Intermediate‐ and High‐Velocity Ionized Gas toward ζ Orionis

We combine UV spectra obtained with the HST/GHRS echelle, IMAPS, andCopernicus to study the abundances and physical conditions in the predominantlyionized gas seen at high (-105 to -65 km/s) and intermediate velocities (-60 to-10 km/s) toward zeta Ori. We have high resolution (FWHM ~ 3.3-4.5 km/s) and/orhigh S/N spectra for at least two significant ions of C, N, Al, Si, S, and Fe-- enabling accurate estimates for both the total N(H II) and the elementaldepletions. C, N, and S have essentially solar relative abundances; Al, Si, andFe appear to be depleted by about 0.8, 0.3-0.4, and 0.95 dex, respectively.While various ion ratios would be consistent with collisional ionizationequilibrium (CIE) for T ~ 25,000-80,000 K, the widths of individualhigh-velocity absorption components indicate that T ~ 9000 K -- so the gas isnot in CIE. Analysis of the C II fine-structure excitation equilibrium yieldsestimated densities (n_e ~ n_H ~ 0.1-0.2 cm^{-3}), thermal pressures (2 n_H T ~2000-4000 cm^{-3}K), and thicknesses (0.5-2.7 pc) for the individual clouds. Wecompare the abundances and physical properties derived for these clouds withthose found for gas at similar velocities toward 23 Ori and tau CMa, and alsowith several models for shocked gas. While the shock models can reproduce somefeatures of the observed line profiles and some of the observed ion ratios,there are also significant differences. The measured depletions suggest that\~10% of the Al, Si, and Fe originally locked in dust in the pre-shock mediummay have been returned to the gas phase, consistent with predictions for thedestruction of silicate dust in a 100 km/s shock. The near-solar gas phaseabundance of carbon, however, seems inconsistent with the predicted longer timescales for the destruction of graphite grains.Comment: 50 pages, 9 figures; aastex; accepted by Ap