(704) Interamnia: a transitional object between a dwarf planet and a typical irregular-shaped minor body^★,★★

¹ Institute of Astronomy, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
e-mail: pepa@sirrah.troja.mff.cuni.cz
² Aix Marseille Univ, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
³ Mathematics and Statistics, Tampere University, 33720 Tampere, Finland
⁴ Space sciences, Technologies and Astrophysics Research Institute, Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium
⁵ IMCCE, Observatoire de Paris, 77 avenue Denfert-Rochereau, 75014 Paris Cedex, France
⁶ Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁸ Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁹ SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Mountain View CA 94043, USA
¹⁰ Center for Solar System Studies, 446 Sycamore Ave., Eaton, CO 80615, USA
¹¹ Geneva Observatory, 1290 Sauverny, Switzerland
¹² Asociación Astronómica Astro Henares, Centro de Recursos Asociativos El Cerro C/ Manuel Azaña, 28823 Coslada, Madrid, Spain
¹³ 490 chemin du Gonnet, 38440 Saint Jean de Bournay, France
¹⁴ Institute of Geology, A. Mickiewicz University, Krygowskiego 12, 61-606 Poznań, Poland
¹⁵ Observatoire de Durtal, 49430 Durtal, France
¹⁶ Observatorio Astronómico de Córdoba, Córdoba, Argentina
¹⁷ 20 parc des Pervenches, 13012 Marseille, France
¹⁸ Instituto de Astrofisica de Andalucia – CSIC. Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁹ I64, SL6 1XE Maidenhead, UK
²⁰ Uranoscope, Avenue Carnot 7, 77220 Gretz-Armainvilliers, France
²¹ Anunaki Observatory, Calle de los Llanos, 28410 Manzanares el Real, Spain
²² Departamento de Astrofisica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
²³ Institute of Theoretical Physics, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
²⁴ Courbes de rotation d’astéroïdes et de comètes, CdR, Geneva Observatory, 1290 Sauverny, Switzerland
²⁵ Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
²⁶ Observatorio Amanecer de Arrakis, Alcalá de Guadaíra, Sevilla, Spain
²⁷ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary
²⁸ Thirty-Meter-Telescope, 100 West Walnut St, Suite 300, Pasadena, CA 91124, USA
²⁹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³⁰ European Space Agency, ESTEC – Scientific Support Office, Keplerlaan 1, Noordwijk 2200 AG, The Netherlands
³¹ Open University, School of Physical Sciences, The Open University, MK7 6AA, UK
³² Laboratoire Atmosphères, Milieux et Observations Spatiales, CNRS & UVSQY, Guyancourt, France
³³ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, 03080 Alicante, Spain
³⁴ Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
³⁵ European Southern Observatory (ESO), Alonso de Cordova 3107, 1900 Casilla Vitacura, Santiago, Chile

Received: 5 September 2019
Accepted: 21 November 2019

Abstract

Context. With an estimated diameter in the 320–350 km range, (704) Interamnia is the fifth largest main belt asteroid and one of the few bodies that fills the gap in size between the four largest bodies with D > 400 km (Ceres, Vesta, Pallas and Hygiea) and the numerous smaller bodies with diameter ≤200 km. However, despite its large size, little is known about the shape and spin state of Interamnia and, therefore, about its bulk composition and past collisional evolution.

Aims. We aimed to test at what size and mass the shape of a small body departs from a nearly ellipsoidal equilibrium shape (as observed in the case of the four largest asteroids) to an irregular shape as routinely observed in the case of smaller (D ≤ 200 km) bodies.

Methods. We observed Interamnia as part of our ESO VLT/SPHERE large program (ID: 199.C-0074) at thirteen different epochs. In addition, several new optical lightcurves were recorded. These data, along with stellar occultation data from the literature, were fed to the All-Data Asteroid Modeling algorithm to reconstruct the 3D-shape model of Interamnia and to determine its spin state.

Results. Interamnia’s volume-equivalent diameter of 332 ± 6 km implies a bulk density of ρ = 1.98 ± 0.68 g cm⁻³, which suggests that Interamnia – like Ceres and Hygiea – contains a high fraction of water ice, consistent with the paucity of apparent craters. Our observations reveal a shape that can be well approximated by an ellipsoid, and that is compatible with a fluid hydrostatic equilibrium at the 2σ level.

Conclusions. The rather regular shape of Interamnia implies that the size and mass limit, under which the shapes of minor bodies with a high amount of water ice in the subsurface become irregular, has to be searched among smaller (D ≤ 300 km) less massive (m ≤ 3 × 10¹⁹ kg) bodies.