Equilibrium rotation states of doubly synchronous binary asteroids

Abstract

A doubly synchronous binary asteroid simultaneously experiences YORP and BYORP, the former being independent of the radius of the orbit, while the latter linearly dependent on the radius. In many systems YORP and BYORP can compensate each other at some radius, causing an equilibrium state, to which other orbits converge in the course of their evolution. We derive mathematical formalism of such equilibria in terms of dimensionless YORP and BYORP coefficients. We compute YORP and BYORP coefficients for a set of photometric and radar shape models, and find equilibria to be relatively common.