The diffuse interstellar clouds toward 23 Orionis

Spectra obtained with the Hubble Space Telescope Goddard High Resolution Sp ectrograph are combined with high-resolution optical spectra and UV spectra from Copernicus to study the abundances and physical conditions in the dif fuse interstellar clouds seen along the line of sight to the star 23 Ori. M ultiple absorption components are present for each of several distinct type s of gas, which are characterized by different relative abundance and deple tion 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 ne utral and singly ionized species at + 20 km s(-1) less than or similar to u psilon. less than or similar to + 26 km s(-1). Most typically severely depl eted species are less depleted by factors of 2-4, compared to the "cold, de nse cloud" pattern found, for example, in the main components toward zeta O ph. For the two strongest SLV components, T similar to 100 K and the therma l pressure log (n(H)T) similar to 3.1 cm(-3) K; we thus have n(H) similar t o 10-15 cm(-3) and a total thickness of 12-16 pc. The adopted average SLV e lectron density, n(e) = 0.15 +/- 0.05 cm(-3), implies a relatively large n( e)/n(H) similar to 0.01 and thus some ionization of hydrogen in these predo minantly neutral components. Weaker low-velocity (WLV) absorption, probably largely due to warmer neutra l gas, is seen primarily for various singly ionized species at + 0 km s(-1) less than or similar to upsilon. less than or similar to + 30 km s(-1). Th e depletions in the WLV gas are typically less severe by a factor of 2-3 th an 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 molecul ar fraction. If T similar to 3000 K for the WLV components, then we have lo g (n(H) T) similar to 4.7-4.8 cm(-3) K, n(H) similar to 15-20 cm(-3), n(e) similar to 0.2 cm(-3), n(e)/n(H) similar to 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) less than or similar to ups ilon. less than or similar to -83 km s(-1). While the depletions in these i onized components are uncertain owing to unobserved ionization stages, alum inum (typically severely depleted) is probably depleted there by only a fac tor similar to 3, even at cloud velocities in excess of 100 km s(-1). The i ndividual high-velocity components typically have T similar to 8000 +/- 200 0 K, n(e) = n(H) similar to 0.4-0.5 cm(-3), thermal pressure log (2n(e) T) similar to 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) less than or similar t o upsilon. less than or similar to -4 km s(-1)). Broad, weak absorption fro m the higher ions S III, C IV, Si IV, and N V is centered at -5 km s(-1) le ss than or similar to upsilon. less than or similar to + 6 km s(-1). No obv ious absorption is discerned from a circumstellar H n region around 23 Ori itself. The large range in n, (from 0.04 to 0.95 cm(-3)) derived independently from nine pairs of neutral and singly ionized species in the SLV gas suggests t hat additional processes besides simple photoionization and radiative recom bination affect the ionization balance. Charge exchange with protons may re duce the abundances of S I, Mn I, and Fe I; dissociative recombination of C H+ 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 regi on, to mixing of neutral and ionized gas at the boundary of the Orion-Erida nus bubble, or perhaps (in part) to charge exchange between singly ionized atomic species and large molecules (in which case the true n(e) would be so mewhat smaller). Comparisons of the SLV depletions and n, with those found for the strong "component B" (upsilon. similar to -14 km s(-1)) blend towar d zeta Oph hint at a possible relationship between depletion and local dens ity for relatively cold interstellar clouds. Calcium appears to be more sev erely depleted in warm, low density gas than has generally been assumed. An appendix summarizes the most reliable oscillator strengths currently avail able for a number of the interstellar absorption lines analyzed in this wor k.