(216) Kleopatra, a low density critically rotating M-type asteroid

[en] Context. The recent estimates of the 3D shape of the M/Xe-type triple asteroid system (216) Kleopatra indicated a density of ~5 g cm^−3, which is by far the highest for a small Solar System body. Such a high density implies a high metal content as well as a low porosity which is not easy to reconcile with its peculiar "dumbbell" shape. Aims: Given the unprecedented angular resolution of the VLT/SPHERE/ZIMPOL camera, here, we aim to constrain the mass (via the characterization of the orbits of the moons) and the shape of (216) Kleopatra with high accuracy, hence its density. Methods: We combined our new VLT/SPHERE observations of (216) Kleopatra recorded during two apparitions in 2017 and 2018 with archival data from the W. M. Keck Observatory, as well as lightcurve, occultation, and delay-Doppler images, to derive a model of its 3D shape using two different algorithms (ADAM, MPCD). Furthermore, an N-body dynamical model allowed us to retrieve the orbital elements of the two moons as explained in the accompanying paper. Results: The shape of (216) Kleopatra is very close to an equilibrium dumbbell figure with two lobes and a thick neck. Its volume equivalent diameter (118.75 ± 1.40) km and mass (2.97 ± 0.32) × 10^18 kg (i.e., 56% lower than previously reported) imply a bulk density of (3.38 ± 0.50) g cm^−3. Such a low density for a supposedly metal-rich body indicates a substantial porosity within the primary. This porous structure along with its near equilibrium shape is compatible with a formation scenario including a giant impact followed by reaccumulation. (216) Kleopatra's current rotation period and dumbbell shape imply that it is in a critically rotating state. The low effective gravity along the equator of the body, together with the equatorial orbits of the moons and possibly rubble-pile structure, opens the possibility that the moons formed via mass shedding. Conclusions: (216) Kleopatra is a puzzling multiple system due to the unique characteristics of the primary. This system certainly deserves particular attention in the future, with the Extremely Large Telescopes and possibly a dedicated space mission, to decipher its entire formation history.

Disciplines :

Space science, astronomy & astrophysics

Author, co-author :

Marchis, F.; SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Suite 200, Mountain View CA, 94043, USA ; Aix Marseille Univ, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France

Jorda, L.; Aix Marseille Univ, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France

Vernazza, P.; Aix Marseille Univ, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France

Brož, M.; Institute of Astronomy, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000, Prague, Czech Republic

Hanuš, J.; Institute of Astronomy, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000, Prague, Czech Republic

Ferrais, Marin ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Origines Cosmologiques et Astrophysiques (OrCa)

Vachier, F.; IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ Paris 06, Univ. Lille, France

Rambaux, N.; IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ Paris 06, Univ. Lille, France

Marsset, M.; Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA

Viikinkoski, M.; Mathematics & Statistics, Tampere University, PO Box 553, 33101, Tampere, Finland

More authors (29 more)

Language :

English

Title :

(216) Kleopatra, a low density critically rotating M-type asteroid

Publication date :

01 September 2021

Journal title :

Astronomy and Astrophysics

ISSN :

0004-6361

eISSN :

1432-0746

Publisher :

EDP Sciences, Les Ulis, France

Volume :

653

Pages :

A57

Peer reviewed :

Peer Reviewed verified by ORBi

Additional URL :

https://ui.adsabs.harvard.edu/abs/2021A&A...653A..57M

Available on ORBi :

since 13 September 2021

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