The Lifetimes of Phases in High-Mass Star-Forming Regions
  High-mass stars form within star clusters from dense, molecular regions, but
is the process of cluster formation slow and hydrostatic or quick and dynamic?
We link the physical properties of high-mass star-forming regions with their
evolutionary stage in a systematic way, using Herschel and Spitzer data. In
order to produce a robust estimate of the relative lifetimes of these regions,
we compare the fraction of dense, molecular regions above a column density
associated with high-mass star formation, N(H2) > 0.4-2.5 x 10^22 cm^-2, in the
'starless (no signature of stars > 10 Msun forming) and star-forming phases in
a 2x2 degree region of the Galactic Plane centered at l=30deg. Of regions
capable of forming high-mass stars on ~1 pc scales, the starless (or embedded
beyond detection) phase occupies about 60-70% of the dense, molecular region
lifetime and the star-forming phase occupies about 30-40%. These relative
lifetimes are robust over a wide range of thresholds. We outline a method by
which relative lifetimes can be anchored to absolute lifetimes from large-scale
surveys of methanol masers and UCHII regions. A simplistic application of this
method estimates the absolute lifetimes of the starless phase to be 0.2-1.7 Myr
(about 0.6-4.1 fiducial cloud free-fall times) and the star-forming phase to be
0.1-0.7 Myr (about 0.4-2.4 free-fall times), but these are highly uncertain.
This work uniquely investigates the star-forming nature of high-column density
gas pixel-by-pixel and our results demonstrate that the majority of high-column
density gas is in a starless or embedded phase.
