Surveying the Giant H II Regions of the Milky Way with SOFIA: V. DR7 and K3-50

We present our fifth set of results from our mid-infrared imaging survey of Milky Way Giant H II (GH II) regions with our detailed analysis of DR7 and K3-50. We obtained 20/25 and 37 μm imaging maps of both regions using the FORCAST instrument on the Stratospheric Observatory For Infrared Astronomy. We investigate the multiscale properties of DR7 and K3-50 using our data in conjunction with previous multiwavelength observations. Near- to far-infrared spectral energy distributions of individual compact infrared sources were constructed and fitted with massive young stellar object (MYSO) models. We find eight out of the ten (80%) compact sources in K3-50 and three out of the four (75%) sources in DR7 are likely to be MYSOs. We derived luminosity-to-mass ratios of the extended radio subregions of DR7 and K3-50 to estimate their relative ages. The large spread in evolutionary state for the subregions in K3-50 likely indicates that the star-forming complex has undergone multiple star-forming events separated more widely in time, whereas the smaller spread in DR7 likely indicates the star formation subregions are more co-eval. DR7 and K3-50 have Lyman continuum photon rates just above the formal threshold criterion for being categorized as a GH II region (10⁵⁰ photons s^-1) but with large enough errors that this classification is uncertain. By measuring other observational characteristics in the infrared, we find that K3-50 has properties more akin to previous bona fide GH II regions we have studied, whereas DR7 has values more like those of the non-GH II regions we have previously studied.