How star clusters could survive low star formation efficiencies

Abstract

After the stars of a new, embedded star cluster have formed they blow the remaining gas out of the cluster. Especially winds of high mass stars and definitely the on-set of the first super novae can remove the residual gas from a cluster. This leads to a very violent mass-loss and leaves the cluster out of virial equilibrium. Standard models predict that the star formation efficiency (SFE) has to be about 33 per cent for sudden (within one crossing-time of the cluster) gas expulsion to retain some of the stars in a bound cluster. If the efficiency is lower the stars of the cluster disperse completely. Recent observations reveal that in strong star bursts star clusters do not form in isolation but in complexes containing dozens and up to several hundred star clusters (super-clusters). By carrying out numerical experiments we demonstrate that in these environments (i.e. the deeper potential of the star cluster complex and the merging process of the star clusters within these super-clusters) the SFEs could be as low as 20 per cent, leaving a gravitationally bound stellar population. We demonstrate that the merging of the first clusters happens faster than the dissolution time therefore enabling more stars to stay bound within the merger object. Such an object resembles the outer Milky Way globular clusters and the faint fuzzy star clusters recently discovered in NGC 1023.