Interstellar Iron and Silicon Depletions in Translucent Sight Lines

We report interstellar FeII and SiII column densities toward six translucentsight lines (A_V >~ 1) observed with the Space Telescope Imaging Spectrograph(STIS). The abundances were determined from the absorption of SiII] at 2335Angstroms, and several weak Fe transitions including the first reporteddetections of the 2234 Angstrom line. We derive an empirical f-value for theFeII 2234 Angstrom transition of log(f lambda) = -1.54 +/- 0.05. The observedsight lines sample a variety of extinction characteristics as indicated bytheir R_V values, which range from 2.6 - 5.8. The dust-phase abundances of bothSi and Fe are positively correlated with the small-grain population (effectiveradii smaller than a few hundred micron) toward the targets. The physicalconditions along the sight lines suggest that this relationship may be due todifferences in the survival of small particles in some interstellarenvironments. The chemical composition of the small grains could eitherresemble dust mantles or be silicate rich.Comment: accepted for publication in Ap