Probing the Dust Responsible for Small Magellanic Cloud Extinction

Abstract

We explore the abundancesand depletion levelsof elementsimportant todustcomposition infourSmall Magellanic Cloud (SMC) sight lines observed with the Hubble Space Telescope. The apparent optical depth method is used to interpret the high-resolution ultraviolet absorption data toward AzV 18, AzV 456, Sk 108, and Sk 155; the latter two sightlineshavebeenpreviouslyexploredusingcomponentfitting.Incontrasttopreviousstudies,wefindevidencethat silicon isdepletedinthe SMC’s interstellar medium (ISM). This makes itimprobable thatSMC-like extinction,i.e., no 2175 8 bump and a steep far-ultraviolet rise, results from a lack of silicate grains in the SMC. Notable differences betweenoursightlinesthathaveSMC-likeandMilkyWay‐likeextinction,AzV18andAzV456,respectively,arethat the former has a substantially larger gas-to-dust ratio and smaller fractional H2 abundance. Iron abundances and depletions in the SMC’s ISM often diverge from the pattern shown by Si and Mg. This is evidence that Fe is not tied to the same grains as silicon, and therefore that most silicate grains are likely magnesium based. The presence of iron depletion in the SMC then suggeststhatthiselement isprobably incorporatedinto grain types suchas metals oroxides. Subject headings: dust, extinction — galaxies: ISM — ISM: abundances — Magellanic Clouds — ultraviolet: ISM