Surveying the Giant HII Regions of the Milky Way with SOFIA: III. W49A
  We present our third set of results from our mid-infrared imaging survey of
Milky Way Giant HII (GHII) regions with our detailed analysis of W49A, one of
the most distant, yet most luminous, GHII regions in the Galaxy. We used the
FORCAST instrument on the Stratospheric Observatory For Infrared Astronomy
(SOFIA) to obtain 20 and 37$\mu$m images of the entire ~5.0' x 3.5'
infrared-emitting area of W49A at a spatial resolution of ~3". Utilizing these
SOFIA data in conjunction with previous multi-wavelength observations from the
near-infrared to radio, including Spitzer-IRAC and Herschel-PACS archival data,
we investigate the physical nature of individual infrared sources and
sub-components within W49A. For individual compact sources we used the
multi-wavelength photometry data to construct spectral energy distributions
(SEDs) and fit them with massive young stellar object (MYSO) SED models, and
find 22 sources that are likely to be MYSOs. Ten new sources are identified for
the first time in this work. Even at 37$\mu$m we are unable to detect infrared
emission from the sources on the western side of the extremely extinguished
ring of compact radio emission sources known as the Welch Ring. Utilizing
multi-wavelength data, we derived luminosity-to-mass ratio and virial
parameters of the extended radio sub-regions of W49A to estimate their relative
ages and find that overall the sub-components of W49A have a very small spread
in evolutionary state compared to our previously studied GHII regions.
