Double white dwarf binary population in MOCCA star clusters -- Comparisons with observations of close and wide binaries

TL;DR

This study uses MOCCA star cluster simulations to analyze double white dwarf binaries, comparing their properties with observations, and highlights the significant role of dynamics in their formation within globular clusters.

Contribution

The paper introduces a large statistical dataset of DWDs from MOCCA models including multiple stellar populations, emphasizing the impact of dynamics on DWD formation and evolution.

Findings

Escaping DWDs outnumber in-cluster binaries

Good agreement in separations of escaped and isolated binaries

Limitations in reproducing extremely low mass WDs

Abstract

There could be a significant population of double white dwarf binaries (DWDs) inside globular clusters (GCs), however, these are often too faint to be individually observed. We have utilized a large number GC models evolved with the Monte Carlo Cluster Simulator (MOCCA) code, to create a large statistical dataset of DWDs. These models include multiple-stellar populations, resulting in two distinct initial populations: one dense and another less dense. Due to the lower density of one population, a large number of objects escape during the early GC evolution, leading to a high mass-loss rate. In this dataset we have analysed three main groups of DWDs, namely in-cluster binaries, escaped binaries, and isolated evolution of primordial binaries. We compared the properties of these groups to observations of close and wide binaries. We find that the number of escaping DWDs is significantly larger than the number of in-cluster binaries and those that form via the isolated evolution of all promiridial binaries in our GC models. This suggests that dynamics play an important role in the formation of DWDs. For close binaries, we found a good agreement in the separations of escaped binaries and isolated binaries, but in-cluster binaries showed slight differences. We could not reproduce the observed extremely low mass WDs due to the limitations of our stellar and binary evolution prescriptions. For wide binaries, we also found a good agreement in the separations and masses, after accounting for observational selection effects. We conclude that, even though the current observational samples of DWDs are extremely biased and incomplete, our results compare reasonably well with observations.