AL-III, SI-IV, AND C-IV ABSORPTION TOWARD ZETA-OPHIUCHI - EVIDENCE FOR PHOTOIONIZED AND COLLISIONALLY IONIZED-GAS

We present Goddard High-Resolution Spectrograph observations at 3.5 km s(-1) resolution and signal-to-noise ratios of 30 to 60 for the Al II I, Si IV, C IV, and N V absorption lines in the far-ultraviolet spectr um of the 09.5 V star xi Ophiuchi. The measurement reveal three types of highly ionized gas along the 140 pc line of sight. 1. Narrow compon ents of Al III (b = 4.3 km s(-1) [upsilon(helio)] = - 7.8 km s(-1) b = 3.2 km s(-1) [upsilon(helio)] = - 14.4 km s(-1)) and Si IV (b = 5.3 k m s(-1), [upsilon(helio)] = -15.0 km s(-1)) trace photoionized gas in the expanding H II region surrounding xi Oph. The observed magnitude a nd direction of the velocity offset between the Al III and Si IV profi les can be explained by models of H II regions that incorporate expans ion. Narrow C IV absorption associated with the H II region is not det ected. Predictions of the expected amounts of Si IV and C IV overestim ate the column densities of these ions by factors of 30 and more than 10, respectively. The discrepancy may be due to the effects of element al depletions in the gas and/or to the interaction of the stellar wind with surrounding matter. 2. Broad (b = 15 to 18 km s(-1)) and weak Si IV and C IV absorption components are detected near [upsilon(helio)] = -26 km s(-1). The high-ionization species associated with these abso rption components are probably produced by electron collisional ioniza tion in a heated gas. This absorption may be physically related to the xi Oph bow shock or to a cloud complex situated within the local inte rstellar medium at d < 60 pc. The C IV to Si IV column density ratio i n this gas is 8, a factor of 6 less than conductive interface models p redict, but this discrepancy may be removed by considering the effects of self-photoionization within the cooling gas in the model calculati ons. 3. A broad (b = 13 km s(-1)) and weak C IV absorption feature det ected at [upsilon(helio)] = -61 km s(-1) is not seen in other species. We tentatively ascribe this absorption to gas in a postshock region o f an optically thin shock in the xi Oph stellar wind.