SOFIA-FORCAST Survey Toward the Giant HII regions of the Galaxy

The chemical input and high energy feedback of massive star formation are important mechanisms for Galactic ecology. Despite of the importance, we barely understand the massive star formation since they are typically far at several kpc distance and deeply embedded in the densest parts of molecular clouds. The Giant HII regions (GHII) possess various evolutionary stages of massive star and star cluster formation thus have been good laboratories for the observational constraints of their formation mechanisms. We present the first results and the current status of the SOFIA-FORCAST MIR (20 and 37μm) imaging survey toward the GHII regions of Milky Way. The bands fill the wavelength gaps between Spitzer and Herschel with comparable angular resolution thus we can achieve the complete SED fits for YSO models as well as the dust graybody assumption of proto-clusters. From the first target of the survey, W51A, we find 47 MIR point sources that we investigate as massive YSO candidates while about half of the sources have not been specified yet. The SED fits show 41/47 sources as promising massive YSOs where 20 of them do not have radio counterparts indicating their earliest evolutionary stages. We determined the relative ages of porto-clusters in W51A via comparing L/M to virial parameter. The analysis implies the formation history and relationship among different porto-clusters in W51A. We also discuss the preliminary results from different survey targets.