Open cluster stability and the effects of binary stars

TL;DR

This study examines the stability of Galactic open clusters using the age versus mass-to-light ratio diagram, highlighting the impact of binary stars and IMF stochasticity on cluster longevity and observational diagnostics.

Contribution

It provides constraints on binary fractions in open clusters and discusses how IMF stochasticity affects age and stability assessments.

Findings

Binary fractions influence cluster stability diagnostics.

IMF stochasticity introduces significant uncertainties.

Location near evolutionary sequences relates to cluster survival.

Abstract

The diagnostic age versus mass-to-light ratio diagram is often used in attempts to constrain the shape of the stellar initial mass function (IMF), and the potential longevity of extragalactic young to intermediate-age massive star clusters. Here, we explore its potential for Galactic open clusters. On the basis of a small, homogenised cluster sample we provide useful constraints on the presence of significant binary fractions. Using the massive young Galactic cluster Westerlund 1 as a key example, we caution that stochasticity in the IMF introduces significant additional uncertainties. We conclude that for an open cluster to survive for any significant length of time, and in the absence of substantial external perturbations, it is a necessary but not a sufficient condition to be located close to or (in the presence of a significant binary population) somewhat BELOW the predicted photometric evolutionary sequences for "normal" simple stellar populations (although such a location may be dominated by a remaining "bound" cluster core and thus not adequately reflect the overall cluster dynamics).