The Diffuse Interstellar Clouds toward 23 Orionis

Spectra obtained with the Hubble Space Telescope GHRS are combined with high-resolution optical spectra and UV spectra from Copernicus to study the abundances and physical conditions in the diffuse interstellar clouds seen along the line of sight to the star 23 Ori. Strong low-velocity (SLV) absorption, due to cool, moderately dense neutral gas and representing about 92% of the total N(H I), is seen for various neutral and singly ionized species at +20 km/s < v < +26 km/s. Weaker low-velocity absorption, probably largely due to warmer neutral gas, is seen primarily for various singly ionized species at 0 km/s < v < +30 km/s. Absorption from a number of singly and doubly ionized species, perhaps due to a radiative shock, is seen at -108 km/s < v < -83 km/s. Weak absorption components from ionized gas are seen at intermediate velocities (-43 km/s < v < -4 km/s). The large range in n_e (from 0.04 cm^{-3} to 0.95 cm^{-3}) derived independently from nine pairs of neutral and singly ionized species in the SLV gas suggests that additional processes besides simple photoionization and radiative recombination affect the ionization balance. The adopted average SLV electron density, n_e = 0.15+/-0.05 cm^{-3}, implies a relatively large n_e/n_H ~ 0.01, and thus some ionization of hydrogen in these predominantly neutral components. Comparisons of the SLV depletions and n_H with those found for the strong ``component B'' (v ~ -14 km/s) blend toward zeta Oph hint at a possible relationship between depletion and local density for relatively cold interstellar clouds. Calcium appears to be more severely depleted in warm, low density gas than has generally been assumed. An appendix summarizes the most reliable oscillator strengths currently available for a number of the interstellar absorption lines analyzed in this work.
73 pages, Latex, aaspp4.sty; to appear in ApJS, vol. 124 (1999 Oct); also available from ftp://astro.uchicago.edu/pub/astro/welty/23ori/23ori9.ps.gz