Linking globular cluster structural parameters and their evolution: multiple stellar populations

TL;DR

This study investigates how the structural properties and evolution of globular clusters influence the fraction of second stellar populations, highlighting the role of cluster mass, escape velocity, and tidal stripping in shaping multiple populations.

Contribution

It reveals that cluster mass and escape velocity are key factors in the formation and evolution of multiple stellar populations within globular clusters, supported by new observational correlations.

Findings

Higher cluster mass correlates with increased 2P star fraction.

Clusters with higher escape velocity tend to have higher 2P fractions.

Correlations differ between low and high mass clusters, indicating bimodal behavior.

Abstract

Globular clusters (GCs) are known to host multiple stellar populations showing chemical anomalies in the content of light elements. The origin of such anomalies observed in Galactic GCs is still debated. Here we analyse data compiled from the Hubble Space Telescope, ground-based surveys and Gaia DR2 and explore relationships between the structural properties of GCs and the fraction of second population (2P) stars. Given the correlations we find, we conclude that the main factor driving the formation/evolution of 2P stars is the cluster mass. The existing strong correlations between the 2P fraction and the rotational velocity and concentration parameter could derive from their correlation with the cluster mass. Furthermore, we observe that increasing cluster escape velocity corresponds to higher 2P fractions. Each of the correlations found is bimodal, with a different behaviour detected for low and high mass (or escape velocity) clusters. These correlations could be consistent with an initial formation of more centrally concentrated 2P stars in deeper cluster potentials, followed by a long-term tidal stripping of stars from clusters outskirts. The latter are dominated by the more extended distributed first population (1P) stars, and therefore stronger tidal stripping would preferentially deplete the 1P population, raising the cluster 2P fraction. This also suggests a tighter distribution of initial 2P fractions than observed today. In addition, higher escape velocities allow better retention of low-velocity material ejected from 1P stars, providing an alternative/additional origin for the observed differences and the distributions of 2P fractions amongst GCs.