The effect of gas loss on the formation of bound stellar clusters

The effect of gas ejection on the structure and binding energy of newly for med stellar clusters is investigated. The star formation efficiency (SFE), necessary for forming a gravitationally bound stellar cluster, is determine d. Two sets of numerical N-body simulations are presented. As a first simplifi ed approach we treat the residual gas as an external potential. The gas exp ulsion is approximated by reducing the gas mass to zero on a given time-sca le, which is treated as a free parameter. In a second set of simulations we use smoothed particle hydrodynamics (SPH) to follow the dynamics of the ou tflowing residual gas self-consistently. We investigate cases where gas out flow is induced by an outwards propagating shock front and where the whole gas cloud is heated homogeneously, leading to ejection. If the stars are in virial equilibrium with the gaseous environment initial ly, bound clusters only form in regions where the local SFE is larger than 50 per cent or where the gas expulsion time-scale is long compared with the dynamical time-scale. A small initial velocity dispersion of the stars lea ds to a compaction of the cluster during the expulsion phase and reduces th e SFE needed to form bound clusters to less than 10 per cent.