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 lambda 1356 and H I Ly alpha 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 sub(2)), over 2 orders of magnitude in < n sub(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 sub(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 sub(H) > = 1.5 cm super(-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 super(log)10 (O/H) = -3.41 plus or minus 0.01 (or 390 plus or minus 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 super(log)10 (O/H) = -3.55 plus or minus 0.02 (284 plus or minus 12 ppm). The data points for low-< n sub(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.