Binaries and the dynamical mass of star clusters

TL;DR

This paper investigates how binary stars influence the estimation of star cluster masses, revealing significant overestimations in less massive clusters due to binary orbital motions.

Contribution

It quantifies the impact of binary stars on dynamical mass measurements, highlighting when these effects are significant based on cluster mass and size.

Findings

Binaries cause overestimation of mass by a factor of two in clusters with M < 10^5 Msun.

In clusters with M > 10^5 Msun, binaries have negligible impact if the cluster is compact.

The effect depends on cluster mass and size, with larger effects in less massive, more diffuse clusters.

Abstract

The total mass of a distant star cluster is often derived from the virial theorem, using line-of-sight velocity dispersion measurements and half-light radii, under the implicit assumption that all stars are single (although it is known that most stars form part of binary systems). The components of binary stars exhibit orbital motion, which increases the measured velocity dispersion, resulting in a dynamical mass overestimation. In this article we quantify the effect of neglecting the binary population on the derivation of the dynamical mass of a star cluster. We find that the presence of binaries plays an important role for clusters with total mass M < 10^5 Msun; the dynamical mass can be significantly overestimated (by a factor of two or more). For the more massive clusters, with Mcl > 10^5 Msun, binaries do not affect the dynamical mass estimation significantly, provided that the cluster is significantly compact (half-mass radius < 5 pc).