New insights on the origin of multiple stellar populations in globular clusters

TL;DR

This study presents chemical evolution models for globular clusters that explain multiple stellar populations and their chemical signatures, addressing the mass budget problem and the origin of helium-rich stars in the Milky Way bulge.

Contribution

It introduces a novel model assuming supernova blast wave blowout and local retention of stellar winds, successfully reproducing observed chemical patterns without ad-hoc assumptions.

Findings

Reproduces Na-O anti-correlation in metal-poor GCs.

Alleviates the mass budget problem without arbitrary parameters.

Explains helium-rich stars in the bulge through chemical enrichment.

Abstract

In order to investigate the origin of multiple stellar populations in the halo and bulge of the Milky Way, we have constructed chemical evolution models for the low-mass proto-Galactic subsystems such as globular clusters (GCs). Unlike previous studies, we assume that supernova blast waves undergo blowout without expelling the pre-enriched gas, while relatively slow winds of massive stars (WMS), together with the winds and ejecta from low and intermediate mass asymptotic-giant-branch stars (AGBs), are all locally retained in these less massive systems. We find that the observed Na-O anti-correlation in metal-poor GCs can be reproduced, when multiple episodes of starbursts are allowed to continue in these subsystems. Specific star formation history (SFH) with decreasing time intervals between the stellar generations, however, is required to obtain this result, which is in good agreement with the parameters obtained from our stellar evolution models for the horizontal-branch. The "mass budget problem" is also much alleviated by our models without ad-hoc assumptions on star formation efficiency (SFE) and initial mass function (IMF). We also applied these models to investigate the origin of super-helium-rich red clump stars in the metal-rich bulge as recently suggested by Lee et al. (2015). We find that chemical enrichments by the WMS can naturally reproduce the required helium enhancement (dY/dZ = 6) for the second generation stars. Disruption of proto-GCs in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field.