Infant mortality in the hierarchical merging scenario: dependence on gas expulsion time-scales

We examine the effects of gas expulsion on initially substructured and out-of-equilibrium star clusters. We perform N-body simulations of the evolution of star clusters in a static background potential before adjusting that potential to model gas expulsion. We investigate the impact of varying the rate at which the gas is removed, and the instant at which gas removal begins. Reducing the rate at which the gas is expelled results in an increase in cluster survival. Quantitatively, this dependence is approximately in agreement with previous studies, despite their use of smooth and virialized initial stellar distributions. However, the instant at which gas expulsion occurs is found to have a strong effect on cluster response to gas removal. We find if gas expulsion occurs prior to one crossing time, cluster response is poorly described by any global parameters. Furthermore, in real clusters the instant of gas expulsion is poorly constrained. Therefore, our results emphasize the highly stochastic and variable response of star clusters to gas expulsion.