The effect of the dynamical state of clusters on gas expulsion and infant mortality

The star-formation efficiency (SFE) of a star cluster is thought to be the critical factor in determining if the cluster can survive for a significant (> 50 Myr) time. There is an often quoted critical SFE of similar to 30% for a cluster to survive gas expulsion. I reiterate that the SFE is not the critical factor; rather, it is the dynamical state of the stars (as measured by their virial ratio) immediately before gas expulsion. If the stars in a star cluster are born in an (even slightly) cold dynamical state, then the survivability of a cluster can be greatly increased.