Space telescope imaging spectrograph observations of interstellar oxygen and krypton in translucent clouds

We have obtained high-resolution Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) observations of O I λ1356 and Kr I λ1236 absorption in 11 sight lines characterized by high extinction, large H I column densities, and/or long path lengths. Previous Goddard High Resolution Spectrograph (GHRS) measurements of these weak features in seven relatively nearby diffuse clouds have shown no evidence for density-dependent depletion of either oxygen or krypton and have yielded a weighted mean gas-phase abundance ratio of log [N(O)/N(Kr)]_GHRS = 5.56 ± 0.04. Our STIS measurements yield a lower weighted mean of log [N(O)/N(Kr)]_STIS = 5.48; the difference is due primarily to several translucent sight lines in the STIS data set that diverge from the GHRS value. These translucent cloud sight lines pass near dense, star-forming regions, notably the p Oph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas, should not be depleted much into grains, these cases suggest a trend toward the enhanced oxygen depletion predicted for denser ISM clouds.