The ages of Galactic globular clusters in the context of self-enrichment

TL;DR

This paper investigates the formation and evolution of multiple stellar populations in globular clusters, focusing on chemical anomalies, dynamical processes, and implications for age estimates, using dynamical models to explain observed star proportions.

Contribution

It introduces a dynamical model accounting for star loss and gas expulsion to explain the observed fractions of anomalous stars in globular clusters.

Findings

Initial anomalous star fraction of 10% can evolve to 25% due to dynamical effects.

Gas expulsion mechanisms can produce higher fractions of anomalous stars.

He-rich second generation stars influence globular cluster age determinations.

Abstract

A significant fraction of stars in globular clusters (about 70%-85%) exhibit peculiar chemical patterns with strong abundance variations in light elements along with constant abundances in heavy elements. These abundance anomalies can be created in the H-burning core of a first generation of fast rotating massive stars and the corresponding elements are convoyed to the stellar surface thanks to rotational induced mixing. If the rotation of the stars is fast enough this matter is ejected at low velocity through a mechanical wind at the equator. It then pollutes the ISM from which a second generation of chemically anomalous stars can be formed. The proportion of anomalous to normal star observed today depends on at least two quantities : (1) the number of polluter stars; (2) the dynamical history of the cluster which may lose during its lifetime first and second generation stars in different proportions. Here we estimate these proportions based on dynamical models for globular clusters. When internal dynamical evolution and dissolution due to tidal forces are accounted for, starting from an initial fraction of anomalous stars of 10% produces a present day fraction of about 25%, still too small with respect to the observed 70-85%. In case gas expulsion by supernovae is accounted for, much higher fraction is expected to be produced. In this paper we also address the question of the evolution of the second generation stars that are He-rich, and deduce consequences for the age determination of globular clusters.