Exploring the dynamical evolution of binary stars in multiple-population globular clusters

Exploring the dynamical evolution of binary stars in multiple-population globular clusters

ABSTRACT

The presence of multiple stellar populations in globular clusters leads to a complex dynamical environment that significantly influences the evolution of binary stars, which in turn impacts the evolution of the cluster itself. For this study, we used a series of Monte Carlo simulations run with the MOCCA code to investigate the long-term dynamical evolution of binary stars in globular clusters hosting two distinct stellar populations. We explored how global binary properties such as incidence, fraction, and spatial distribution evolve over time due to the unique dynamical environment associated with each population. Our results show how binaries in the more centrally concentrated second population experience increased rates of hardening and disruption relative to the first population, leading to distinct radial profiles in binary incidence and fraction. We also demonstrate the difference in spatial mixing timescales for binaries compared to single stars, where binary stars in each population retain some memory of their initial configurations even after complete single star mixing. Additionally, we investigated the formation and evolution of mixed binaries, which form primarily within the core through dynamical interactions. Finally, we studied main sequence-white dwarf binaries and find that they represent a larger fraction of binaries in the first population compared to the second. The results of this paper highlight the interplay between cluster dynamics and the evolution of binary stars and how binaries can act as tracers of the cluster's initial conditions and dynamical evolution.

One. Introduction

Galactic globular clusters were traditionally considered to be simple stellar populations. However, spectroscopic and photometric studies have shown that the majority of globular clusters host at least two stellar populations, characterized by variations in their light element abundances. The origin and dynamical evolution of these multiple populations are not yet completely understood, and is an important open question in the field.

Several theoretical models have been proposed regarding the formation of multiple populations, most of which involve the second population forming from a combination of reaccreted pristine gas and a pollutant such as ejecta from asymptotic giant branch stars, supermassive stars, or massive binary stars. A common prediction of these models is that second population stars form in a more concentrated central subsystem within a more extended first population; this has been observed in numerous clusters.

The initial structural differences between the multiple populations has many important implications for the dynamical evolution of the cluster, particularly in the case of binary stars.

The increased stellar density of the second population results in shorter local relaxation timescales and increased encounter rates, affecting the properties and overall survival of binary systems. Previous studies have shown that these environmental differences can have significant effects on the survival of binaries and the evolution of their dynamical properties.

For this paper we investigated the long-term dynamical evolution of binary stars in multiple-population globular clusters using a series of Monte Carlo simulations run with the MOCCA code. We focused on how the distinct environments inhabited by binaries in each population affect the local and global binary fractions and incidences, the differing degrees of spatial mixing relative to the single stars of each population, and the formation and evolution of more exotic binaries. This paper builds on and extends previous studies by focusing on the evolution of the first and second population binary incidences, the formation and dynamics of mixed binaries, and main sequence-white dwarf binaries and their radial variation within a cluster.

The structure of the paper is as follows. Section Two details the simulation setup for this paper. In Section Three we report the results obtained through analyzing the binary fractions and incidences, spatial mixing of the populations, as well as the formation and evolution of mixed binaries and main sequence-white dwarf binaries. Finally, Section Four summarizes and discusses the main findings of the paper.