An anisotropic distribution of spin vectors in asteroid families^⋆

¹ Astronomical Institute, Faculty of Mathematics and Physics, Charles University in Prague, V Holešovičkách 2, 18000 Prague, Czech Republic
e-mail: hanus.home@gmail.com
² Palmer Divide Observatory, 17995 Bakers Farm Rd., Colorado Springs, CO 80908, USA
³ Via Capote Observatory, Thousand Oaks, CA 91320, USA
⁴ Shed of Science Observatory, 5213 Washburn Ave. S, Minneapolis, MN 55410, USA
⁵ Hunters Hill Observatory, 7 Mawalan Street, ACT 2913 Ngunnawal, Australia
⁶ 980 Antelope Drive West, Bennett, CO 80102, USA
⁷ Kingsgrove, NSW, Australia
⁸ 4438 Organ Mesa Loop, Las Cruces, NM 88011, USA
⁹ Center for Solar System Studies, 9302 Pittsburgh Ave, Suite 105, Rancho Cucamonga, CA 91730, USA
¹⁰ Observatory of Bassano Bresciano, via San Michele 4, Bassano Bresciano (BS), 25020 Brescia, Italy

Received: 30 May 2013
Accepted: 16 September 2013

Abstract

Context. The current number of ~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, β) 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 β > 0° are usually found to the right of the family center; (iii) on the other hand, asteroids with β < 0° to the left of the center; (iv) the majority of asteroids have large pole-ecliptic latitudes (|β| ≳ 30°); and finally (v) some families have a statistically significant excess of asteroids with β > 0° or β < 0°. 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).