MSX Mid Infrared Imaging of Massive Star Birth Environments. I: Ultracompact H ii Regions

We present mid-IR 21μm images of a sample of radio selected Ultracompact H ii (UCH ii) regions, obtained with the Midcourse Space Experiment (MSX). All, with one possible exception, are detected at mid-IR wavelengths, sampling the warm dust emission of the cocoons of the OB star central exciting sources. Many of the UCH ii regions have nearby (up to ≈ few pc distant) companion dust emission sources, which represent other potential star birth sites. In some objects the companion dominates the IRAS point source catalogue entry for the UCH ii region. We compare the midand far-IR dust emission, measuring the embedded hot star luminosity, with published UCH ii radio emission, measuring the Lyman continuum luminosity. We find a spectral type dependence, as predicted by the standard model of an ultracompact ionized hydrogen region, surrounded by a natal dust shell, with some scatter, which can be understood by consideration of: 1) dust absorption of some fraction of the emitted Lyman continuum photons; 2) fainter companion stars within the UCH ii region; 3) the structure of the UCH ii regions differing from star to star. Overall, the higher spatial resolution offered by MSX alleviates difficulties often encountered by comparison of IRAS far-IR fluxes with radio derived ionizing fluxes for UCH ii regions.