THE DUST ENVIRONMENT OF COMET LEVY-1990XX

Two wide-field plates taken with the U.K. Schmidt Telescope Unit of th e perspective dust antitail of comet Levy 1990XX are analysed by means of the inverse Monte-Carlo model of dust tails (Fulle M., Astron. Ast rophys. 217, 283, 1989). The model considers dust grains of diameters between 2 mum and 2 cm ejected from 5 months before perihelion to 3 mo nths after. The observation geometry allows a very accurate estimate o f the dust ejection velocity for the pre-perihelion times. which resul ts to be 30 +/- 10 m s-1 for grains of 0.1 mm diameter and strongly co nstrains coma models interpreting pre-perihelion observations. At peri helion the velocity increases to about 120 m s-1, and then decreases t o pre-perihelion values. The mass loss rate reaches a wide maximum of 3 x 10(7) g s-1 10 days before perihelion. Only a negligible fraction (lower than 2%) of the observed produced dust mass (4 +/- 1 10(14) g) appears to be injected into bound orbits; however, such an estimate fo r the meteoroid contribution to the zodiacal cloud should be a crude l ower limit, since the 1990XX antitail mainly concerns quite small grai ns. Nevertheless, as for other already analysed new comets (in the Oor t sense, C/1987VII, Cremonese. G. and Fulle, M., Astron. J. 100, 1285, 1990; C/1990V, Fulle, M. et al.. Astron. Astrophys., in press, 1994), the time-averaged size distribution is dominated by large grains, wit h a power index of -3.2 +/- 0.1.