2014A&A...563A.123G

Given the rarity of young O star candidates, compact HII regions embedded in dense molecular cores continue to serve as potential sites for peering into the details of high-mass star formation. We uncover the ionizing sources of the most luminous and compact HII regions embedded in the RCW106 and RCW122 giant molecular clouds, known to be relatively nearby (2-4kpc) and isolated, thus providing an opportunity to examine spatial scales of a few hundred to a thousand AU in size. High spatial resolution (0.3''), mid-infrared spectra (R=350), including the fine structure lines [ArIII] and [NeII], were obtained for four luminous compact HII regions embedded inside the dense cores within the RCW106 and RCW122 molecular cloud complexes. At this resolution, these targets reveal point-like sources surrounded by the nebulosity of different morphologies, thereby uncovering details at spatial dimensions of <1000AU. The point-like sources display [ArIII] and [NeII] lines - the ratios of which are used to estimate the effective temperature of the embedded sources. The derived temperatures are indicative of mid-late O type objects for all the sources with [ArIII] emission. Previously known characteristics of these targets from the literature, including evidence of disk or accretion, suggest that the identified sources may grow more to become early-type O stars by the end of the star formation process.