SCYON Abstract

Received on June 02 2011

In search of massive single--population Globular Clusters

Authors	V. Caloi (¹) and F. D'Antona (²)
Affiliation	(¹) INAF, IASF-Roma, via Fosso del Cavaliere 100, I-00133 Roma, Italy (²) INAF, Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monteporzio Catone (Roma), Italy
Accepted by	Monthly Notices of the Royal Astronomical Society
Contact	dantona@oa-roma.inaf.it
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Abstract

The vast majority of globular clusters so far examined shows the chemical signatures of hosting (at least) two stellar populations. According to recent ideas, this feature requires a two-step process, in which the nuclearly processed matter from a "first generation" (FG) of stars gives birth to a "second generation" (SG) bearing the fingerprint of a fully CNO-cycled matter. Since, as observed, the present population of most globular clusters is made up largely of SG stars, a substantial fraction of the FG (>≈ 90%) must be lost. Nevertheless, two types of clusters dominated by a simple stellar population (FG clusters) should exist: either clusters initally too small to be able to retain a cooling flow and form a second generation (FG-only clusters), or massive clusters that could retain the CNO processed ejecta and form a SG, but were unable to lose a significant fraction of their FG (mainly-FG clusters). Identification of mainly-FG clusters may provide an estimate of the fraction of the initial mass involved in the formation of the SG.
We attempt a first classification of FG clusters, based on the morphology of their horizontal branches (HBs), as displayed in the published catalogues of photometric data for 106 clusters. We select, as FG candidates, the clusters in which the HB can be reproduced by the evolution of an almost unique mass. We find that less than 20% of clusters with [Fe/H]<-0.8 appear to be FG, but only ~10% probably had a mass sufficient to form at all an SG. This small percentage confirms on a wider database the spectroscopic result that the SG is a dominant constituent of today's clusters, suggesting that its formation is an ingredient necessary for the survival of globular clusters during their dynamical evolution in the Galactic tidal field.
In more detail we show that Pal 3 turns out to be a good example of FG-only cluster. Instead, HB simulations and space distribution of its components, indicate that M 53 is a "mainly-FG" cluster, that evolved in dynamical isolation and developed a small SG in its core thanks to its large mass. Mainly-FG candidates may be also NGC 5634, NGC 5694 and NGC 6101. In contrast, NGC 2419 contains >30% of SG stars, and its present dynamical status bears less information on its formation process than the analysis of the chemical abundances of its stars and of its HB morphology.