Massive Stars in the W33 Giant Molecular Complex
  Rich in HII regions, giant molecular clouds are natural laboratories to study
massive stars and sequential star formation. The Galactic star forming complex
W33 is located at l=~12.8deg and at a distance of 2.4 kpc, has a size of ~10 pc
and a total mass of (~0.8 - ~8.0) X 10^5 Msun. The integrated radio and IR
luminosity of W33 - when combined with the direct detection of methanol masers,
the protostellar object W33A, and protocluster embedded within the radio source
W33 main - mark the region out as a site of vigorous ongoing star formation. In
order to assess the long term star formation history, we performed an infrared
spectroscopic search for massive stars, detecting for the first time fourteen
early-type stars, including one WN6 star and four O4-7 stars. The distribution
of spectral types suggests that this population formed during the last ~2-4
Myr, while the absence of red supergiants precludes extensive star formation at
ages 6-30 Myr. This activity appears distributed throughout the region and does
not appear to have yielded the dense stellar clusters that characterize other
star forming complexes such as Carina and G305. Instead, we anticipate that W33
will eventually evolve into a loose stellar aggregate, with Cyg OB2 serving as
a useful, albeit richer and more massive, comparator. Given recent distance
estimates, and despite a remarkably similar stellar population, the rich
cluster Cl 1813-178 located on the north-west edge of W33 does not appear to be
physically associated with W33.
