An alternative method to study star cluster disruption

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: mgieles@eso.org
² Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK e-mail: bastian@star.ucl.ac.uk

Received: 23 October 2007
Accepted: 16 February 2008

Abstract

Many embedded star clusters do not evolve into long-lived bound clusters. The most popular explanation for this “infant mortality” of young (few Myrs) clusters is the expulsion of natal gas by stellar winds and supernovae, which perturbs the clusters' potential and leaves up to 90% of them unbound. A cluster disruption model has recently been proposed in which this mass-independent disruption of clusters proceeds for another Gyr after gas expulsion. In this scenario, the survival chances of massive clusters are much smaller than in the traditional mass-dependent disruption models. The most common way to study cluster disruption is to use the cluster age distribution, which, however, can be heavily affected by incompleteness. To avoid this pitfall we introduce a new method of studying cluster disruption based on size-of-sample effects, namely the relation between the most massive cluster, M_max, and the age range sampled. Assuming that clusters are stochastically sampled from a power-law cluster initial mass function, with index –2 and that the cluster formation rate is constant, M_max scales with the age range sampled, such that the slope in a log vs. log(age) plot is equal to unity. This slope decreases if mass-independent disruption is included. For 90% mass-independent cluster disruption per age dex, the predicted slope is zero. For the solar neighbourhood, SMC, LMC, M 33, and M 83, based on ages and masses taken from the literature, we find slopes consistent with the expected size-of-sample correlations for the first 100 Myr, hence ruling out the 90% mass-independent cluster disruption scenario. For M 51, however, the increase of log with log(age) is slightly shallower and for the Antennae galaxies it is flat. This simple method shows that the formation and/or disruption of clusters in the Antennae must have been very different from that of the other galaxies studied here, so it should not be taken as a representative case.