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C IV and SI IV in the interstellar medium
Column-densities are presented of C IV and Si IV along the line of sightto 49 galactic O-stars, derived from IUE spectra. Since thesecolumn-densities appear to be uncorrelated with the distance to thebackground star a truly interstellar origin of these lines can be ruledout. The fact that large column-densities are preferentially observedtowards O-stars in open clusters and towards O-stars associated withnebulosity strongly suggests that these lines are formed in interstellargas close to the observed star. To interpret the observedcolumn-densities quantitatively, ionization models have been calculatedof H II regions excited by stars with NLTE atmospheres. The predictedcolumn-densities turn out to be significantly larger than thosepreviously found for LTE models. The calculations show thatphotoionization can indeed explain the magnitude and the ratio of theobserved column-densities. Good agreement between theory and observationis obtained for depletion factors of silicon and carbon observed in thegeneral interstellar medium. Theoretical objections against aphotoionization origin based on the fact that the inner parts of an H IIregion will be cleared out by a stellar wind can be circumvented bypostulating that the interstellar medium in which the stellar windbubble develops is clumpy. In this picture the C IV and Si IV lines areformed in slowly evaporating ionized clouds which are embedded in thehot stellar wind bubble.
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