Evolution of two stellar populations in globular clusters

II. Effects of primordial gas expulsion

¹ Argelander Institute for Astronomy (AIfA), Auf dem Hügel 71, 53121 Bonn, Germany e-mail: decressin@astro.uni-bonn.de
² Geneva Observatory, University of Geneva, 51 ch. des Maillettes, 1290 Versoix, Switzerland
³ Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS UMR 5572, Université de Toulouse, 14 Av. E. Belin, 31400 Toulouse, France

Received: 19 November 2009
Accepted: 23 March 2010

Abstract

Aims. We investigate the early evolution of two distinct populations of low-mass stars in globular clusters under the influence of primordial gas expulsion driven by supernovae and study whether this process can increase the fraction of second generation stars at the level required by observations.

Methods. We analyse N-body models that take the effect of primordial gas expulsion into account. We divide the stars into two populations that mimic the chemical and dynamical properties of stars in globular clusters so that second-generation stars start with a more centrally concentrated distribution.

Results. The main effect of gas expulsion is to eject mostly first-generation stars while second-generation stars remain bound to the cluster. In the most favourable cases, second-generation stars can account for 60% of the bound stars we see today. We also find that, at the end of the gas expulsion phase, the radial distribution of the two populations is still different, so that long-term evolution will further increase the fraction of second generation stars.

Conclusions. The large fraction of chemically anomalous stars is readily explainable as a second generation of stars formed out of the slow winds of rapidly rotating massive stars if globular clusters suffer explosive residual gas expulsion for a star formation efficiency of about 0.33.