Recombination lines of C I and N II in the interstellar medium

A model potential method has been used to calculate non-hydrogenic transition probabilities and radiative electron recombination coefficients of singly excited states of C I and N II. Comparison with other theoretical calculations show large differences in some cases. The results are applied to the calculation of emission spectrum produced by recombination in the interstellar medium. The spectrum produced by resonant fluorescence from the ground and metastable states is also calculated for comparison with the recombination spectrum. The calculated emission rates are used to interpret observations of forbidden and permitted lines of C I and N II in planetary nebulae, H II regions and nova shells using a minimum number of assumptions concerning the physical conditions in those objects. Comparisons of relative line intensities allows the determination of the excitation mechanism in cases in which several lines of the same ion have been observed. When the excitation is by recombination, an abundance, which is weakly dependent on temperature, can be determined. It is found that the [C I] λλ9850, 9823, and λ8727 lines are not produced by recombination in planetary nebulae unless large undetected quantities of cold neutral material containing ionized carbon exist in those objects. The [C I] lines, however, are probably produced by recombination in the C II region surrounding the M42 and NGC 2024 H II regions. The suggestion that N II permitted lines are excited by fluorescence is examined in detailed in some planetary nebulae. It appears that the intensity of some of the N II permitted lines observed in planetary nebula can only be explained with excitation by radiative recombination