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Evolution of cometary grains from studies of comet images L. Kolokolovaa, B.Â.S. Gustafsona, K. Jockersb and G. Lichtenberg15 aDept. of Astronomy, University of Florida, Gainesville, FL, USA bMax-Planck Institut für Aeronomie, Katlenburg-Lindau, D-37191, Germany

CCD colourimetric and polarimetric observations of comets, obtained using the two-channel focal reducer of MPAe, allowed to analyze change in colour and polarization with the distance from the nucleus that may indicate dust evolution in the coma. This paper concentrates on trends along the solar-antisolar direction for comets Hale-Bopp, Hyakutake, and Tabur in the innermost coma where the observed comets demonstrate a correlation between colour and polarization. We show that the observed correlation is most likely a result of evaporation/destruction of a dark (organic) material. The evolution and properties of the dust in the central coma of these comets are studied using a tentative model of cometary dust as an ensemble of core-mantle particles with sublimating mantles. Calculations for a variety of optical properties, core-mantle mass ratios and dust size distributions showed that this model could describe the observed change in colour and polarization in cometary comae at realistic ranges of input parameters. The best-fit particles have silicate cores of radius 0.05-0.2 fxm with a porous organic-carbon mantle of the mantle/core mass ratio 0.5-1. Polarimetric colour can further restrict the range of satisfactory refractive indices of both the core and the mantle materials.

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