Probing the dust responsible for Small Magellanic Cloud extinction

We explore the abundances and depletion levels of elements important to dust composition in four Small 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 sight lines have been previously explored using component fitting. In contrast to previous studies, we find evidence that silicon is depleted in the SMC's interstellar medium ( ISM). This makes it improbable that SMC-like extinction, i. e., no 2175 angstrom bump and a steep far- ultraviolet rise, results from a lack of silicate grains in the SMC. Notable differences between our sight lines that have SMC-like and Milky Way - like extinction, AzV 18 and AzV 456, respectively, are that the former has a substantially larger gas-to-dust ratio and smaller fractional H-2 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 suggests that this element is probably incorporated into grain types such as metals or oxides.