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

Abstract

The C II lines $$2s2p^2 \enspace^2D \rightarrow 2s^22p\enspace ^2P^0$$, λ 1334.53 and λλ 1335.66, 1335.71 (blended) are observed in high-dispersion spectra of IC 418, and have profiles of P Cygni Type. The fluxes in these lines are larger than would be expected if they were collisionally excited. Storey has shown that they have large rate coefficients for excitation by di-electronic recombination. Assuming this mechanism, the calculated ratio of the flux in C IIλ 1335 relative to that in C IIλ 4267 is found to be about twice the observed ratio. This discrepancy is probably due to some absorption of the C IIλ 1335 resonance lines by dust internal to the nebula. The required dust optical depth, at the wavelength of the lines, is τ_D = 0.08. The ratio F(λλ 1335.66, 1335.71)/F(λ 1334.53) obtained from the observations is 1.5 ± 0.4, compared with the value of 2.0 to be expected for LS coupling. It is possible that departures from LS coupling are important in the dielectronic recombination process.