Abundances and physical conditions in the warm neutral medium toward mu Columbae

We present ultraviolet interstellar absorption-line measurements for the si ghtline toward the O9.5 V star mu Columbae (l = 237.degrees 3, b = -27.degr ees 1; d approximate to 400 pc, z approximate to 180 pc; [n(H I)] approxima te to 0.06 cm(-3)) obtained with the Goddard High Resolution Spectrograph ( GHRS) on board the Hubble Space Telescope. These archival data represent th e most complete GHRS interstellar absorption-line measurements for any line of sight toward an early-type star. The 3.5 km s(-1) resolution of the ins trument allows us to accurately derive the gas-phase column densities of ma ny important ionic species in the diffuse warm neutral medium, including ac counting for saturation effects in the data and for contamination from ioni zed gas along this sightline. For the low-velocity material (-20 less than or similar to v(LSR) less than or similar to +15 km s(-1)), we use the appa rent column density method to derive column densities. For the individual a bsorbing components at v(LSR) approximate to -28.8, +20.1, +31.0, and +41.2 km s(-1), we apply component fitting techniques to derive column densities and b-values. We have also used observations of interstellar Ly alpha abso rption taken with the GHRS intermediate resolution gratings to accurately d erive the H I column density along this sightline. The resulting interstell ar column density, log N(H I) = 19.86 +/- 0.015, is in agreement with other determinations but is significantly more precise. The low-velocity materia l shows gas-phase abundance patterns similar to the warm cloud (cloud A) to ward the disk star zeta Ophiuchi, while the component at v(LSR) approximate to +20.1 km s(-1) shows gas-phase abundances similar to those found in war m halo clouds. We find that the velocity-integrated gas-phase abundances of Zn, P, and S relative to H along this sightline are indistinguishable from solar system abundances. We discuss the implications of our gas-phase abun dance measurements for the composition of interstellar dust grains. We find a dust-phase abundance [(Fe + Mg)/Si](d) = 2.7-3.3 in the low-velocity gas ; therefore the dust cannot be composed solely of common silicate grains, b ut must also include oxides or pure iron grains. The low-velocity material along this sightline is characterized by T approximate to 6000-7000 K with n(e) approximate to 0.3 cm(-3), derived from the ionization equilibrium of Mg and Ca. The relative ionic column density ratios of the intermediate-vel ocity components at v(LSR) = +31.0 and +41.2 km s(-1) show the imprint both of elemental incorporation into grains and (photo)ionization. These clouds have low total hydrogen column densities [log N(H) similar to 17.4-17.7], and our component fitting b-values constrain the temperature in the highest velocity component to be T = 4000 + 700 K. The electron density of this cl oud is n(e) approximate to 0.6 cm(-3), derived from the P-2(1/2) to P-2(3/2 ) fine structure excitation of C II. The components at v(LSR) approximate t o -30 and -48 km s(-1) along this sightline likely trace shocked gas with v ery low hydrogen column densities. The v(LSR) approximate to -30 km s(-1) c omponent is detected in a few strong low-ionization lines, while both are e asily detected in Si III. The relative column densities of the -30 km s(-1) suggest that the gas is collisionally ionized at moderate temperatures (T approximate to 25,000 K). This is consistent with the measured b-values of this component, though non thermal motions likely contribute significantly to the observed breadths.