Spin distribution of asteroids - Statistical model revisited

The distribution of the rotational frequencies of asteroids is believed to carry important information on their formation and the subsequent evolutionary processes (Burns, 1975). In particular, it is commonly considered that during their formation stage the larger asteroids in the Main Belt have attained a statistical equilibrium (canonical ensemble) in the 3-dimensional isotropic velocity vector space. Subsequently, especially the smaller objects, suffered from various dynamical processes, such as collisions, fragmentation and YORP effect, for example, which modified their spin velocity and direction. In this work we re-examine the spin distribution of asteroids using more recent data and focusing on its statistical aspects, in particular, the dimensionality of the phase space. We find that the presently observed spin distribution of asteroids of any diameter bin is clearly consistent with a 2-dimensional phase space, or even less for the smaller objects. This is true also for those objects with diameter larger than 50 km, whose distribution is usually believed to be consistent with isotropic 3-dimensional Maxwellian. The present result casts open questions on the origin of the asteroids spin.