Gas expulsion and the destruction of massive young clusters

Abstract

We examine the luminosity and dynamical mass estimates for young massive stellar clusters. For many young (<50 Myr) clusters, the luminosity and dynamical mass estimates differ by a significant amount. We explain this as being due to many young clusters being out of virial equilibrium (which is assumed in dynamical mass estimates) because the clusters are undergoing violent relaxation after expelling gas not used in star formation. We show that, if we assume that luminous mass estimates are correct (for a standard IMF), at least 50 per cent of young clusters for which dynamical masses are known are likely to be destroyed within a few 10s Myr of their formation. Even clusters which will retain a bound core may lose a large fraction of their stellar mass. We also show that the core radius and other structural parameters change significantly during the violent relaxation that follows gas expulsion and that they should be considered instantaneous values only, not necessarily reflecting the final state of the cluster. In particular we note that the increasing core radii observed in young LMC/SMC clusters can be well explained as an effect of rapid gas loss.Comment: 8 pages, 5 figures. MNRAS, in pres

The Age, Mass, and Size Distributions of Star Clusters in M51

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Extracting parameters from colour–magnitude diagrams

“…Goodwin 2009;Lamers, Baumgardt & Gieles 2010), infant mortality 1 (e.g. Lada & Lada 2003;Goodwin & Bastian 2006), star-formation rate in the Galaxy (e.g. Lamers & Gieles 2006;Bonatto & Bica 2011), among others.…”

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“…For example it has been suggested that stars may form with initially sub-virial velocities that might allow the star clusters that form to remain in virial equilibrium even after gas dispersion (Offner et al, 2009;Smith et al, 2011). Alternatively if the star formation efficiency (the fraction of gas converted into stars) in these clusters was sufficiently high (>50%) then more gas would have been converted into stars during their formation and therefore the cluster may not have become unbound by the removal of the remaining gas potential (Goodwin & Bastian, 2006). Finally it has also been suggested that stars may naturally become spatially decou- showing the detailed structure revealed by these observations.…”