Depletions of Fe, Si and P in the Neutral Interstellar Medium Determined from IMAPS Spectra
Session 35 -- ISM: Abundances and Dust
Display presentation, Wednesday, June 14, 1995, 9:20am - 6:30pm

## [35.05] Depletions of Fe, Si and P in the Neutral Interstellar Medium Determined from IMAPS Spectra

U. J. Sofia (NRC, NASA/GSFC), C. L. Joseph (Univ. of Wisc. SAL)

Abundances and depletions are measured for iron, silicon, and phosphorus in the neutral interstellar medium (ISM) toward 9 lightly reddened stars observed with the Interstellar Medium Absorption Profile Spectrograph (IMAPS). Spectra obtained with IMAPS have a resolution $\lambda/\Delta\lambda \ge$ 200,000 ($\sim$2 km s$^{-1}$) and cover the wavelength range 940 - 1160 \AA. The high-resolution of these data allows individual absorbing clouds to be studied, and helps to alleviate the problem of blended saturated lines which is often a large source of error in abundance determinations.

Fe and Si are very important constituents of dust grains. The majority of Fe in diffuse neutral interstellar clouds is always incorporated into dust, therefore it is likely to reside primarily in resilient grain cores. Silicates are known to exist in the ISM, however, it is not clear whether most of the Si is in the cores or mantles of dust. P generally shows low to moderate levels of depletion from the gas-phase ISM, and is probably only present in grain mantles. The different manner in which these elements are incorporated into interstellar grains can be used to explore the characteristics of dust in clouds with measured depletions. We discuss preliminary results from our study of the sightlines toward $\epsilon$ Ori, $\zeta$ Ori, $\kappa$ Ori, $\zeta$ Pup, $\gamma$ Cas, $\gamma^{2}$ Vel, $\beta$ Cen, $\beta$ Cru and $\alpha$ Eri.