ON THE TIME VARIABILITY OF THE STAR FORMATION EFFICIENCY

A star formation efficiency per free-fall time that evolves over the lifetime of giant molecular clouds (GMCs) may have important implications for models of supersonic turbulence in molecular clouds or for the relation between the star formation rate and H-2 surface density. We discuss observational data that could be interpreted as evidence of such a time variability. In particular, we investigate a recent claim based on measurements of H-2 and stellar masses in individual GMCs. We show that this claim depends crucially on the assumption that H-2 masses do not evolve over the lifetimes of GMCs. We exemplify our findings with a simple toy model that uses a constant star formation efficiency and, yet, is able to explain the observational data.