Shapes of large main belt asteroids, deviation from hydrostatic equilibria and geophysical properties.

Abstract

A recent high-angular resolution imaging survey of 42 large main belt asteroids, with the SPHERE/ZIMPOL instrument of VLT, has provided shape models with unprecedented kilometre accuracy. The aim of this paper is to determine the equilibrium figures of these asteroids from their shape models, in order to define a size limit between dwarf planets and small bodies. Therefore, we quantify the deviation of asteroids from homogeneous hydrostatic equilibria (Maclaurin and Jacobi ellipsoids) using a spherical harmonic analysis and a minimization criterion computation. Then, this deviation is interpreted through a study of the physical and geophysical properties of asteroids. As a result, although 80% of asteroids tend towards spheroidal figures, only nine volatile-rich asteroids are compatible with Maclaurin ellipsoids. Among the remaining asteroids, about 30% could be differentiated or fossil shape, and 50% require failure criteria to describe their shape, damaged by interactions with their environments (families, satellites, collisions). The geophysical parameters calculated for these bodies are: topography perceptible through high and odd degree coefficients, internal friction angles between [Formula: see text] and [Formula: see text], stress deviation tensors higher than 0.2 MPa, strength lower than 5 MPa.This article is part of the theme issue 'Major advances in planetary sciences thanks to stellar occultations'.