2017ApJ...847..102Y

We use Hubble Space Telescope (HST) observations of red clump stars taken as part of the Small Magellanic Cloud Investigation of Dust and Gas Evolution (SMIDGE) program to measure the average dust extinction curve in a ∼200 pc x 100 pc region in the southwest bar of the Small Magellanic Cloud (SMC). The rich information provided by our eight-band ultraviolet through near-infrared photometry allows us to model the color-magnitude diagram of the red clump accounting for the extinction curve shape, a log-normal distribution of A_V, and the depth of the stellar distribution along the line of sight. We measure an extinction curve with R₄₇₅ =A₄₇₅/(A₄₇₅–A₈₁₄)=2.65±0.11. This measurement is significantly larger than the equivalent values of published Milky Way (MW) R_V = 3.1 (R₄₇₅=1.83) and SMC Bar R_V = 2.74 (R₄₇₅=1.86) extinction curves. Similar extinction curve offsets in the Large Magellanic Cloud (LMC) have been interpreted as the effect of large dust grains. We demonstrate that the line-of-sight depth of the SMC (and LMC) introduces an apparent "gray" contribution to the extinction curve inferred from the morphology of the red clump. We show that no gray dust component is needed to explain extinction curve measurements when FWHM depth of 10 ± 2 kpc in the stellar distribution of the SMC (5 ± 1 kpc for the LMC) is considered, which agrees with recent studies of Magellanic Cloud stellar structure. The results of our work demonstrate the power of broadband HST imaging for simultaneously constraining dust and galactic structure outside the MW.