Echelle spectroscopy of interstellar absorption toward mu Columbae with the Goddard high resolution spectrograph

Goddard High Resolution Spectrograph echelle-mode observations of the inter stellar absorption lines of Mg II, Si IV, C IV, and N V toward mu Columbae (HD 38666) are presented. The observations have a spectral resolution of 3. 5 km s(-1) and signal-to-noise ratios (S/Ns) of 20-200. The mu Col sight li ne (l = 237.degrees 3, b = -27.degrees 1, d = 0.40 kpc, z = -0.18 kpc) exte nds though the Local Bubble and the warm neutral, warm ionized, and hot ion ized phases of the interstellar medium (ISM). The high-ionization column de nsities toward mu Col are log N(Si IV) = 12.16 +/- 0.05, log N(C IV) = 12.8 8 +/- 0.02, and log N(N V) = 11.8-12.3. Profile fits to Copernicus satellit e measures of O VI absorption toward mu Col yield log N(O VI) = 13.82+/-0.0 1 and b = 38.7 km s(-1). This implies N(C IV)/N(O VI) = 0.11 +/- 0.01, whic h is typical of the values found for the hot ISM of the Galactic disk. The O vr profile is twice as broad as the C rv and N V profiles, even though th ese species have roughly similar average velocities. Some of the C IV, N V, and O VI absorption toward mu Col may occur at the interface of the Local Cloud and Local Bubble, although additional contributions to these ions pro bably also occur in more distant gas along the sight line. A substantial pa rt of the Si Iv absorption likely arises in warm photoionized gas in an H I I region surrounding mu Col. The profile width differences among the high-i onization lines of C IV, N V, and O VI could be produced if the line of sig ht passes through a highly evolved supernova remnant. The observations for mu Col and for other stars observed at high resolution with the GHRS reveal that multiple gas types (warm and hot) contribute to the absorption by the highly ionized atoms along both nearby and distant sight lines. Disentangl ing the relative contributions from the different gas types requires high-r esolution and high-S/N observations. The Mg II observations, combined with a solar Mg reference abundance, imply that the Mg depletion toward mu Col i s -0.31 dex. As observed for other sight lines through the warm neutral med ium, the gas-phase observations of Mg, when combined with results for Fe an d Si, suggest that Mg and Fe are more deficient from the gas phase than one would expect if these elements are only contained in silicate dust grains.