Evolution of two stellar populations in globular clusters II. Effects of primordial gas expulsion

TL;DR

This study uses N-body simulations to show that primordial gas expulsion can preferentially eject first-generation stars from globular clusters, increasing the relative number of second-generation stars to observed levels.

Contribution

It demonstrates through models that gas expulsion can significantly enhance the second-generation star fraction in globular clusters, aligning with observational data.

Findings

Second generation stars can comprise up to 60% of bound stars after gas expulsion.

Gas expulsion preferentially ejects first-generation stars, altering population ratios.

Radial distributions of populations remain distinct after gas expulsion, influencing long-term evolution.

Abstract

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 to study if this process can increase the fraction of second generation stars at the level required by observations. We analyse N-body models that take into account the effect of primordial gas expulsion. We divide the stars into two populations which mimic the chemical and dynamical properties of stars in globular clusters so that second generation stars start with a more centrally concentrated distribution. The main effect of gas expulsion is to eject preferentially 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. 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.