Surveying the Giant HII Regions of the Milky Way with SOFIA: I. W51A
  We discuss the first results from our mid-infrared imaging survey of Milky
Way Giant HII regions with our detailed analysis of W51A, which is one of the
largest GHII regions in our Galaxy. We used the FORCAST instrument on SOFIA to
obtain 20 and 37$\mu$m images of the central $10' \times 20'$ area, which
encompasses both of the G49.5-0.4 and G49.4-0.3 sub-regions. Based on these new
data, and in conjunction with previous multi-wavelength observations, we
conjecture on the physical nature of several individual sources and
sub-components within W51A. We find that extinction seems to play an important
role in the observed structures we see in the near- to mid-infrared, both
globally and locally. We used the SOFIA photometry combined with Spitzer-IRAC
and Herschel-PACS photometry data to construct spectral energy distributions
(SEDs) of sub-components and point sources detected in the SOFIA images. We fit
those SEDs with young stellar object models, and found 41 sources that are
likely to be massive young stellar objects, many of which are identified as
such in this work for the first time. Close to half of the massive young
stellar objects do not have detectable radio continuum emission at cm
wavelengths, implying a very young state of formation. We derived
luminosity-to-mass ratio and virial parameters of the extended radio
sub-regions of W51A to estimate their relative ages.
