Overview and First Results of the SOFIA Mid-Infrared Imaging Survey toward Giant H II Regions

We present an overview of the on-going large-scale infrared imaging survey at 20 and 37 micrometers of Milky Way Giant H II (GH II) regions with the FORCAST instrument on SOFIA. We discuss the first results from this survey with our detailed analysis of W51A, which is one of the largest GH II regions in our Galaxy and host to prolific high-mass star formation activity. In addition to the SOFIA-FORCAST observations, we utilized multi-wavelength archival data from several facilities in order to investigate the detailed physical properties of individual sources within W51A, and of the larger-scale star-forming sub-structures. From this multi-band photometry we created spectral energy distributions (SEDs), and using young stellar object (YSO) models, we found several dozen compact sources in the region that are likely to be massive YSOs, many of which are revealed as such for the first time. We also found about half of these sources do not have a radio continuum counterpart which likely indicate the sources are at their very early stages of star formation. We determined the relative ages of the sub-regions of W51A analytically, and find that W51A possesses star-forming regions in various evolutionary stages, though none show strong evidence of being internally triggered.