Evidence for two classes of comets from their polarimetric properties at large phase angles.

Abstract

The dependence of the linear polarization of solar light scattered by cometary dust upon the phase angle has been studied, through appropriate fits, from a data base of 22 comets and 750 points (mainly in the green and red spectral domains). Although the whole coma polarization is found to slightly increase with the wavelength, all the data obtained at a given phase angle compare fairly well up to about 45deg. For larger phase angles, a dichotomy is clearly pointed out between comets which have, in the red domain, a maximum of polarization, P_max_, almost equal to 30 per cent (e.g. West 1976 VI, P/Halley 1986 III, Bradfield 1987 XXIX, and Levy 1990 XX), and comets whose P_max_ is below 20 per cent (e.g. Kobayashi-Berger-Milon 1975 IX and Austin 1990 V). The comets with high P_max_ are noticed to be mainly comets that have been observed by Hanner et al. (1994 a, b) to display structured silicate features. The existence of these two polarimetric classes is most likely a clue to some significant differences in the bulk properties (albedo, size distribution, porosity) of dust in different comets.