Color and polarization as indicators of comet dust properties and evolution in the near-nucleus coma

We present CCD colorimetric and polarimetric observations of comets Hale-Bopp, Hyakutake, and Tabur and an analysis of their change in color and polarization with the distance from the nucleus that may indicate dust evolution in the coma. We concentrate on trends along the solar-antisolar direction in the innermost coma, where all three observed comets demonstrate a correlation between color and polarization. Comparisons to laboratory studies of scattering by complex dust structures reveal that the observed correlation is most likely a result of evaporation/destruction of a dark (organic) material. We study the evolution and properties of the dust in the central coma of these comets 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 color and polarization in cometary comae when realistic ranges of input parameters are used. The best fit particles have silicate cores of radius 0.05-0.2 mum with a porous organic-carbon mantle of mantle/core mass ratio 0.5-1. Within the frames of the model the peculiarity of the color and polarization for comet Hale-Bopp (both increase with the distance from the nucleus unlike the decrease observed in comets Hyakutake and Tabur) can be understood in terms of a compact organic mantle on the Hale-Bopp dust grains, whereas the Hyakutake and Tabur dust have more porous mantles. We show how polarimetric color restricts the range of satisfactory refractive indices of both the core and the mantle materials.