The effect of binaries on the dynamical mass determination of star clusters

¹ Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK e-mail: [t.kouwenhoven;r.degrijs@sheffield.ac.uk]
² National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China

Received: 22 October 2007
Accepted: 20 December 2007

Abstract

The total mass of distant star clusters is often derived from the virial theorem, using line-of-sight velocity dispersion measurements and half-light radii. Although most stars form in binary systems, this is mostly ignored when interpreting the observations. The components of binary stars exhibit orbital motion, which may increase the measured velocity dispersion, and may therefore result in a dynamical mass overestimation. In this paper we quantify the effect of neglecting the binary population on the derivation of the dynamical mass of a star cluster. We simulate star clusters numerically, and study the dependence of the derived dynamical mass on the properties of the binary population. We find that the presence of binaries plays a crucial role for very sparse clusters with a stellar density comparable to that of the field star population (~0.1 stars pc^-3), as the velocity dispersion is fully dominated by the binary orbital motion. For such clusters, the dynamical mass may overestimate the true mass by up to an order of magnitude. For very dense clusters (10⁷ stars pc^-3), binaries do not affect the dynamical mass estimation significantly. For clusters of intermediate density (0.1–10⁷ stars pc^-3), the dynamical mass can be overestimated by 10–100%, depending on the properties of the binary population.