The Homogeneity of Interstellar Oxygen in the Galactic Disk

We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of O I λ1356 and H I Lyman-α 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(H2), over 2 orders of magnitude in 〈nH〉, 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 HighResolution 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 (〈nH〉) 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 〈nH〉 = 1.5cm −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 log10(O/H) = -3.41±0.01 (or 390±10 ppm),