Dust evolution of comet C/1995 O1 (Hale-Bopp) by imaging polarimetric observations

Comet C / 1995 O1 (Hale-Bopp) has been observed over a large range of phase angles (7degrees-47degrees) by the imaging polarimetric method from June 1996 to April 1997. Polarization maps were obtained and long-term evolution ( month) of the coma structures followed. Short-term evolution ( 20 min) during the April 1997 observations has allowed us to measure the period of rotation of the central arc on the polarization maps. The comparison between the polarization maps and the intensity structures confirms a greater polarization in the jets. Phase curves are obtained for different regions of the coma and compared to those obtained for the whole coma. For the region around the nucleus, the phase curve is always underneath the phase curve corresponding to the whole coma, and the polarization at phase angles smaller than 25degrees is negative (P-min = - 5%). In the jets, the phase curve is always above the whole coma phase curve, and the polarization is always positive. Whole coma phase curves are obtained with all the available data sets in different wavelengths. A comparison with other diagnostics and laboratory experiments leads us to suggest that small grains build up fluffy aggregates of high porosity; the light scattered by such aggregates is highly polarized, as observed in this active comet.