COLLISIONAL EVOLUTION OF ASTEROID FAMILIES

Most asteroid dynamical families are thought to be the outcomes of col lisional disruption of parent asteroids destroyed by high-velocity imp acts with other astroids. However, subsequent collisions modify both t he sizes and the orbits of family members, so the distributions that w e see today may be very different from those following the breakup of the parent body. We study the postbreakup evolution of family asteroid s with a numerical model which keeps track of both the sizes and the o rbits of the fragments as they collisionally interact with the field p opulation of asteroids. Using this model we visualize how the family a ppears at different evolutionary stages. In particular we find that th e size distribution of a family becomes less steep with time. We have simulated the possible evolutionary history of the three most populous Hirayama families, Koronis, Eos, and Themis. By matching the distribu tion of sizes and orbits with those observed for the families, we obta in significant constraints on the properties of their parent bodies an d on some collisional response parameters, together with the evolution ary ages of the families. The Themis family appears as the outcome of the catastrophic disruption of one of the largest asteroids, probably a unique event over the history of the Solar System. On the other hand , the Koronis and Eos families appear to have been formed from smaller parent bodies, but peculiar features may require specific processes o r events. Koronis' size distribution has several bodies of comparable size at the large diameter end, which can be explained if the largest fragment of the initial breakup underwent subsequent fragmentation. Th e ''anisotropic'' orbital distribution of the Eos family requires eith er a peculiar fragment velocity field or the action of poorly understo od dynamical processes on the orbits of its members. For both the Koro nis and the Themis families we derive an estimate of the age of the or der of 2 Byr. The uncertainties affecting our estimates of family ages and of the properties of the parent bodies are mainly due to the pres ent limited understanding of collisional breakup processes for bodies hundreds of kilometers in size and to the poor knowledge of the size d istribution of small asteroids. (C) 1995 Academic Press, Inc.