Constraints on the near-Earth asteroid obliquity distribution from the Yarkovsky effect

C Tardioli,D Cotto-Figueroa,S R Chesley,T S Statler,B Rozitis,D Farnocchia,M Vasile

doi:10.1051/0004-6361/201731338

Abstract

Aims. From lightcurve and radar data we know the spin axis of only 43 near-Earth asteroids. In this paper we attempt to constrain the spin axis obliquity distribution of near-Earth asteroids by leveraging the Yarkovsky effect and its dependence on an asteroid’s obliquity. Methods. By modeling the physical parameters driving the Yarkovsky effect, we solve an inverse problem where we test different simple parametric obliquity distributions. Each distribution results in a predicted Yarkovsky effect distribution that we compare with a X2 test to a dataset of 125 Yarkovsky estimates. Results. We find different obliquity distributions that are statistically satisfactory. In particular, among the considered models, the best-fit solution is a quadratic function, which only depends on two parameters, favors extreme obliquities, consistent with the expected outcomes from the YORP effect, has a 2:1 ratio between retrograde and direct rotators, which is in agreement with theoretical predictions, and is statistically consistent with the distribution of known spin axes of near-Earth asteroids.

Highlights

The complex motion of near-Earth asteroids (NEAs) is dominated by the gravitational perturbations of the Sun and planets
The Yarkovsky perturbation can be modeled as a transverse acceleration A2/r2 (Farnocchia et al 2013a), where r is the distance from the Sun in au and A2 is the sum of two terms, one corresponding to the diurnal effect due to the asteroid’s rotation and one corresponding to the seasonal effect due to the asteroid’s orbital motion: A2
All of the models give a retrograde to direct rotators ratio (RR/D) that is statistically consistent with the 2+−10.7 ratio found

Summary

Introduction

The complex motion of near-Earth asteroids (NEAs) is dominated by the gravitational perturbations of the Sun and planets. The Yarkovsky effect is a subtle nongravitational acceleration due to the anisotropic emission of thermal radiation of Solar System objects that causes a secular drift in the semi-major axis (Bottke et al 2006). This perturbation is important to understand the long-term dynamics of the asteroid population since it is a driving factor for feeding resonances in the main belt and transporting asteroids to the inner Solar System (Farinella et al 1998; Morbidelli & Vokrouhlicky 2003; Bottke et al 2002b). This technique was introduced, with a preliminary application to a similar dataset, in Cotto-Figueroa (2013)

Yarkovsky modeling

Probability distribution of physical parameters

Models for the obliquity distribution

Solution of the inverse problem

Results

Conclusions