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 epsilon_GMC = 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
epsilon_ff = 1. Weighting by ionizing luminosity, we find an average of
epsilon_ff = 0.16 - 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 =
0.008. We show that the lifetimes of giant molecular clouds at the mean mass
found in our sample is 17 plus or minus 4 Myr, about two 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
