VLA Observations of Strong IRAS Point Sources Associated with Compact H II Regions

Abstract

We present multifrequency radio continuum observations made with the Very Large Array of a sample of 16 strong IRAS point sources associated with compact H II regions. Twelve sources were observed at 1.5, 4.9, and 15.0 GHz, with angular resolution of ˜15", 5", and 3", respectively. Three were observed at 1.5 and 4.9 GHz, and one at 1.5 GHz. The radio continuum brightness distribution was resolved for all sources. Nine objects show simple morphologies: core-halo (2), shell (3), cometary (3), and bipolar (1), consistent with their being excited by a single star. The remaining seven sources show complex radio structure. The average infrared luminosity of the complex sources is 3 times larger than that of the simple sources. The morphologies of the complex H II regions, which can be decomposed into multiple components, together with their large IR luminosities, suggest that these regions are excited by a cluster of stars. Furthermore, the observed infrared and radio properties suggest that the cluster contains only massive stars (0 and early B spectral types). About 70% of the individual regions of ionized gas have linear diameters between 0.1 and 1 pc and electron densities between 103 and 104 cm-3, corresponding to H II regions of class II in the classification scheme of Habing & Israel (1979). The electron densities and diameters follow a power-law relation of the form Ne ∝ D-1. For the sources showing simple morphologies, the ratio between the total infrared and Lyman-alpha luminosities ranges from 6 to 21. If single stars excite these simple H II regions, then the infrared excesses are fully accounted for by dust emission in large, cool envelopes heated by stellar photons longward of the Lyman continuum limit and by optical/near-infrared photons emitted by hot dust within the H II regions. The fraction of Lyman continuum photons absorbed by dust within the H II regions is typically 55%.