Abundances and Depletions of Interstellar Oxygen

We report on the abundance of interstellar neutral oxygen (O I) for 26 sight lines, using data from the Far Ultraviolet Spectroscopic Explorer, the International Spectroscopic Explorer, and the Hubble Space Telescope. O I column densities are derived by measuring the equivalent widths of several ultraviolet absorption lines and subsequently fitting those to a curve of growth. We consider both our general sample of 26 sight lines and a more restrictive sample of 10 sight lines that utilize HST data for a measurement of the weak 1355 Å line of oxygen and are thus better constrained owing to our sampling of all three sections of the curve of growth. The column densities of our HST sample show ratios of O/H that agree with the current best solar value if dust is considered, with the possible exception of one sight line (HD 37903). We note some very limited evidence in the HST sample for trends of increasing depletion with respect to RV and f(H2), but the trends are not conclusive. Unlike a recent result from Cartledge et al., we do not see evidence for increasing depletion with respect to ⟨nH⟩, but our HST sample contains only two points more dense than the critical density determined in that paper. The column densities of our more general sample show some scatter in O/H, but most agree with the solar value to within errors. We discuss these results in the context of establishing the best method for determining interstellar abundances, the unresolved question of the best value for O/H in the interstellar medium, the O/H ratios observed in Galactic stars, and the depletion of gas-phase oxygen onto dust grains.