Coupled Spin and Shape Evolution of Small Rubble-Pile Asteroids: Self-Limitation of the YORP Effect

TL;DR

This study presents self-consistent simulations showing that small rubble-pile asteroids experience YORP effect self-limitation, which constrains their spin rates and shape evolution, affecting their long-term rotational behavior and binary formation potential.

Contribution

First self-consistent simulations of coupled spin-shape evolution of rubble-pile asteroids under YORP, revealing self-limitation and diverse evolutionary behaviors.

Findings

YORP self-limitation confines asteroid spin rates.

Diverse behaviors include stochastic, self-governed, and stagnating YORP.

Mass loss occurs in about one-third of simulations.

Abstract

We present the first self-consistent simulations of the coupled spin-shape evolution of small gravitational aggregates under the influence of the YORP effect. Because of YORP's sensitivity to surface topography, even small centrifugally driven reconfigurations of aggregates can alter the YORP torque dramatically, resulting in spin evolution that can differ qualitatively from the rigid-body prediction. One third of our simulations follow a simple evolution described as a modified YORP cycle. Two-thirds exhibit one or more of three distinct behaviors---stochastic YORP, self-governed YORP, and stagnating YORP---which together result in YORP self-limitation. Self-limitation confines rotation rates of evolving aggregates to far narrower ranges than those expected in the classical YORP cycle, greatly prolonging the times over which objects can preserve their sense of rotation. Simulated objects are initially randomly packed, disordered aggregates of identical spheres in rotating equilibrium, with low internal angles of friction. Their shape evolution is characterized by rearrangement of the entire body, including the deep interior. They do not evolve to axisymmetric top shapes with equatorial ridges. Mass loss occurs in one-third of the simulations, typically in small amounts from the ends of a prolate-triaxial body. We conjecture that YORP self-limitation may inhibit formation of top-shapes, binaries, or both, by restricting the amount of angular momentum that can be imparted to a deformable body. Stochastic YORP, in particular, will affect the evolution of collisional families whose orbits drift apart under the influence of Yarkovsky forces, in observable ways.