VLA OBSERVATIONS OF STRONG IRAS POINT SOURCES ASSOCIATED WITH COMPACTH-II REGIONS

We present multifrequency radio continuum observations made with the V ery 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 approximately 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), she ll (3), cometary (3), and bipolar (1), consistent with their being exc ited 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 component s, together with their large IR luminosities, suggest that these regio ns are excited by a cluster of stars. Furthermore, the observed infrar ed and radio properties suggest that the cluster contains only massive stars (O and early B spectral types). About 70% of the individual reg ions of ionized gas have linear diameters between 0.1 and 1 pc and ele ctron densities between 10(3) and 10(4) cm-3, corresponding to H II re gions of class II in the classification scheme of Habing & Israel (197 9). The electron densities and diameters follow a power-law relation o f the form N(e) is-proportional-to D-1. For the sources showing simple morphologies, the ratio between the total infrared and Lyman-alpha lu minosities 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 emit ted by hot dust within the H II regions. The fraction of Lyman continu um photons absorbed by dust within the H II regions is typically 55%.