Near-infrared emission of neutral carbon from photon-dominated regions

Detailed calculations are reported of the intensities of the near-infrared lines of neutral carbon atoms at 985.0, 982.3, and 872.7 nm emitted from dense clouds subjected to intense radiation fields. There is a small contribution to the excitation of the metastable levels by electron impacts in the heated edge zones of the clouds but it is insignificant compared to that from the recombination of the C(+) ions produced by photoionization. The lines observed in M42 and NGC 2024 can be interpreted as arising in gas with densities in excess of 100,000/cu cm and radiation fields with intensities between a thousand and a million times the average interstellar field intensity. Radiative recombination of C(+) ions may be an important source of the emission lines detected in the planetary nebulae NGC 6270 and NGC 7027. 21 refs.