Dynamical constraints on star formation efficiency

Dynamical constraints are placed on the local star formation efficiency during the formation of star clusters. Virial models are used to examine the expected changes in cluster velocity dispersion, density, and radius during the period when the gas not converted into stars is removed from the system. Comparison of observed initial and final states of open clusters indicate local efficiencies of about 30 percent if the gas dispersal is slow relative to the dynamical crossing time and 55 percent if the gas loss is rapid. Efficiencies are somewhat dependent on support mechanisms of moleclar clouds, but it is argued that observations of young clusters are in accord with the assumptions of the models used here.