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Session 44 - The Local Diffuse ISM.
Display session, Tuesday, June 11
Great Hall,

[44.05] GHRS Observations of the Interstellar Medium toward 23 Ori

D. E. Welty, L. M. Hobbs, J. T. Lauroesch (U. Chicago), D. C. Morton (Herzberg Inst.), L. Spitzer (Princeton U.), D. G. York (U. Chicago)

High-resolution optical spectra, GHRS echelle and G160M spectra, and Copernicus spectra are combined to study the abundances and physical conditions in the diffuse interstellar clouds seen toward 23 Ori. High-resolution (0.3--2.0 km s^-1) spectra of Na I, Ca I, Ca II, and K I are used to define the underlying component structure that is unresolved in the UV spectra, enabling more accurate individual cloud abundances and physical conditions to be derived from the numerous transitions found in the UV. Multiple absorption components are present for each of several distinct types of gas. Strong low-velocity (SLV) absorption, due to cool, moderately dense neutral gas, is seen for neutral and singly ionized species at 20 km s^-1 \la v_ødot \la 30 km s^-1. Weaker low-velocity (WLV) absorption, probably due to warmer gas, is seen primarily for singly ionized species at 0 km s^-1 \la v_ødot \la 20 km s^-1. Weak absorption from C II, Mg II, and Si III is seen at -45 km s^-1 \la v_ødot \la 0 km s^-1, and from more highly ionized species at -5 km s^-1 \la v_ødot \la 25 km s^-1. Absorption from singly and doubly ionized species is seen at -110 km s^-1 \la v_ødot \la -80 km s^-1; this apparently shock-ionized gas is not in collisional ionization equilibrium, having cooled more quickly than it could recombine. The various types of gas present along this line of sight are characterized by different relative abundance / depletion patterns; these depletion patterns are compared with the physical conditions derived for the different components. For the SLV components, most typically depleted species are less depleted by factors of 2--4, compared to the ``cold, dense cloud'' pattern found, for example, toward \zeta Oph. The depletions in the WLV gas are typically another factor of 2--3 less than in the SLV gas, similar to the ``warm cloud'' pattern seen in less reddened lines of sight. While there is some uncertainty, due to unobserved ionization stages, as to the depletions in the high-velocity gas, Si and Al apparently are depleted by less than factors of \sim2 and \sim5, respectively, relative to C, N, and S.

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