PROBING HYPERGIANT MASS LOSS WITH ADAPTIVE OPTICS IMAGING AND POLARIMETRY IN THE INFRARED: MMT-Pol AND LMIRCam OBSERVATIONS OF IRC+10420 AND VY CANIS MAJORIS

We present 2-5 mu m adaptive optics (AO) imaging and polarimetry of the famous hypergiant stars IRC + 10420 and VY Canis Majoris. The imaging polarimetry of IRC + 10420 with MMT-Pol at 2.2 mu m resolves nebular emission with intrinsic polarization of 30%, with a high surface brightness indicating optically thick scattering. The relatively uniform distribution of this polarized emission both radially and azimuthally around the star confirms previous studies that place the scattering dust largely in the plane of the sky. Using constraints on scattered light consistent with the polarimetry at 2.2 mu m, extrapolation to wavelengths in the 3-5 mu m band predicts a scattered light component significantly below the nebular flux that is observed in our Large Binocular Telescope/LMIRCam 3-5 mu m AO imaging. Under the assumption this excess emission is thermal, we find a color temperature of similar to 500 K is required, well in excess of the emissivity-modified equilibrium temperature for typical astrophysical dust. The nebular features of VY CMa are found to be highly polarized (up to 60%) at 1.3 mu m, again with optically thick scattering required to reproduce the observed surface brightness. This star's peculiar nebular feature dubbed the Southwest Clump is clearly detected in the 3.1 mu m polarimetry as well, which, unlike IRC + 10420, is consistent with scattered light alone. The high intrinsic polarizations of both hypergiants' nebulae are compatible with optically thick scattering for typical dust around evolved dusty stars, where the depolarizing effect of multiple scatters is mitigated by the grains' low albedos.