Compact H II regions in the far-infrared

Abstract

Far-infrared (40-150 microns) observations of W51 (G49.5-0.4), K3-50, DR 21, NGC 7538, and W3(OH) are presented and discussed. At these wavelengths each source is dominated by a small bright component closely associated with a compact H II region. Their spectra can be explained as emission by dust at temperatures of 30-50 K. Some of the objects appear to be optically thick out to wavelengths of about 60 microns. Of the 10 source components in the five regions, at least four may be powered by pre-main-sequence stars. The far-infrared data are consistent with depletion of volatile components of the dust within compact H II regions, but do not clearly require destruction or expulsion of refractory grains. The bulk of the dust which emits primarily in the far-infrared must lie in a dense shell immediately outside the ionized zone. The sources are possible precursers to open clusters with stellar masses in the range from 1,000 to 10,000 solar masses.