As in Of stars the lines of N iii 3p ` `D XX4634, 4641,4642 are always in emission, one has in the past been looking for special excitation mechanisms. With 3d `D H 3p P in emission it also seemed strange that the most obvious decay 3P `P H 3s `S through X4097, 4103 does not produce emission lines. Population of 3d `D and 3s `S by absorption of continuous radiation and decay of 3p `P mainly by two-electron transitions to terms other than 3s `S can explain this puzzle. Population of 3d 2D through the Bowen mechanism is shown to be improbable. The possibility of exciting 2s3d 1D, the upper level of C ITT 5696, by photoionization of 2s'3d by He ii X304 or indirectly by resonance absorption of N iv X322.57 is examined and ruled out as ineffective. Calculations taking into account effects of optical depth predict C TI' X5696 to be in emission but the triplet at X4649 to be in absorption without having to stipulate an extended atmosphere.
No takes yet. Share an insight, caveat, or question.
H. Nussbaumer (1971) studied this question.