An Anisotropic Distribution of Spin Vectors in Asteroid Families

Context. The current number of similar to 500 asteroid models derived from the disk-integrated photometry by the lightcurve inversion method allows us to study the spin-vector properties of not only the whole population of main-belt asteroids, but also of several individual collisional families.Aims. We create a data set of 152 asteroids that were identified by the hierarchical clustering method (HCM) as members of ten collisional families, among which are 31 newly derived unique models and 24 new models with well-constrained pole-ecliptic latitudes of the spin axes. The remaining models are adopted from the DAMIT database or a few individual publications.Methods. We revised the preliminary family membership identification by the HCM according to several additional criteria: taxonomic type, color, albedo, maximum Yarkovsky semi-major axis drift, and the consistency with the size-frequency distribution of each family, and consequently we remove interlopers. We then present the spin-vector distributions for asteroidal families Flora, Koronis, Eos, Eunomia, Phocaea, Themis, Maria, and Alauda. We use a combined orbital- and spin-evolution model to explain the observed spin-vector properties of objects among collisional families.Results. In general, for studied families we observe similar trends in (a(p),beta) space (proper semi-major axis vs. ecliptic latitude of the spin axis): (i) larger asteroids are situated in the proximity of the center of the family; (ii) asteroids with beta > 0 degrees are usually found to the right of the family center; (iii) on the other hand, asteroids with beta < 0 degrees to the left of the center; (iv) the majority of asteroids have large pole-ecliptic latitudes (vertical bar beta vertical bar >= 30 degrees); and finally (v) some families have a statistically significant excess of asteroids with beta > 0 degrees or beta < 0 degrees. Our numerical simulation of the long-term evolution of a collisional family is capable of reproducing the observed spin-vector properties well. Using this simulation, we also independently constrain the age of families Flora (1.0 +/- 0.5 Gyr) and Koronis (2.5-4 Gyr).