Massive Star Cluster Formation with Binaries. I. Evolution of Binary Populations

TL;DR

This study uses advanced simulations to analyze how binary star populations evolve during the formation of massive star clusters, revealing that binary fractions decrease and orbital properties change significantly, especially in denser environments.

Contribution

It introduces a comprehensive simulation framework coupling stellar dynamics, magnetohydrodynamics, and stellar evolution to study binary populations during cluster formation, highlighting the impact of hierarchical assembly.

Findings

Binary fraction decreases during cluster formation across all cloud masses.

Orbital properties of binaries change more rapidly in denser, more massive clouds.

Most binary disruptions involve wide binaries wider than 100 au.

Abstract

We study the evolution of populations of binary stars within massive cluster-forming regions. We simulate the formation of young massive star clusters within giant molecular clouds with masses ranging from 2 x 10$^{4}$ to 3.2 x 10$^{5}$ M$_{\odot}$. We use Torch, which couples stellar dynamics, magnetohydrodynamics, star and binary formation, stellar evolution, and stellar feedback through the AMUSE framework. We find that the binary fraction decreases during cluster formation at all molecular cloud masses. The binaries' orbital properties also change, with stronger and quicker changes in denser, more massive clouds. Most of the changes we see can be attributed to the disruption of binaries wider than 100 au, although the close binary fraction also decreases in the densest cluster-forming region. The binary fraction for O stars remains above 90%, but exchanges and dynamical hardening are ubiquitous, indicating that O stars undergo frequent few-body interactions early during the cluster formation process. Changes to the populations of binaries are a by-product of hierarchical cluster assembly: most changes to the binary population take place when the star formation rate is high and there are frequent mergers between sub-clusters in the cluster-forming region. A universal primordial binary distribution based on observed inner companions in the Galactic field is consistent with the binary populations of young clusters with resolved stellar populations, and the scatter between clusters of similar masses could be explained by differences in their formation history.