Infrared polarimetry of Mrk 231: Scattering off hot dust grains in the
  central core
  We present high-angular (0.17$-$0.35 arcsec) resolution imaging polarimetric
observations of Mrk 231 in the 3.1 $\mu$m filter using MMT-Pol on the 6.5-m
MMT, and in the 8.7 $\mu$m, 10.3 $\mu$m, and 11.6 $\mu$m filters using
CanariCam on the 10.4-m Gran Telescopio CANARIAS. In combination with already
published observations, we compile the 1$-$12 $\mu$m total and polarized
nuclear spectral energy distribution (SED). The total flux SED in the central
400 pc is explained as the combination of 1) a hot (731 $\pm$ 4 K) dusty
structure, directly irradiated by the central engine, which is at 1.6 $\pm$ 0.1
pc away and attributed to be in the pc-scale polar region, 2) an
optically-thick, smooth and disk-like dusty structure (`torus') with an
inclination of 48 $\pm$ 23$^{\circ}$ surrounding the central engine, and 3) an
extinguished (A$_{\mbox{V}} =$ 36 $\pm$ 5 mag) starburst component. The
polarized SED decreases from 0.77 $\pm$ 0.14 per cent at 1.2 $\mu$m to 0.31
$\pm$ 0.15 per cent at 11.6 $\mu$m and follows a power-law function,
$\lambda^{\sim0.57}$. The polarization angle remains constant
($\sim$108$^{\circ}$) in the 1$-$12 $\mu$m wavelength range. The dominant
polarization mechanism is explained as scattering off hot dust grains in the
pc-scale polar regions.
