Rapid Circumstellar Disk Evolution and an Accelerating Star Formation
  Rate in the Infrared Dark Cloud M17 SWex
  We present a catalog of 840 X-ray sources and first results from a 100 ks
Chandra X-ray Observatory imaging study of the filamentary infrared dark cloud
G014.225$-$00.506, which forms the central regions of a larger cloud complex
known as the M17 southwest extension (M17 SWex). In addition to the rich
population of protostars and young stellar objects with dusty circumstellar
disks revealed by Spitzer Space Telescope archival data, we discover a
population of X-ray-emitting, intermediate-mass pre--main-sequence stars (IMPS)
that lack infrared excess emission from circumstellar disks. We model the
infrared spectral energy distributions of this source population to measure its
mass function and place new constraints on the inner dust disk destruction
timescales for 2-8 $M_{\odot}$ stars. We also place a lower limit on the star
formation rate (SFR) and find that it is quite high ($\dot{M}\ge
0.007~M_{\odot}$ yr$^{-1}$), equivalent to several Orion Nebula Clusters in
G14.225$-$0.506 alone, and likely accelerating. The cloud complex has not
produced a population of massive, O-type stars commensurate with its SFR. This
absence of very massive (${\ge}20~M_{\odot}$) stars suggests that either (1)
M17 SWex is an example of a distributed mode of star formation that will
produce a large OB association dominated by intermediate-mass stars but
relatively few massive clusters, or (2) the massive cores are still in the
process of accreting sufficient mass to form massive clusters hosting O stars.
