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K. Ohtsuki (University of Colorado)
Gravitational accretion of ring particles is characterized by impact velocity, coefficient of restitution, and a parameter rp/rH, the ratio of the sum of the radii of two colliding particles to their mutual Hill radius. Ohtsuki (1993, Icarus 106, 228) performed orbital integrations including collisions and gravitational encounters and found that the probability of accretion dropped rapidly for rp/rH > 0.7, because the particles overflow their mutual Hill sphere. Assuming random impact orientation, Canup and Esposito (1995, Icarus 113, 331) obtained a simple expression of capture criteria which can approximately reproduce the above numerical results. However, Ohtsuki (1993) investigated accretion probability in the Roche zone only for a limited range of parameters (several values of rp/rH in the case of particles on circular orbits, and two different values of rp/rH in the case with particles' relative random velocity equal to their escape velocity). To better understand dynamical evolution of planetary rings in the Roche zone, we examine accretion probability of colliding particles by three-body orbital integrations for a much wider range of parameters. Preliminary results with e=2i (e and i are orbital eccentricity and inclination of particles) show that the accretion probability for rp/rH=0.5 - 0.8 is as low as 0.2-0.5 when the random velocity is as large as the escape velocity, even when the restitution coefficient in normal direction is as small as 0.1. Thus, more than 50% of colliding particles fail to accrete when their random velocity is as large as their escape velocity, unless collisions are much more dissipative.