Homogeneous internal structure of CM-like asteroid (41) Daphne^★,★★

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
e-mail: benoit.carry@oca.eu
² IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ Paris 06, Paris, France
³ Astrophysics Research Centre, Queen’s University Belfast, BT7 1NN, UK
⁴ Aix-Marseille Université, CNRS, LAM, Laboratoire d’Astro- physique de Marseille, Marseille, France
⁵ School of Physical Sciences, The Open University, MK7 6AA, UK
⁶ Southwest Research Institute, Boulder, CO 80302, USA
⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire GéoAzur, France
⁸ Large Binocular Telescope Observatory, University of Arizona, Tucson, AZ 85721, USA
⁹ Faculty of Physics, Astronomical Observatory Institute, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
¹⁰ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
¹¹ Institute of Physics, University of Szczecin, Wielkopolska 15, 70-453 Szczecin, Poland
¹² Department of Mathematics, Tampere University of Technology, PO Box 553, 33101 Tampere, Finland
¹³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
¹⁵ Thirty-Meter-Telescope, 100 West Walnut St, Suite 300, Pasadena, CA 91124, USA
¹⁶ Leidos, Starfire Optical Range, AFRL, Kirtland AFB, NM 87117, USA
¹⁷ Binary Astronomy, Aurora, CO 80012, USA
¹⁸ CdR & CdL Group: Lightcurves of Minor Planets and Variable Stars, Observatoire de Genève, 1290 Sauverny, Switzerland
¹⁹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
²⁰ European Space Agency, ESTEC – Scientific Support Office, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
²¹ Observatoire du Bois de Bardon, 16110 Taponnat, France
²² Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium
²³ SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Mountain View CA 94043, USA
²⁴ Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Apartado 1761, Lima, Peru
²⁵ Center for Solar System Studies, 446 Sycamore Ave., Eaton, CO 80615, USA
²⁶ European Southern Observatory (ESO), Alonso de Cordova 3107, 1900 Casilla Vitacura, Santiago, Chile
²⁷ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
²⁸ Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile

Received: 18 July 2018
Accepted: 7 January 2019

Abstract

Context. CM-like asteroids (Ch and Cgh classes) are a major population within the broader C-complex, encompassing about 10% of the mass of the main asteroid belt. Their internal structure has been predicted to be homogeneous, based on their compositional similarity as inferred from spectroscopy and numerical modeling of their early thermal evolution.

Aims. Here we aim to test this hypothesis by deriving the density of the CM-like asteroid (41) Daphne from detailed modeling of its shape and the orbit of its small satellite.

Methods. We observed Daphne and its satellite within our imaging survey with the Very Large Telescope extreme adaptive-optics SPHERE/ZIMPOL camera and complemented this data set with earlier Keck/NIRC2 and VLT/NACO observations. We analyzed the dynamics of the satellite with our Genoid meta-heuristic algorithm. Combining our high-angular resolution images with optical lightcurves and stellar occultations, we determine the spin period, orientation, and 3D shape, using our ADAM shape modeling algorithm.

Results. The satellite orbits Daphne on an equatorial, quasi-circular, prograde orbit, like the satellites of many other large main-belt asteroids. The shape model of Daphne reveals several large flat areas that could be large impact craters. The mass determined from this orbit combined with the volume computed from the shape model implies a density for Daphne of 1.77 ± 0.26 g cm⁻³ (3 σ). This densityis consistent with a primordial CM-like homogeneous internal structure with some level of macroporosity (≈ 17%).

Conclusions. Based on our analysis of the density of Daphne and 75 other Ch/Cgh-type asteroids gathered from the literature, we conclude that the primordial internal structure of the CM parent bodies was homogeneous.