Dust environment of Comet Hyakutake 1996B2.

Narrow-band wide-field CCD images of the dust tail of Comet Hyakutake 1996B2 are analysed by means of the inverse dust tail model (Fulle, 1989A&A...217..283F). The dust tail fits allow us to estimate the ejection velocity, mass loss rate and size distribution of the dust grains ejected during the months of March and April, 1996 (when the heliocentric distance was decreasing from 1.5AU to 0.5AU), in the size range between 1μm and 1mm. The best fits of the dust tail images are obtained for a power index of the velocity size dependence in agreement with classical models of the dust drag by gas in the inner coma (Gombosi 1986, in Exploration of Halley's comet (Vol. 3), ESA SP-250, Paris, 167, and Crifo 1991, in Comets in the Post-Halley Era, Kluwer, Dordrecht, 937). The same fits do not show a clear dependence on the dust ejection anisotropy from the inner coma. The dust ejection velocity of 10μm sized grains increases from about 50m/s at the beginning of March, to about 500m/s at the end of April. The mass loss rate increases from about 2x10^3^kg/s at the beginning of March, to about 10^4^kg/s in mid April. These loss rates depend linearly on the dust scattering efficiency (assumed equal to 1) and inversely on the albedo (assumed equal to 0.04). Twenty days before perihelion, both the size distribution power index and the mass loss rate show a strong drop, possibly due either to an increase of dust fragmentation processes inside the inner coma, or to a strong size dependence of the size distribution power index. The same time marks a difference in the observed dust environment, passing from a dust mass dominated by large grains to a dust mass dominated by small ones. The time averaged size distribution power index (equal to -3.6+/-0.2) and the dust to gas ratio (at least one for the assumed dust albedo and scattering efficiency) point out that C/Hyakutake was a dusty comet. About 1% of the released dust mass goes to replenish the isotropic meteoric cloud (6+/-3x10^3^kg of dust were injected into bound orbits).