C IV and SI IV in the interstellar medium.

Abstract

Column-densities are presented of C IV and Si IV along the line of sight to 49 galactic O-stars, derived from IUE spectra. Since these column-densities appear to be uncorrelated with the distance to the background star a truly interstellar origin of these lines can be ruled out. The fact that large column-densities are preferentially observed towards O-stars in open clusters and towards O-stars associated with nebulosity strongly suggests that these lines are formed in interstellar gas close to the observed star. To interpret the observed column-densities quantitatively, ionization models have been calculated of H II regions excited by stars with NLTE atmospheres. The predicted column-densities turn out to be significantly larger than those previously found for LTE models. The calculations show that photoionization can indeed explain the magnitude and the ratio of the observed column-densities. Good agreement between theory and observation is obtained for depletion factors of silicon and carbon observed in the general interstellar medium. Theoretical objections against a photoionization origin based on the fact that the inner parts of an H II region will be cleared out by a stellar wind can be circumvented by postulating that the interstellar medium in which the stellar wind bubble develops is clumpy. In this picture the C IV and Si IV lines are formed in slowly evaporating ionized clouds which are embedded in the hot stellar wind bubble.