The Onset of Massive Star Formation: The Evolution of Temperature and
  Density Structure in an Infrared Dark Cloud
  We present new NH3 (1,1), (2,2), and (4,4) observations from the Karl G.
Jansky Very Large Array (VLA) compiled with work in the literature to explore
the range of conditions observed in young, massive star-forming regions. To
sample the effects of evolution independent from those of distance/resolution,
abundance, and large-scale environment, we compare clumps in different
evolutionary stages within a single Infrared Dark Cloud (IRDC), G32.02+0.06. We
find that the early stages of clustered star formation are characterized by
dense, parsec-scale filamentary structures interspersed with complexes of dense
cores (<0.1 pc cores clustered in complexes separated by ~1 pc) with masses
from about 10 to 100 Msun. The most quiescent core is the most extended while
the star-forming cores are denser and more compact, showing very similar column
density structure before and shortly after the onset of massive star formation,
with peak surface densities >~ 1 g cm^-2. Quiescent cores and filaments show
smoothly varying temperatures from 10-20 K, rising to over 40 K in star-forming
cores. We calculate the virial parameters for 16 cores and find that the level
of support provided by turbulence is generally insufficient to support them
against gravitational collapse (alpha_vir ~ 0.6). The star-forming filaments
show smooth velocity fields, punctuated by discontinuities at the sites of
active star formation. We discuss the Massive Molecular Filament (MMF; M > 10^5
Msun, l > 60 pc) hosting the IRDC, hypothesizing that it may have been shaped
by previous generations of massive stars.
