SIMBAD references

We present mid-infrared 21-µm images of a sample of radio-selected ultracompact HII (UCHII) regions, obtained with the Midcourse Space Experiment (MSX). With one possible exception, all are detected at mid-infrared wavelengths, sampling the warm dust emission of the cocoons of the OB star central exciting sources. Many of the UCHII 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 UCHII region. We compare the mid- and far-infrared dust emission, measuring the embedded hot star luminosity, with published UCHII radio emission, measuring the Lyman continuum (LyC) 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 UCHII region; and (3) the structure of the UCHII regions differing from star to star. Overall, the higher spatial resolution offered by MSX alleviates difficulties often encountered by comparison of IRAS far-infrared fluxes with radio-derived ionizing fluxes for UCHII regions.