Abstract
We present new VLA observations at 1.3 cm wavelengths and new Br..gamma.., Pf..gamma.., and Br..cap alpha.. IR line observations of very compact molecular cloud IR sources that show evidence of ionized gas. We develop a model for their radio continuum and IR line emission in terms of a dense, ionized circumstellar envelope that is the result of mass loss. The effects of a finite radius of the ionized region of flow are specifically included in our analysis and prove to be important for interpreting the radio spectra of these objects. This analysis permits us to discriminate between those IR sources that seem to be ultracompact H II regions and those whose ionized regions are best characterized as flowing circumstellar envelopes with their ionized regions extending to only a few tens of AU from the central star. These flows apparently are often so massive that both their radio continuum and hydrogen IR line emission are optically thick. A model for the photoionization of the dense envelopes by the Balmer photon flux of the central stars of these objects is developed. It is shown that photoionization from the hydrogen n = 2 energy level can be an important ionization mechanism for thesemore » very dense circumstellar envelopes. Thus, all the IR sources we have observed can be understood as powered by a nearly spectral type star on or near the main sequence. The different types among these IR sources may represent different stages of the star's interaction with its molecular cloud environment.« less
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