ON THE UNUSUAL DEPLETIONS TOWARD Sk 155, OR WHAT ARE THE SMALL MAGELLANIC CLOUD DUST GRAINS MADE OF?

The dust in the Small Magellanic Cloud (SMC), an ideal analog of primordial galaxies at high redshifts, differs markedly from that in the Milky Way by exhibiting a steeply rising far-ultraviolet extinction curve, an absence of the 2175 extinction feature, and a local minimum at ∼12 mm in its infrared emission spectrum, suggesting Å the lack of ultrasmall carbonaceous grains (i.e., polycyclic aromatic hydrocarbon molecules), which are ubiquitously seen in the Milky Way. While current models for the SMC dust all rely heavily on silicates, recent observations of the SMC line of sight toward Sk 155 have indicated that Si and Mg are essentially undepleted and that the depletions of Fe range from mild to severe, suggesting that metallic grains or iron oxides, instead of silicates, may dominate the SMC dust. However, here we apply the Kramers-Kronig relation to demonstrate that neither metallic grains nor iron oxides are capable of accounting for the observed extinction; silicates remain as an important contributor to the extinction, consistent with current models for the SMC dust. Subject headings: dust, extinction — ISM: abundances — Magellanic Clouds — stars: individual (Sk 155)