The evolution of binary populations in cool, clumpy star clusters

TL;DR

This study uses N-body simulations to examine how initial substructure and cool conditions in star clusters influence the evolution and processing of binary star systems, aligning with observations of the Orion Nebula Cluster.

Contribution

It demonstrates that initial substructure significantly affects binary processing and that primordial binary fractions may need to be near 100% to match observations.

Findings

Primordial binary populations can match observed binary fractions and separations.

Highly substructured clusters process more binaries due to dense pockets.

Binary processing occurs before the cluster reaches maximum density.

Abstract

Observations and theory suggest that star clusters can form in a subvirial (cool) state and are highly substructured. Such initial conditions have been proposed to explain the level of mass segregation in clusters through dynamics, and have also been successful in explaining the origin of trapezium-like systems. In this paper we investigate, using N-body simulations, whether such a dynamical scenario is consistent with the observed binary properties in the Orion Nebula Cluster (ONC). We find that several different primordial binary populations are consistent with the overall fraction and separation distribution of visual binaries in the ONC (in the range 67 - 670 au), and that these binary systems are heavily processed. The substructured, cool-collapse scenario requires a primordial binary fraction approaching 100 per cent. We find that the most important factor in processing the primordial binaries is the initial level of substructure; a highly substructured cluster processes up to 20 per cent more systems than a less substructured cluster because of localised pockets of high stellar density in the substructure. Binaries are processed in the substructure before the cluster reaches its densest phase, suggesting that even clusters remaining in virial equilibrium or undergoing supervirial expansion would dynamically alter their primordial binary population. Therefore even some expanding associations may not preserve their primordial binary population.