The strength of the C IV 1550 A resonance lines in planetary nebulae.

Abstract

The strengths of the C IV resonance lines (1548/51 A) in NGC 6210, NGC 7009,. NGC 3242 and IC 2003 obtained from IUE spectra are compared with the predictions of ionization models constructed with standard assumptions. The ratios of observed to computed fluxes range from 0.4 to 0.034. In order to understand these discrepancies, detailed transfer calculations are employed to investigate the influence of dust and possible additional decay channels for the upper level on the radiation field of the line. It is found that dust can only account for a part of the discrepancy. The effects of additional deexcitation are negligible. The consequences of a central cavity for the carbon ionization equilibrium are then discussed. By adjusting the size of the inner hole it is possible to reduce the ionization sufficiently so that the observed C IV and C III lines are reproduced with a very small amount of dust. It is therefore suggested that these geometry effects are the main reason for the observed weakness of the lines.