EVIDENCE FOR 2 CLASSES OF COMETS FROM THEIR POLARIMETRIC PROPERTIES AT LARGE PHASE ANGLES

The dependence of the linear polarization of solar light scattered by cometary dust upon the phase angle has been studied, through appropria te fits, from a data base of 22 comets and 750 points (mainly in the g reen and red spectral domains). Although the whole coma polarization i s found to slightly increase with the wavelength, all the data obtaine d at a given phase angle compare fairly well up to about 45 degrees. F or larger phase angles, a dichotomy is clearly pointed out between com ets which have, in the red domain, a maximum of polarization, P-max, a lmost equal to 30 per cent (e.g. West 1976 VI, P/Halley 1986 III, Brad field 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 like ly a clue to some significant differences in the bulk properties (albe do, size distribution, porosity) of dust in different comets.