An anisotropic distribution of spin vectors in asteroid families

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 spinvector 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).

Escape of asteroids from the main belt

“…Since da/dt(D, γ) ∝ cos γ, we might expect da/dt uniformly spans the interval of values [−(da/dt) 0 , (da/dt) 0 ] for an isotropic distribution of spin axes among asteroids and D = D 0 . However, small asteroids typically have spin axes normal to the ecliptic (cos γ ±1) (e.g., Hanuš et al 2011Hanuš et al , 2013, and a bimodal distribution da/dt ±(da/dt) 0 better matches observations. Moreover, since our goal with this variant of the code is primarily to locate relevant escape routes from the main belt, the extreme da/dt values enable us to do so faster.…”

Characterizing the original ejection velocity field of the Koronis family

“…Perhaps this may suggest that the stochastic YORP effect may not be that "stochastic", meaning that longer timescales for choosing new asteroid shape models (and consequently, more reorientation events) could be closer to what happens in nature. A static YORP effect is actually needed to explain the ecliptic latitude distribution of main belt asteroids (Hanuš et al, 2013) or the Slivan states of some Koronis family members (Vokrouhlický et al, 2003). Alternatively, if we only consider the results of our dynamical simulation for the a < 2.88 au population, the initial ejection velocity field could have been rather unisotropical, with lower values of δv W and larger values of δv r and δv t .…”

Photometric survey, modelling, and scaling of long-period and low-amplitude asteroids

“…However, so far only the spin-up of the rotation period has been directly detected (e.g. Lowry et al 2007Lowry et al , 2014; Kaasalainen et al 2007;Ďurech et al 2008) The spatial distribution of asteroid spin axes suggests that the largest bodies generally preserved their primordial, prograde spin, while smaller ones, with diameters less than 30 km, seem to be strongly affected by the YORP effect that pushes these axes towards extreme values of obliquities (Hanuš et al 2013). The spins of prograde rotators under the YORP effect influence can be captured into spin-orbit resonances, sometimes even forming spin clusters (Slivan 2002;Kryszczyńska et al 2012).…”