High-precision star formation efficiency measurements in nearby clouds
  On average molecular clouds convert only a small fraction epsilon_ff of their
mass into stars per free-fall time, but differing star formation theories make
contrasting claims for how this low mean efficiency is achieved. To test these
theories, we need precise measurements of both the mean value and the scatter
of epsilon_ff, but high-precision measurements have been difficult because they
require determining cloud volume densities, from which we can calculate
free-fall times. Until recently, most density estimates assume clouds as
uniform spheres, while their real structures are often filamentary and highly
non-uniform, yielding systematic errors in epsilon_ff estimates and smearing
real cloud-to-cloud variations. We recently developed a theoretical model to
reduce this error by using column density distributions in clouds to produce
more accurate volume density estimates. In this letter, we apply this model to
recent observations of 12 nearby molecular clouds. Compared to earlier
analyses, our method reduces the typical dispersion of epsilon_ff within
individual clouds from 0.35 dex to 0.31 dex, and decreases the median value of
epsilon_ff over all clouds from ~ 0.02 to ~ 0.01. However, we find no
significant change in the ~ 0.2 dex cloud-to-cloud dispersion of epsilon_ff,
suggesting the measured dispersions reflect real structural differences between
clouds.
