We combine near-UV spectra obtained with the Hubble Space Telescope GHRS echelle with far-UV spectra obtained with IMAPS and Copernicus to study the abundances and physical conditions in the predominantly ionized gas seen at high velocity ( - 105 km s ^ { -1 } \lesssim v _ { \odot } \lesssim - 65 km s ^ { -1 } ) and at intermediate velocity ( - 60 km s ^ { -1 } \lesssim v _ { \odot } \lesssim - 10 km s ^ { -1 } ) along the line of sight to the star \zeta Ori .
We have high resolution ( FWHM \sim 3.3–4.5 km s ^ { -1 } ) 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 , relative to C , N , and S. While various ion ratios would be consistent with collisional ionization equilibrium ( CIE ) at temperatures of 25,000–80,000 K , the widths of individual high-velocity absorption components indicate that T \sim 9000 \pm 2000 K — so the gas is not in CIE .
Analysis of the C ii fine-structure excitation equilibrium , at that temperature , yields estimates for the densities ( n _ { e } \sim n _ { H } \sim 0.1–0.2 cm ^ { -3 } ) , thermal pressures ( 2 n _ { H } T \sim 2000–4000 cm ^ { -3 } K ) , and thicknesses ( 0.5–2.7 pc ) characterizing 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 different 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 between the models and the data .
The measured depletions suggest that roughly 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 recent predictions for the destruction of silicate dust in a 100 km s ^ { -1 } shock .
The observed near-solar gas phase abundance of carbon , however , appears to be inconsistent with the predicted longer time scales for the destruction of graphite grains .