Imaging Polarimetry of the Yellow Hypergiant IRC+10420 at 2.2 µm with MMTPOL

We present imaging polarimetry of the circumstellar environment of the yellow hypergiant star IRC +10420 obtained as part of first light observations with MMTPOL, the 1 - 5 µm imaging polarimeter commissioned in Fall 2012 at the 6.5 m MMT telescope on Mount Hopkins, AZ. Achieving an angular resolution at λ = K (2.2 µm) of 0.2″ (near the diffraction limit despite windy non-photometric conditions), MMTPOL has resolved polarized emission from IRC +10420’s circumstellar nebula to within ~ 0.5″ of the bright central star. The polarization position angles exhibit a clear circosymmetric pattern consistent with scattering from dust grains formed during a period of sustained mass loss. We map fractional polarizations from ~ 5% to 50% for radii between 0.5″ to 3.0″ from the star (2,500 to 15,000 AU at 5 kpc), with the highest polarization found at a radius of 1.6″. Assuming scattering from astronomical silicate grains with average radius ~ 0.3 µm (a typical size as found recently for the hypergiant star VY CMa), this high polarization is consistent with scattering angles of either ~ 60° and/or ~ 120°. This places the grains at ~ 30° in front of the plane of the sky or behind it, respectively.