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 lambda1356 and H I Lyalpha 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 log(10)(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 (Andre 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 log(10)(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.