Polarimetric Observations of Comets C/1995 O1 Hale–Bopp and C/1996 B2 Hyakutake

Abstract

We present polarimetric observations of Comet C/1995 O1 Hale–Bopp. Polarimetric data was obtained with a Pockels cell polarimeter at the Observatoire du Mont Mégantic, between July 1996 and May 1997. An intermediate bandpass red filter and five cometary filters, in the continuum and in emission bands, have been used. The comet was measured in linear and circular polarimetry, in the nuclear region and in the coma and tail, and through apertures of different sizes. The polarization was always parallel or perpendicular to the scattering plane within 4° or 5°. The curve of polarization as a function of the phase angle β has a steeper slope than usually seen in comets: a trigonometric fit gives 0.303±0.014% per degree. The crossover value of the phase angle, β co, is evaluated to be around 20°, which is typical for comets. Polarization and the angle of polarization show some variations when using different apertures and also as calculated in concentric rings centered on the nuclear region. Polarization was found to be significantly lower in the blue in the CO + and C 3 emission filters (426 and 406 nm) than in an adjacent 484-nm continuum filter and significantly higher in the red in the H 2O + emission filter (700 nm) than in the adjacent 684-nm continuum filter. Polarization has been measured in the coma and tail up to a distance of about 200″, corresponding to 2.2×10 5 km, in two continuum filters; the polarization increases and then decreases when going away from the nuclear region. Circular polarization at a maximum value of 0.2% was detected and was found to be variable. We present also a few observations of Comet C/1996 B2 Hyakutake that show that P max≈28.6% occurs at β≈91°.