The Homogeneity of Interstellar Oxygen in the Galactic Disk

We present an analysis of high-resolution Hubble Space Telescope ( HST ) Space Telescope Imaging Spectrograph (STIS) observations of O I λ1356 and H I Lyα absorption in 36 sight lines that probe a variety of Galactic disk environments and include paths that range over nearly 4 orders of magnitude in f (H 2 ), over 2 orders of magnitude in ⟨ n H ⟩, and that extend up to 6.5 kpc in length. Since the majority of these sight lines have also been observed by the Far Ultraviolet Spectroscopic Explorer ( FUSE ), we have undertaken the study of gas-phase O/H abundance ratio homogeneity using the current sample and previously published Goddard High Resolution Spectrograph (GHRS) results. Two distinct trends are identified in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance with increasing mean sight-line density (⟨ n H ⟩) and a gap between the mean O/H ratio for sight lines shorter and longer than about 800 pc. The first effect is a smooth transition between two depletion levels associated with large mean density intervals; it is centered near ⟨ n H ⟩ = 1.5 cm -3 and is similar to trends evident in gas-phase abundances of other elements. Paths less dense than the central value exhibit a mean O/H ratio of (O/H) = -3.41 ± 0.01 (or 390 ± 10 ppm), which is consistent with averages determined for several long low-density paths observed by STIS (André et al. 2003) and short low-density paths observed by FUSE (Moos et al. 2002). Sight lines of higher mean density exhibit an average O/H value of (O/H) = -3.55 ± 0.02 (284 ± 12 ppm). The data points for low-⟨ n H ⟩ paths are scattered more widely than those for denser sight lines, because O/H ratios for such paths shorter than 800 pc are generally about 0.10 dex lower than the values for longer ones. Scenarios that would be consistent with these results include a recent infall of metal-poor gas onto the local Galactic disk and an interstellar environment toward Orion that is conducive to reducing the apparent gas-phase oxygen abundance.