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

We have obtained high-resolution Hubble Space Telescope (HST) Space Telesco pe Imaging Spectrograph (STIS) observations of O I lambda 1356 and Kr I lam bda 1236 absorption in 11 sight lines characterized by high extinction, lar ge H I column densities, and/or long path lengths. Previous Goddard High Re solution Spectrograph (GHRS) measurements of these weak features in seven r elatively nearby diffuse clouds have shown no evidence for density-dependen t depletion of either oxygen or krypton and have yielded a weighted mean ga s-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 sig ht lines pass near dense, star-forming regions, notably the rho Oph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas, should not be deple ted much into grains, these cases suggest a trend toward the enhanced oxyge n depletion predicted for denser ISM clouds.