Infrared Imaging Polarimetry of Comet C/1995 01 (Hale–Bopp)

We present 2.2 μm ( K ) imaging infrared (IR) polarimetry of Comet Hale–Bopp at three different epochs: 1996 April and August and 1997 April. We also present 1.5- and 1.7-μm imaging polarimetry and photometry from 1.7 to 3.8 μm in 1997 April. In 1996 April and August the comet was ≤ 0.5% polarized in the K band. By 1997 April the comet was strongly polarized from 1.5 to 2.2 μm—more polarized than is typical for a comet at optical wavelengths. This suggests that the grains or grain aggregates are behaving more like Rayleigh particles at λ=1.5 μm. The percentage polarization was not uniform across the coma. It was higher in the jet, in particular at the leading edge of the jet and in the more distant (older) jet arc or halo. This behavior is most easily explained by a population of small grains present in the jet that are continuing to break up several hours after release from the nucleus. The 1.7- to 3.8-μm color images of the comet are consistent with this conclusion, although changes in the grain population in the jet between epochs are also probably a factor.