Do binaries in clusters form in the same way as in the field?

TL;DR

This study investigates how binary star systems are affected by their cluster environment, revealing that dense clusters destroy wide binaries quickly, and that the field binary population results from a combination of clustered and isolated star formation.

Contribution

It provides a detailed analysis of binary destruction in clusters of varying densities and links binary properties to the dynamical history of star clusters.

Findings

High-density clusters destroy binaries with separations > 10^3 AU rapidly.

The Orion Nebula Cluster was denser in the past, inferred from its binary population.

Field binaries with separations < 10^2 AU are unaffected by cluster dynamics.

Abstract

We examine the dynamical destruction of binary systems in star clusters of different densities. We find that at high densities (10^4 - 10^5 Msun pc^-3) almost all binaries with separations > 10^3 AU are destroyed after a few crossing times. At low densities (order(10^2) Msun pc^-3) many binaries with separations > 10^3 AU are destroyed, and no binaries with separations > 10^4 AU survive after a few crossing times. Therefore the binary separations in clusters can be used as a tracer of the dynamical age and past density of a cluster. We argue that the central region of the Orion Nebula Cluster was around 100 times denser in the past with a half-mass radius of only 0.1 - 0.2 pc as (a) it is expanding, (b) it has very few binaries with separations > 10^3 AU, and (c) it is well-mixed and therefore dynamically old. We also examine the origin of the field binary population. Binaries with separations < 10^2 AU are not significantly modified in any cluster, therefore at these separations the field reflects the sum of all star formation. Binaries with separations in the range 10^2 - 10^4 AU are progressively more and more heavily affected by dynamical disruption in increasingly dense clusters. If most star formation is clustered, these binaries must be over-produced relative to the field. Finally, no binary with a separation > 10^4 AU can survive in any cluster and so must be produced by isolated star formation, but only if all isolated star formation produces extremely wide binaries.