DYNAMICAL FRICTION ON RADIAL ORBITS

Using N-body simulations, we probe the effects of dynamical friction o f a softened point-mass secondary object falling: on a nearly radial o rbit, onto a cuspy many-body primary object, 1 to 100 its mass, and co mparing to the equivalent two body-problem, for which we compute exact ly the interaction potential (given the primary density profile and th e softening kernel). Not only does the dissipation of orbital energy o ccur principally at pericenter, but the effective strength of dynamica l friction, measured as an effective instantaneous Coulomb logarithm, is also maximum at pericenter. Averaging over the orbit, the effective Coulomb logarithm is of order unity, increasing with the initial orbi tal angular momentum of the secondary, and decreasing with the ratio o f secondary to primary mass, but less so than usually assumed.