Evidence for Delayed Massive Star Formation in the M17 Proto-OB
  Association
  Through analysis of archival images and photometry from the Spitzer GLIMPSE
and MIPSGAL surveys combined with 2MASS and MSX data, we have identified 488
candidate young stellar objects (YSOs) in the giant molecular cloud M17 SWex,
which extends ~50 pc southwest from the prominent Galactic H II region M17. Our
sample includes >200 YSOs with masses >3 Msun that will become B-type stars on
the main sequence. Extrapolating over the stellar initial mass function (IMF),
we find that M17 SWex contains >1.3x10^4 young stars, representing a proto-OB
association. The YSO mass function is significantly steeper than the Salpeter
IMF, and early O stars are conspicuously absent from M17 SWex. Assuming M17
SWex will form an OB association with a Salpeter IMF, these results reveal the
combined effects of (1) more rapid circumstellar disk evolution in more massive
YSOs and (2) delayed onset of massive star formation.
