SPIN AXIS ORIENTATION OF 2060-CHIRON FROM DUST COMA MODELING

The numerical inverse model of comet dust tails (Fulle 1989) is improv ed in order to take into account possible effects of the nucleus rotat ion on the dust shell axis orientation, and is applied to the asymmetr ic dust coma image of 2060 Chiron (West 1991). Tests of the model allo w to prove that the dust shell axis orientation (i) strongly affects t he coma image fits, (ii) is uniquely determined and (iii) introduces c hanges of the second order in the solutions (loss rates and size distr ibution) with respect to other non-linear free parameters (time and si ze dependence of the dust velocity and ejection anisotropy). The compa rison of our results with those of coma photometry allows to infer the most probable spin axis orientation: retrograde rotation and obliquit y of about 135-degrees. The strongest activity occurs 9 +/- 4 min afte r the Sun culmination. The ejection velocity of grains of 40 mum diame ter is 5 +/- 1 m s-1, much lower than the probable escaping velocity o f CO or CO2, thus suggesting a strong deceleration due to the nucleus gravity. The correlation between nucleus rotation and coma shape seems to exclude significant orbital escaping from a possible bound coma cl ose to the nucleus surface (Meech & Belton 1990). The mass loss rate i s higher than the estimate of Luu & Jewitt (1990), reaching 20 +/- 10 kg s-1 between 1987 and 1990, and is dominated by grains of 0.1 mm siz e, as confirmed by the time-averaged size distribution, the power inde x of which is -3.2 +/- 0.1 for diameters between 1 mum and 1 mm.