The SOFIA Mid-Infrared Imaging Survey toward Giant HII Regions: W49A

Our Milky Way Galaxy harbors about four dozen giant HII (GHII) regions which are where the highest concentrations of star formation activity is presently occurring. We have been systematically studying these GHII regions with SOFIA to better understand their properties individually and as a population. This study concentrates on W49A, which is one of the largest and most luminous GHII regions in the Galaxy. Embedded within its wide-spread (~5′×4′) extended radio continuum and thermal dust emission is one of the highest concentrations of compact HII and ultra-compact HII regions in the Galaxy. They are distributed in a ring-like structure referred to as the Welsh Ring, whose most dominant member is the so-called G region. The western side of the Welsh Ring has compact HII regions that are easily seen in cm radio continuum emission, but are not visible in the near-infrared. Our SOFIA observations probe even longer infrared wavelengths (37 microns), and still these western sources are not detectable, confirming extremely high levels of extinction that are thought to exist toward this sub-region. We use our new SOFIA 20 and 37 micron imaging data in conjunction with Spitzer 3-8 micron data and Hershel 70 and 160 micron data to create SEDs of the compact sources in W49A. By utilizing SED models, we find the best fits to the SEDs to estimate the mass and luminosity of each source, allowing us to identify and study those sources that are likely massive young stellar objects. For the larger regions of W49A, we calculate two independent evolutionary tracers for the major sub-regions of W49A, allowing us to quantitatively discern relative ages. We determine that the G region is the youngest and W49 South is one of the most evolved.