Open cluster stability and the effects of binary stars

¹ Department of Physics & Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK e-mail: [R.deGrijs;S.Goodwin;T.Kouwenhoven]@sheffield.ac.uk
² National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China
³ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53347 Bonn, Germany e-mail: pavel@astro.uni-bonn.de

Received: 23 May 2008
Accepted: 19 September 2008

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 necessary but not sufficient 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.