In search of massive single-population Globular Clusters

TL;DR

This study classifies globular clusters based on their stellar populations, revealing that most are dominated by second-generation stars, with only a small fraction being first-generation only, highlighting the importance of second-generation formation for cluster survival.

Contribution

It introduces a classification scheme for globular clusters based on horizontal branch morphology to identify first-generation-only clusters and analyze their properties.

Findings

Less than 20% of clusters with [Fe/H]<-0.8 are first-generation.

Approximately 10% of clusters had enough mass to form a second generation.

Most clusters are dominated by second-generation stars, essential for their survival.

Abstract

Most Globular Clusters so far examined host (at least) two stellar populations. 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 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 initially too small to be able to retain a cooling flow and form a SG (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). We attempt a classification of FG clusters, based on the morphology of their horizontal branches (HBs), as displayed in photomectric catalogues for 106 clusters. FG candidates are the clusters in which the HB can be reproduced by the evolution of an almost unique mass. <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 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. Pal3 turns out to be a good example of FG-only cluster. HB simulations and space distribution of its components, indicate that M 53 is a "mainly-FG" cluster. Mainly-FG candidates may be also NGC5634, NGC5694 and NGC6101. 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.