3D shape of asteroid (6) Hebe from VLT/SPHERE imaging: Implications for the origin of ordinary H chondrites ^⋆

¹ Astrophysics Research Centre, Queen’s University Belfast, BT7 1NN, UK
e-mail: michael.marsset@qub.ac.uk
² Université Côte d’Azur, Observatoire de la Côte d’Azur, 06304 Lagrange, CNRS, France
³ IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ Paris 06, Univ. Lille, France
⁴ TMT Observatory, 100 W. Walnut Street, Suite 300, Pasadena, CA 91124, USA
⁵ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
⁶ Department of Mathematics, Tampere University of Technology, PO Box 553, 33101 Tampere, Finland
⁷ Aix Marseille Univ, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13013 Marseille, France
⁸ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁹ Space sciences, Technologies and Astrophysics Research Institute, Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium
¹⁰ Open University, School of Physical Sciences, The Open University, MK7 6AA, UK
¹¹ European Southern Observatory (ESO), Alonso de Córdova 3107, 1900 Casilla Vitacura, Santiago, Chile
¹² ONERA – the French Aerospace Lab, 92322 Châtillon, France
¹³ Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
¹⁴ CdR & CdL Group: Lightcurves of Minor Planets and Variable Stars, Observatoire de Genève, 1290 Sauverny, Switzerland

Received: 22 April 2017
Accepted: 19 May 2017

Abstract

Context. The high-angular-resolution capability of the new-generation ground-based adaptive-optics camera SPHERE at ESO VLT allows us to assess, for the very first time, the cratering record of medium-sized (D ~ 100–200 km) asteroids from the ground, opening the prospect of a new era of investigation of the asteroid belt’s collisional history.

Aims. We investigate here the collisional history of asteroid (6) Hebe and challenge the idea that Hebe may be the parent body of ordinary H chondrites, the most common type of meteorites found on Earth (~34% of the falls).

Methods. We observed Hebe with SPHERE as part of the science verification of the instrument. Combined with earlier adaptive-optics images and optical light curves, we model the spin and three-dimensional (3D) shape of Hebe and check the consistency of the derived model against available stellar occultations and thermal measurements.

Results. Our 3D shape model fits the images with sub-pixel residuals and the light curves to 0.02 mag. The rotation period (7.274 47 h), spin (ECJ2000 λ, β of 343°, +47°), and volume-equivalent diameter (193 ± 6 km) are consistent with previous determinations and thermophysical modeling. Hebe’s inferred density is 3.48 ± 0.64 g cm^-3, in agreement with an intact interior based on its H-chondrite composition. Using the 3D shape model to derive the volume of the largest depression (likely impact crater), it appears that the latter is significantly smaller than the total volume of close-by S-type H-chondrite-like asteroid families.

Conclusions. Our results imply that (6) Hebe is not the most likely source of H chondrites. Over the coming years, our team will collect similar high-precision shape measurements with VLT/SPHERE for ~40 asteroids covering the main compositional classes, thus providing an unprecedented dataset to investigate the origin and collisional evolution of the asteroid belt.