Unusual Depletions toward the SMC Star S[CLC]k[/CLC] 155—Differences in Dust Composition in the SMC Interstellar Medium?

Abstract

We report initial results from an analysis of Hubble Space Telescope/Space Telescope Imaging Spectrograph echelle spectra of interstellar absorption toward Sk 155, located in the wing region of the SMC. There are at least 12 Milky Way (disk and halo) and at least 25 SMC components discernible in the profiles of various neutral and singly ionized species. Fits to the line profiles yield column densities and relative elemental abundances for the individual components. In the SMC components, the depletions of Fe and Ni range from mild ([Fe, Ni/Zn] ~ -0.3 to -0.8 dex) to severe ([Fe, Ni/Zn] ~ -1.7 dex); Mg and Si, however, appear to be essentially undepleted ([Mg, Si/Zn] ~ -0.1 to +0.2 dex) throughout. The combination of severe depletion for Cr, Mn, Fe, and Ni and negligible depletion for Mg and Si has not been seen in any Galactic sight line. If Si is generally undepleted in the SMC, then current models of the SMC dust—which rely heavily on silicates—will have to be reconsidered; oxides and/or metallic grains may dominate. We briefly discuss possible the implications of these SMC depletions for understanding the elemental abundances in QSO absorption-line systems.