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Z. M. Leinhardt, D. C. Richardson (Dept. of Astronomy, University of Maryland)
We present results from direct N-body simulations of collisions between gravitational aggregates of varying size as part of a study to parameterize planetesimal growth in the solar system. We find that as the ratio of projectile to target mass departs from unity the impact angle (angular momentum) has less effect on the outcome. At the same time, the probability of planetesimal growth increases. Conversely, for a fixed impact energy, collisions between impactors with mass ratio near unity are more dispersive than those with impactor mass ratio far from unity. We derive an expression for the accretion probability as a function of mass ratio. For a mass ratio of 1:5 we find an accretion probability of ~ 60% over all impact parameters. We also compute the critical specific dispersal energy Q*D as a function of projectile size. Extrapolating to a projectile size of 1 m with a 1 km target we find Q*D = 103--104 J kg-1, in agreement with several other collision models that use fundamentally different techniques. Our model assumes that the components of each gravitational aggregate are identical and indestructible over the range of sampled impact speeds. In future work we hope to incorporate a simple fracture model to extend the range of applicable speeds and plan to implement our results into a large-scale planetesimal evolution code.