Puzzling Asteroid Families

The analysis of recent observational data has clarified several general properties of the collisional processes originating the asteroid dynamical families. However, a few puzzling physical problems remain open. According to the observations, the fragments are usually ejected at high velocities; the available theoretical models of catastrophic fragmentation, based on hydrodynamical simulations, do not reproduce this kind of result. Whenever high velocities are obtained in the models, the family is formed by very small-and thus unobservable-fragments. In this paper we discuss the general features of the problem, and we focus on the action of self-gravity, which may cause a later reaccumulation of originally small fragments. We have performed several N-body simulations, showing that the self-gravitational reaccumulation is unable to act for more than one-or few-selected fragments. Thus an original deep shattering of the parent body, followed by a significant recollection driven by self-gravity, cannot reproduce the size distribution observed in various real families, in which the large-size end includes several fragments of almost the same size. Moreover the ejection velocities required by the model to form, at least, a reaccumulated largest remnant as massive as that observed are usually too small in comparison to those estimated from the observations. We propose possible alternative explanations for this puzzling inconsistency between model outcomes and real world.