VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis^{★,★★,★★★}

¹ Aix Marseille Université, CNRS, CNES, Laboratoire d’Astrophysique de Marseille, Marseille, France
e-mail: pierre.vernazza@lam.fr
² Institute of Astronomy, Charles University, Prague, V Holešovičkách 2, 18000 Prague 8, Czech Republic
³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁴ Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁵ Mathematics and Statistics, Tampere University, 33720 Tampere, Finland
⁶ SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Mountain View CA 94043, USA
⁷ IMCCE, CNRS, Observatoire de Paris, PSL Université, Sorbonne Université, Paris, France
⁸ Astronomical Institute of the Romanian Academy, 5-Cuţitul de Argint, 040557 Bucharest, Romania
⁹ Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland
¹⁰ NASA Goddard Space Flight Center, University of Maryland College Park, USA
¹¹ Space sciences, Technologies and Astrophysics Research Institute, Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium
¹² Institut de Planetologie et d’Astrophysique de Grenoble, UGA-CNRS, France; Institut Universitaire de France, Paris, France
¹³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁴ European Space Agency, ESTEC - Scientific Support Office, Keplerlaan 1, Noordwijk 2200 AG, The Netherlands
¹⁵ TMT Observatory, 100 W. Walnut Street, Suite 300, Pasadena, CA 91124, USA
¹⁶ Open University, School of Physical Sciences, The Open University, MK7 6AA, UK
¹⁷ Laboratoire Atmosphères, Milieux et Observations Spatiales, CNRS & Université de Versailles Saint-Quentin-en-Yvelines, Guyancourt, France
¹⁸ Departamento de Fisica, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Alicante, Spain
¹⁹ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí Franquès 1, E08028 Barcelona, Spain
²⁰ European Southern Observatory (ESO), Alonso de Cordova 3107, 1900 Casilla Vitacura, Santiago, Chile
²¹ Observatoire des Hauts Patys, 84410 Bédoin, France
²² Observatoire de Chinon, Mairie de Chinon, 37500 Chinon, France
²³ Aix Marseille Université, CNRS, OHP (Observatoire de Haute Provence), Institut Pythéas (UMS 3470), 04870 Saint-Michel-l’Observatoire, France
²⁴ Geneva Observatory, 1290 Sauverny, Switzerland
²⁵ High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
²⁶ B74, Avinguda de Catalunya 34, 25354 Santa Maria de Montmagastrell (Tarrega), Spain
²⁷ I39, Cruz del Sur Observatory, San Justo city, Buenos Aires, Argentina
²⁸ Observatoire du Bois de Bardon, 16110 Taponnat, France
²⁹ Association T60, Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France
³⁰ Hong Kong Space Museum, Tsimshatsui, Hong Kong, PR China
³¹ I64, SL6 1XE Maidenhead, UK
³² Korea Astronomy and Space Science Institute, 776, Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Korea
³³ Chungbuk National University, 1, Chungdae-ro, Seowon-gu, Cheongju-si, Chungcheongbuk-do 28644, Korea
³⁴ Faculty of Physics, Astronomical Observatory Institute, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
³⁵ Observatoire OPERA, 33820 Saint Palais, France
³⁶ Uranoscope, Avenue Carnot 7, 77220 Gretz-Armainvilliers, France
³⁷ Institut d’Astrophysique de Paris, 98 bis boulevard Arago, UMR 7095 CNRS et Sorbonne Universités, 75014 Paris, France
³⁸ Anunaki Observatory, Calle de los Llanos, 28410 Manzanares el Real, Spain
³⁹ Club d’Astronomie de Lyon Ampere (CALA), Place de la Nation, 69120 Vaulx-en-Velin, France
⁴⁰ Kingsgrove, NSW, Australia
⁴¹ Planetary Science Institute, 1700 East Fort Lowell Road, Tucson, AZ 85719, USA
⁴² Rio Cofio Observatory, Robledo de Chavela (Madrid), Spain
⁴³ Cicha 43, 44-144 Nieborowice, Poland

Received: 13 July 2021
Accepted: 3 September 2021

Abstract

Context. Until recently, the 3D shape, and therefore density (when combining the volume estimate with available mass estimates), and surface topography of the vast majority of the largest (D  ≥ 100 km) main-belt asteroids have remained poorly constrained. The improved capabilities of the SPHERE/ZIMPOL instrument have opened new doors into ground-based asteroid exploration.

Aims. To constrain the formation and evolution of a representative sample of large asteroids, we conducted a high-angular-resolution imaging survey of 42 large main-belt asteroids with VLT/SPHERE/ZIMPOL. Our asteroid sample comprises 39 bodies with D  ≥ 100 km and in particular most D  ≥ 200 km main-belt asteroids (20/23). Furthermore, it nicely reflects the compositional diversity present in the main belt as the sampled bodies belong to the following taxonomic classes: A, B, C, Ch/Cgh, E/M/X, K, P/T, S, and V.

Methods. The SPHERE/ZIMPOL images were first used to reconstruct the 3D shape of all targets with both the ADAM and MPCD reconstruction methods. We subsequently performed a detailed shape analysis and constrained the density of each target using available mass estimates including our own mass estimates in the case of multiple systems.

Results. The analysis of the reconstructed shapes allowed us to identify two families of objects as a function of their diameters, namely “spherical” and “elongated” bodies. A difference in rotation period appears to be the main origin of this bimodality. In addition, all but one object (216 Kleopatra) are located along the Maclaurin sequence with large volatile-rich bodies being the closest to the latter. Our results further reveal that the primaries of most multiple systems possess a rotation period of shorter than 6 h and an elongated shape (c∕a ≤ 0.65). Densities in our sample range from ~1.3 g cm⁻³ (87 Sylvia) to ~4.3 g cm⁻³ (22 Kalliope). Furthermore, the density distribution appears to be strongly bimodal with volatile-poor (ρ ≥ 2.7 g cm⁻³) and volatile-rich (ρ ≤ 2.2 g cm⁻³) bodies. Finally, our survey along with previous observations provides evidence in support of the possibility that some C-complex bodies could be intrinsically related to IDP-like P- and D-type asteroids, representing different layers of a same body (C: core; P/D: outer shell). We therefore propose that P/ D-types and some C-types may have the same origin in the primordial trans-Neptunian disk.