How do binaries affect the derived dynamical mass of a star cluster?

TL;DR

This paper investigates how binary stars influence the calculation of a star cluster's dynamical mass, revealing that binaries can cause significant overestimations in low-density clusters but not in high-density ones.

Contribution

It provides a detailed analysis of the impact of binary orbital motion on dynamical mass estimates, highlighting the importance of considering binaries in low-velocity dispersion clusters.

Findings

Binaries significantly affect dynamical mass estimates in clusters with sigma < 1 km/s.

Binaries have negligible impact on clusters with sigma > 10 km/s.

Binaries can cause a downward shift in the log(L/Mdyn) vs. age relation.

Abstract

The dynamical mass of a star cluster can be derived from the virial theorem, using the measured half-mass radius and line-of-sight velocity dispersion of the cluster. However, this dynamical mass may be a significant overestimation of the cluster mass if the contribution of the binary orbital motion is not taken into account. In these proceedings we describe the mass overestimation as a function of cluster properties and binary population properties, and briefly touch the issue of selection effects. We find that for clusters with a measured velocity dispersion of sigma > 10 km/s the presence of binaries does not affect the dynamical mass significantly. For clusters with sigma < 1 km/s (i.e., low-density clusters), the contribution of binaries to sigma is significant, and may result in a major dynamical mass overestimation. The presence of binaries may introduce a downward shift of Delta log(L/Mdyn) = 0.05-0.4 in the log(L/Mdyn) vs. age diagram.