Near-infrared polarimetry and photometry of recent comets

We present near-infrared (2.2 μm) imaging polarimetry and near-infrared photometry for recent comets. The 2.2 μm polarization as a function of phase angle is 1%-2% higher than typical comet visual narrowband red polarization. This implies a general, but weak, trend to higher polarization at wavelengths longer than 0.7 μm. As found with visual narrowband red polarimetry, comets appear to divide into a high- and a low-polarization class at 2.2 μm as well. The high-polarization class is characterized by moderate to strong mid-infrared silicate emission, while the low-polarization class is characterized by weak to absent silicate emission. In contrast to C/1995 O1 (Hale-Bopp) and some other dusty comets, no significant grain population radial gradient is seen in any of the polarization maps for the six comets in our sample with near-infrared imaging polarimetry. Our analysis of previously published Hale-Bopp data shows that the polarization at 40° phase angle varies by only about a factor of 1.6 from 0.3 to 2.2 μm and is nearly flat from 0.7 to 2.2 μm. Any model of comet dust must reproduce this weak wavelength dependence.