The impact of physical processes on the estimation of the ages of asteroid families

One of the methods used to estimate the ages of the asteroid families is based on the interpretation of the distribution of the sizes versus orbital semimajor axes of their members as the result of their post-formation dynamical evolution. The fundamental hypothesis is that the present distribution of the semimajor axes is essentially the product of the Yarkovsky effect. On the other hand, the observable features of the asteroid families can be affected by several physical and dynamical processes. In this paper, we discuss the role of: (1) the initial distribution of the ejection velocities at the time of the primordial break-up event; (2) the possible correlations between the family members ejection direction and the orientation of the rotational axis (which the direction of the Yarkovsky semimajor axis drift depends on); (3) the gravitational reaccumulation of the parent body fragments during the ballistic phase of the formation process; and (4) the collisional re-orientation of the spin axes during the post-formation evolution phase. We show how each of these mechanisms affects the determination of the ages of the asteroid families, and what additional information can be inferred regarding some aspects of the collisional evolution of the rotation axes.