AAS 198th Meeting, June 2001
Session 9. Dwarf, Irregular and Starburst Galaxies
Display, Monday, June 4, 2001, 9:20am-6:30pm, Exhibit Hall

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[9.03] Unusual Depletions toward the SMC Star Sk 155 --- Differences in Dust Composition in the SMC ISM?

D.E. Welty (U. Chicago), J.T. Lauroesch (Northwestern), J.C. Blades (STScI), E.L. Fitzpatrick (Villanova), L.M. Hobbs, D.G. York (U. Chicago)

We report initial results from an analysis of HST/STIS 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 very mild ([Fe,Ni/Zn] > -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 sightline. 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 implications of SMC-like depletions for understanding the elemental abundances in QSO absorption-line systems.

Support for this work has been provided by NASA through grant HST-GO-08145.01-A (administered by STScI) and through LTSA grant NAG5-3228 (both to the University of Chicago).

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The author(s) of this abstract have provided an email address for comments about the abstract: welty@oddjob.uchicago.edu

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