Binary Disruption and Ejected Stars from Hierarchical Star Cluster Assembly

TL;DR

This study uses simulations to explore how star cluster mergers affect binary star systems and the distribution of unbound stars, highlighting the importance of primordial binaries and gas environment in these processes.

Contribution

It demonstrates that realistic gas environments and primordial binaries significantly influence binary disruption and unbound star distribution during cluster mergers.

Findings

Simulations without primordial binaries produce more unbound stars.

Gas-rich sub-cluster mergers disrupt binaries more effectively.

Unbound stars tend to move in preferred directions.

Abstract

We simulate mergers between star clusters embedded within their natal giant molecular cloud. We extract initial conditions from cloud-scale simulations of cluster formation and introduce different prescriptions for primordial binaries. We find that simulations that do not include primordial binaries result in a larger fraction of unbound stars than simulations which include a prescription for binaries based on observations. We also find a preferred direction of motion for stars that become unbound during the merger. Sub-cluster mergers within realistic gas environments promote binary disruption while mergers between idealized, gas-rich spherical clusters do not produce the same disruption. Binary systems with smaller semi-major axes are disrupted in simulations of sub-cluster mergers within their natal environment compared to simulations that do not include the realistic gas environment. We conclude that binary disruption and the production of an anisotropic distribution of unbound stars are the natural consequences of sub-cluster mergers during star cluster assembly.