Intermediate- and High-Velocity Ionized Gas toward zeta Orionis

Abstract

We combine UV spectra obtained with the HST/GHRS echelle, IMAPS, and Copernicus to study the abundances and physical conditions in the predominantly ionized 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/or high 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 elemental depletions. C, N, and S have essentially solar relative abundances; Al, Si, and Fe 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 ionization equilibrium (CIE) for T ~ 25,000-80,000 K, the widths of individual high-velocity absorption components indicate that T ~ 9000 K -- so the gas is not in CIE. Analysis of the C II fine-structure excitation equilibrium yields estimated 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. We compare the abundances and physical properties derived for these clouds with those found for gas at similar velocities toward 23 Ori and tau CMa, and also with several models for shocked gas. While the shock models can reproduce some features 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 medium may have been returned to the gas phase, consistent with predictions for the destruction of silicate dust in a 100 km/s shock. The near-solar gas phase abundance of carbon, however, seems inconsistent with the predicted longer time scales for the destruction of graphite grains.