Hierachical merging - what makes a star cluster survive gas expulsion

Abstract: Within a collapsing molecular cloud, star formation is observed to occur in 
small sub-clumps distributed throughout the cloud. In order to form a massive star 
cluster, it is currently believed that these sub-clumps must hierarchically merge 
together. Stellar feedback induced gas mass loss can result in infant mortality - the 
destruction of fledgling clusters.
We investigate the parameters effecting cluster survival using Nbody simulations. We 
find that the initial star formation efficiency is not a good measure of cluster 
survivability. The stellar distribution can change significantly, causing a large change in 
the relative importance of the stellar and gas potentials, and allowing clusters with low 
initial star formation efficiency to survive. Instead we find that the initial stellar 
distribution and velocity dispersion are far more important parameters controlling 
cluster survival. The dynamical state of the cluster at gas expulsion is also found to be 
influential.