The Panchromatic Hubble Andromeda Treasury. VII. The Steep
  Mid-Ultraviolet to Near-Infrared Extinction Curve in the Central 200 pc of
  the M31 Bulge
  We measure the extinction curve in the central 200 pc of M31 at
mid-ultraviolet to near-infrared wavelengths (from 1928A to 1.5{\mu}m), using
Swift/UVOT and HST WFC3/ACS observations in thirteen bands. Taking advantage of
the high angular resolution of the HST WFC3 and ACS detectors, we develop a
method to simultaneously determine the relative extinction and the fraction of
obscured starlight for five dusty complexes located in the circumnuclear
region. The extinction curves of these clumps (RV =2.4-2.5) are steeper than
the average Galactic one (RV =3.1), but are similar to optical and
near-infrared curves recently measured toward the Galactic Bulge (RV~2.5). This
similarity suggests that steep extinction curves may be common in the inner
bulge of galaxies. In the ultraviolet, the extinction curves of these clumps
are also unusual. We find that one dusty clump (size <2 pc) exhibits a strong
UV bump (extinction at 2175A), more than three standard deviation higher than
that predicted by common models. Although the high stellar metallicity of the
M31 bulge indicates that there are sufficient carbon and silicon to produce
large dust grains, the grains may have been destroyed by supernova explosions
or past activity of the central super-massive black hole, resulting in the
observed steepened extinction curve.
