Spin Axes and Shape Models of Asteroid Pairs: Fingerprints of YORP and a Path to the Density of Rubble Piles

Abstract

An asteroid pair consists of two unbound objects with almost identical heliocentric orbital elements that were formed when a single "rubble pile" asteroid failed to remain bound against an increasing rotation rate. Models suggest that the pairs' progenitors gained the fast rotation due to the YORP effect. Since it was shown that the spin axis vector can be aligned by the YORP effect, such a behavior should be seen on asteroid pairs, if they were indeed formed by the described mechanism. Alternatively, if the pairs were formed by a collision, the spin axes should have a random direction and small or young bodies might have a tumbling rotation. Here I apply the lightcurve inversion method on self-obtained photometric data, in order to derive the rotation axis vectors and shape models of the asteroid pairs 2110, 3749, 5026, 6070, 7343 and 44612. Three asteroids resulted with polar-directed spin axes and three objects with ambiguous results. In addition, the secondary member 44612 presents the same sense of rotation as its primary member 2110, and its spin is not tumbling. Finally, I use a rotational fission model, based on the assumption of an angular momentum conservation, and match it to the measured spin, shape, and mass ratio parameters in order to constrain the density of the primary members in the pairs. Using this method, low density values that are expected from a "rubble pile" are derived. All these results lead to the conclusion that the disruption of these asteroid pairs was most likely the outcome of the YORP effect that spun-up "rubble pile" asteroids.