On microwave recombination lines from HI regions

We observed recombination lines attributed to carbon at 21-, 30-, 36-, and 43-cm wavelengths toward IC 1795 (W3), Orion A, and NGC 2024; and a narrow hydrogen line toward NGC 2024. A comparison of a considerable body of carbon recombination-line and radio molecular-line data indicates that these two types of lines are probably formed in dense contiguous regions the carbon lines in a thin layer facing a hot star and the molecular lines in the rest of the cloud shielded from the stellar radiation by the carbon-emitting slab. This slab should be approximately the same as that responsible for much of the far- infrared flux that emerges from the vicinity of H ii regions. The high-frequency carbon lines toward Orion A are probably due to spontaneous rather than stimulated emission, which might not play an important role in any carbon-line emission region. In NGC 2024 the H I regions responsible for the observed hydrogen and carbon lines are probably dense (11H > 10 cm-3). Previous estimates of the fractional ionization of hydrogen may be inaccurate because different volumes of gas may be observed in the hydrogen and carbon recombination lines. Without further calculations and observations it is not possible to decide whether the narrow hydrogen line originates in a dense "cloud" or in an ionization front. Subject headings: molecules, interstellar - nebulae - radio lines