Impact of Gas Removal on the Evolution of Embedded Clusters

We study both analytically and numerically the disruptive effect of instantaneous gas removal from an embedded cluster. We setup a calculation based on the stellar velocity distribution function, to compute the fraction of stars that remain bound once the cluster has ejected the gas and is out of equilibrium. We find tracks of bound mass-fraction vs star formation efficiency similar to those obtained with N-body calculations. We use these to argue that embedded clusters must develop high-binding energy cores if they are to survive as bound clusters despite a star formation rate as low as 20 per cent or lower suggested by observations.