High-Resolution Mid-Infrared Imaging of Radio Ultracompact H II Regions

We present data from mid-infrared Keck Telescope imaging of 18 radio-selected ultracompact H II region candidates at diffraction-limited resolution. The goal of these observations is to determine the sizes, luminosities, and morphologies of the mid-infrared-emitting dust surrounding the stellar sources. All 18 sources were imaged at 11.7 and 17.65 μm, and 10 of them were also imaged at 24.5 μm. All the sources were resolved. We have generated dust temperature and optical depth maps and combined them with radial velocity measurements and radio data (1.4 and 5 GHz) to constrain the properties of these star-forming regions. Half of our objects are excited by B-type stars, and all our objects have derived types that are later than an O6 star. We find a significant correlation between infrared and radio flux densities and a weaker one between infrared diameters and the central source ionizing photon rates. This latter correlation suggests that the more compact sources result from later spectral types rather than young age. Our new data may suggest a revision to the infrared color selection criteria of ultracompact H II regions at resolutions 1''. These 18 sources are part of a sample of 687 sources dominated by ultracompact H II regions selected by matching radio and infrared maps of the first Galactic quadrant by Giveon and coworkers. The new mid-infrared images constitute a significant improvement in resolving substructure at these wavelengths. If applied to all of this sample, our analysis will improve our understanding of embedded star formation in the Galaxy.