The origin of an overpopulation of contact binary plutinos

Abstract

In the trans-Neptunian region, 10-25% of binary objects are contact binaries. However, the amplitude of the light curves of a sample of Plutinos indicates that there this fraction could reach 50 % or even more. This remarkable difference cannot be explained by the single action of the Kozai cycles and tidal friction (KCTF), nor it can be assumed that it is due to a formation mechanism different from the one that gave rise to the binaries of the other hot populations. In this work we present numerical simulations showing that a super abundance of contact binaries occurs as a natural consequence of the coupling between the dynamical evolution of the mutual orbit of the binaries subject to the action of KCTF, and the variation of the heliocentric orbit of the binary centre of mass due to the perturbations of the giant planets. This same mechanism affects the other populations, but to a lesser extent, because the variations of the heliocentric orbits are much smaller for them than for objects that are within the 3:2 mean motion resonance with Neptune. This mechanism also increases the fraction of tight binaries in the Plutino population, where it could reach ∼ 30%.