High-Resolution Mid-Infrared Imaging of Radio Ultracompact HiiRegions

We present data from mid-infrared Keck Telescope imaging of 18 radio-selectedultra-compact HII region candidates at diffraction-limited resolution. The goalof these observations is to determine the sizes, luminosities, and morphologiesof the mid-infrared emitting dust surrounding the stellar sources. All 18sources were imaged at 11.7um and at 17.65um, and 10 of them were imaged alsoat 24.5um. All the sources were resolved. We have generated dust temperatureand optical depth maps and combine them with radial velocity measurements andradio data (1.4 and 5 GHz) to constrain the properties of these star-formingregions. Half of our objects are excited by B-stars, and all our objects havederived types that are later than an O6 star. We find a significant correlationbetween infrared and radio flux densities, and a weaker one between infrareddiameters and the central source ionizing photon rates. This latter correlationsuggests that the more compact sources result from later spectral types ratherthan young age. Our new data may suggest a revision to infrared color selectioncriteria of ultra-compact HII regions at resolutions <1". These 18 sources arepart of a sample of 687 sources dominated by ultra-compact HII regions selectedby matching radio and infrared maps of the first Galactic quadrant by Giveonand coworkers. The new mid-infrared images constitute a significant improvementin resolving sub-structure at these wavelengths. If applied to all of thissample our analysis will improve our understanding of embedded star-formationin the Galaxy.Comment: 66 pages, 34 figures. Accepted for publication in A