The Small Magellanic Cloud Investigation of Dust and Gas Evolution
  (SMIDGE): The Dust Extinction Curve from Red Clump Stars
  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 8-band ultra-violet 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 $R_{475} = A_{475}/(A_{475}-A_{814})$ = 2.65
$\pm$ 0.11. This measurement is significantly larger than the equivalent values
of published Milky Way $R_{V}$ = 3.1 ($R_{475} = 1.83$) and SMC Bar $R_{V}$ =
2.74 ($R_{475} = 1.86$) 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 a full-width half-max
depth of 10 $\pm$ 2 kpc in the stellar distribution of the SMC (5 $\pm$ 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
broad-band HST imaging for simultaneously constraining dust and galactic
structure outside the Milky Way.
