Interpretation of the carbon recombination line

The ionization equilibrium of carbon and heavier elements in a neutral gas cloud close to an early type star is discussed using a simple model for the transfer of the ultra-violet radiation. The balance between atomic and molecular hydrogen is also considered. It is concluded that the H II region exciting star may be, but should not be assumed to be, the source of the photons which ionize carbon in the region giving rise to the observed recombination lines. The origins of the narrow hydrogen and heavier element lines are also discussed. 1 table, 2 figs, 29 refs