THE TRANSLUCENT MOLECULAR CLOUDS TOWARD HD-154368 .1. EXTINCTION, ABUNDANCES, AND DEPLETIONS

We report the results of a comprehensive analysis of a line of sight o bserved using the Goddard High Resolution Spectrograph (GHRS) on the H ubble Space Telescope (HST), toward HD 154368, an 09.5 Iab star locate d about 800 pc away. The line of sight intersects translucent interste llar cloud material, having a color excess E(B - V) = 0.82 and known h igh molecular abundances from ground-based data. The HST observations consist of high-resolution spectra obtained with grating ECH-B at wave lengths longward of 1800 Angstrom, and moderate-resolution spectra fro m grating G160M at shorter wavelengths. We observed some 19 wavelength settings, covering the positions of numerous atomic and molecular tra nsitions. We also incorporate IUE and ground-based data in the study, the latter coming from several observatories at visible and millimeter wavelengths. In this paper we report on the general goals and results of the study, with emphasis on the atomic abundances and depletions. We find that the column densities of most species are slightly higher, relative to the adopted total gas column density, than in some thinne r clouds in which most of the absorption arises in a single dense comp onent. Consequently the depletions of elements from the gas phase onto the dust are less than in ''typical'' diffuse clouds such as the one toward zeta Oph. Most of the gas toward HD 154368 resides in two main clouds, centered near -3.26 and -20.95 km s(-1) (heliocentric). Our pr ofile analyses show that the depletions in these two clouds are simila r. We discuss the possible relationship of this result to the ultravio let extinction curve toward HD 154368, derived from IUE spectra.