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S.I.B. Cartledge, D.M. Meyer, J.T. Lauroesch (Northwestern U.), U.J. Sofia (Whitman College)
We present HST STIS measurements of the interstellar O I, Mg II, P II, Mn II, Ni II, Cu II, Ge II, and Kr I absorption lines in over 40 sight lines with pathlengths of up to 5 kpc through the Galactic disk. Utilizing targeted and archival FUSE data, we have also obtained H2 measurements for over half of our sight lines. A trend toward lower gas-phase elemental abundance ratios with increasing mean total hydrogen sight line density (< nH > = N(H)/d) is evident for each element except krypton. Although such trends have previously been identified by Copernicus measurements, they are more distinct in our STIS/FUSE data which benefit from the improved measurement capabilities of these instruments. The case of oxygen in low density sight lines (< nH > less than 1 cm-3) is particularly intriguing. Although there is no trend with < nH > in this sample, shorter sight lines (d < 750 pc) have about 0.15 dex lower gas-phase oxygen abundances than for longer sight lines. In addition, we report the first sight line (HD152590) with an apparently solar Kr/H ratio. Since the inferred depletions for other elements in this sight line are characteristic of the cold ISM, the krypton enhancement appears to be a nucleosynthetic effect.
Financial support for this work was provided by the Space Telescope Science Institute through grants to Northwestern Univerisity and Whitman College.