The present distribution of asteroid spin vectors and its relevance to the origin and evolution of main belt asteroids

Abstract

The large main belt asteroids (MBA) have been formed during an accretion process where their original physical properties (e.g. spin rate) depended on the medium they formed from and the details of the growth mechanism. In this context the spin vector (SV) of an asteroid is an important property closely linked to the general evolution of the MBA. New photometric observations have been acquired that yielded models of the SV for 46 asteroids. Together with previously published models the new data set consists of SV parameters for 73 MBA and is close of being complete for the largest objects (D>200 km). The spin vector distribution (SVD) has following basic properties: 1) There exists a significant dominance of asteroids with a prograde sense of rotation and with their spin vectors concentrated towards the north ecliptic pole. 2) A comparison of the SVD between asteroids with different diameters reveals three specific zones: The large asteroids (D>200 km) for which the spin-vectors are moderately anisotropic. An intermediate size range (D~70-150 km) including objects with the strongest SV concentration toward the north ecliptic pole and the smaller asteroids with a completely isotropic distribution. 3) There exists a strong correlation between the SVD are present between asteroids of different taxonomic types. These results are interpreted in an evolutionary scenario and put firm constraints on the accretion phase and collisional evolution of the main belt population.