Ultraviolet spectra of planetary nebulae - the C II 1335 dielectronic recombination lines in IC 418.

Abstract

High-dispersion observations of IC 418 by the IUE satellite show the C II lines (1) 1334.53 A, (2) 1335.66 A, and (3) 1335.71 A. The profiles are of the P Cygni type, a form expected to result from scattering in the expanding nebular shell, with a mean velocity difference between nebular emission lines and interstellar absorption lines of 47 km/sec. The interpretation of flux ratios demonstrates that the 1335 A lines are excited by dielectronic recombination, and comparison with the C II 4267 A recombination line indicates that the flux in 1335 A is attenuated by a factor of 0.54 due to absorption by dust within the nebula. The observed ratio F(2 + 3)/F(1) of fluxes in the C II lines is 1.5 + or - 0.4, compared with the ratio of 2.0 expected for LS coupling. Further research is recommended to determine whether dielectronic recombination can lead to significant departures from the LS coupling ratio.