Linear polarimetry of five comets

Abstract

We made linear polarimetry of five comets, C/1989 X1 (Austin), 109P/Swift-Tuttle, C/1996 B2 (Hyakutake), C/1995 O1 (Hale-Bopp), and C/1996 Q1 (Tabur) simultaneously at eight wavelengths, in order to study phase angle and wavelength dependence of polarization in continuum light, and polarization behavior of molecular origin. Polarization in continuum of Swift-Tuttle and Hyakutake were rather high i.e. similar to that of 1P/Halley, while those of Austin and Tabur were low in the positive branch. Hale-Bopp showed the highest polarization ever observed at the corresponding phase angles in the positive branch. We also made observations scanning across the coma for Hyakutake and Hale-Bopp to investigate polarization structure. For Hale-Bopp, we found: (1) The position of maximum polarization was not at the visible center, and a red part was observed in the central region. (2) Optical properties and/or size spectrum of dust varied in a jet. On the contrary, any peculiar structure was not found for Hyakutake in spite of a much finer spatial resolution. The scanning observation of Hale-Bopp suggests that dust including its size spectrum evolves after the ejection from the nucleus, and that the optical depth in the central region plays an important role.