible. We exemplify how it is possible to infer the presence of a He – rich stellar component in different clusters thanks to different HB features (gaps, RR Lyr periods and period distribution, ratio of blue to red stars, blue tails). In many clusters at least 50% of the stars belong to the second stellar generation, and in some cases we suspect that the stars might all belong to the second generation. We shortly examine the problem of the initial mass function required to achieve the observed number ratios and conclude that: 1) the initial cluster must have been much more massive than today's cluster, and 2) formation of the second stellar generation mainly in the central regions of the cluster may help in obtaining the desired values.

Keywords

globular clusters: general – globular clusters: formation – globular clusters: individual NGC 2808NGC 6388NGC 6441M 3 – Stars: Horizontal Branch
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 3 , Symposium S246: Dynamical Evolution of Dense Stellar Systems , September 2007 , pp. 156 - 160
Copyright
Copyright © International Astronomical Union 2008

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