The Diffuse Interstellar Clouds toward 23 Orionis

Spectra obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph 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. Multiple absorption components are present for each of several distinct types of gas, which are characterized by different relative abundance and depletion patterns and physical conditions. 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-1 ≲ v☉ ≲ +26 km s-1. Most typically severely depleted species are less depleted by factors of 2-4, compared to the "cold, dense cloud" pattern found, for example, in the main components toward ζ Oph.For the two strongest SLV components, T ~ 100 K and the thermal pressure log (nHT) ~ 3.1 cm-3 K; we thus have nH ~ 10-15 cm-3 and a total thickness of 12-16 pc. The adopted average SLV electron density, ne = 0.15 ± 0.05 cm-3, implies a relatively large ne/nH ~ 0.01 and thus some ionization of hydrogen in these predominantly neutral components. Weaker low-velocity (WLV) absorption, probably largely due to warmer neutral gas, is seen primarily for various singly ionized species at 0 km s-1 ≲ v☉ ≲ +30 km s-1. The depletions in the WLV gas are typically less severe by a factor of 2-3 than in the SLV gas and are somewhat similar to the "warm cloud" pattern seen in lines of sight with low reddening, low mean density, and/or low molecular fraction. If T ~ 3000 K for the WLV components, then we have log(nHT) ~ 4.7-4.8 cm-3 K, nH ~ 15-20 cm-3, ne ~ 0.2 cm-3, ne/nH ~ 0.01, and a total thickness of 0.7-0.9 pc. Absorption from a number of singly and doubly ionized species, perhaps due to a radiative shock, is seen at -108 km s-1 ≲ v☉ ≲ -83 km s-1. While the depletions in these ionized components are uncertain owing to unobserved ionization stages, aluminum (typically severely depleted) is probably depleted there by only a factor ~3, even at cloud velocities in excess of 100 km s-1. The individual high-velocity components typically have T ~ 8000 ± 2000 K, ne = nH ~ 0.4-0.5 cm-3, thermal pressure log(2neT) ~ 3.7-4.0 cm-3 K, and thicknesses of order 0.1 pc. Weak absorption components from ionized (H II) gas are seen in C II, Mg II, and Si III at intermediate velocities (-43 km s-1 ≲ v☉ ≲ -4 km s-1). Broad, weak absorption from the higher ions S III, C IV, Si IV, and N V is centered at -5 km s-1 ≲ v☉ ≲ +6 km s-1. No obvious absorption is discerned from a circumstellar H II region around 23 Ori itself. The large range in ne (from 0.04 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. Charge exchange with protons may reduce the abundances of S I, Mn I, and Fe I; dissociative recombination of CH+ may help to enhance C I. The large apparent fractional ionization in the SLV and WLV gas may be due to an enhanced flux of X-rays in the Orion region, to mixing of neutral and ionized gas at the boundary of the Orion-Eridanus bubble, or perhaps (in part) to charge exchange between singly ionized atomic species and large molecules (in which case the true ne would be somewhat smaller). Comparisons of the SLV depletions and nH with those found for the strong "component B" (v☉ ~ -14 km s-1) blend toward ζ 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.