The possible role of stellar mergers for the formation of multiple stellar populations in globular clusters

TL;DR

This paper investigates how stellar mergers, driven by binary evolution and cluster dynamics, could explain the formation of multiple stellar populations in globular clusters, offering a new perspective on their origins.

Contribution

It introduces direct N-body simulations of young globular clusters that incorporate stellar mergers, highlighting their potential role in MSP formation and cluster evolution.

Findings

Stellar mergers are significantly enhanced in binary-rich clusters.

Merged stars can produce gas that leads to multiple stellar populations.

The scenario may explain the high MSP mass fraction and absence in young clusters.

Abstract

Many possible scenarios for the formation of multiple stellar populations (MSP) in globular clusters (GCs) have been discussed so far, including the involvement of asymptotic giant branch stars, fast rotating main sequence stars, very massive main sequence stars and mass-transferring massive binaries based on stellar evolution modelling. But self-consistent, dynamical simulations of very young GCs are usually not considered. In this work, we perform direct $N$-body modelling such systems with total masses up to $3.2\times10^5$ M$_\odot$, taking into account the observationally constrained primordial binary properties, and discuss the stellar-mergers driven both by binary stellar evolution and dynamical evolution of GCs. The occurrence of stellar mergers is enhanced significantly in binary-rich clusters such that stars forming from the gas polluted by mergers-driven ejection/winds would appears as MSPs. We thus emphasize that stellar mergers can be an important process that connects MSP formation with star cluster dynamics, and that multiple MSP formation channels can naturally work together. The scenario studied here, also in view of a possible top-heavy IMF, may be particularly relevant for explaining the high mass fraction of MSPs (the mass budget problem) and the absence of MSPs in young and low-mass star clusters.