Oxygen Gas Phase Abundance Revisited

We present new measurements of the interstellar gas-phase oxygen abundance along the sight lines towards 19 early-type galactic stars at an average distance of 2.6 kpc. We derive O I column densities from HST/STIS observations of the weak 1355 Å intersystem transition. We derive total hydrogen column densities [N(H I)+2N(H2)] using HST/STIS observations of Lyman-α and FUSE observations of molecular hydrogen. The molecular hydrogen content of these sight lines ranges from f(H2) = 2N(H2)/[N(H I)+2N(H2)] = 0.03 to 0.47. The average 〈Htot/EB−V 〉 of 6.3×10 21 cm mag with a standard deviation of 15% is consistent with previous surveys. The mean oxygen abundance along these sight lines, which probe a wide range of galactic environments in the distant ISM, is 10 (O/H)gas = 408 ± 13 (1 σ in the mean). We see no evidence for decreasing gas-phase oxygen abundance with increasing molecular hydrogen fraction and the relative constancy of (O/H)gas suggests that the component of dust containing the oxygen is not readily destroyed. We estimate that, if 60% of the dust grains are resilient against destruction by shocks, the distant interstellar total oxygen abundance can be reconciliated with the solar value derived from the most recent measurements of 10 (O/H)gas⊙ = 517 ± 58 (1 σ). We note that the smaller oxygen abundances derived for the interstellar gas within 500 pc or from nearby B star surveys are consistent with a local elemental deficit. Subject headings: ISM: oxygen abundance, dust, galactic gradient; spectroscopy: far ultraviolet. Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France, [email protected] The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA