Star Formation Efficiencies and Lifetimes of Giant Molecular Clouds in the Milky Way

We use a sample of the 13 most luminous WMAP Galactic free-free sources, responsible for 33% of the free-free emission of the Milky Way, to investigate star formation. The sample contains 40 star-forming complexes; we combine this sample with giant molecular cloud (GMC) catalogs in the literature to identify the host GMCs of 32 of the complexes. We estimate the star formation efficiency epsilon_GMC and star formation rate per free-fall time epsilon_ff. We find that epsilon_GMC ranges from 0.002 to 0.2, with an ionizing luminosity-weighted average langepsilon_GMCrang _Q = 0.08, compared to the Galactic average ≈0.005. Turning to the star formation rate per free-fall time, we find values that range up to ɛ_ff ≡ τ _ff \cdot \dot{M}_*/M_GMC≈ 1. Weighting by ionizing luminosity, we find an average of langepsilon_ffrang _Q = 0.14-0.24 depending on the estimate of the age of the system. Once again, this is much larger than the Galaxy-wide average value epsilon_ff = 0.006. We show that the lifetimes of GMCs at the mean mass found in our sample is 27 ± 12 Myr, a bit less than three free-fall times. The GMCs hosting the most luminous clusters are being disrupted by those clusters. Accordingly, we interpret the range in epsilon_ff as the result of a time-variable star formation rate; the rate of star formation increases with the age of the host molecular cloud, until the stars disrupt the cloud. These results are inconsistent with the notion that the star formation rate in Milky Way GMCs is determined by the properties of supersonic turbulence.