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

J. Hanuš (1) , P. Vernazza (2) , M. Viikinkoski (3) , M. Ferrais (4, 2) , N. Rambaux (5) , E. Podlewska-Gaca (6) , A. Drouard (2) , L. Jorda (2) , E. Jehin (4) , B. Carry (7) , M. Marsset (8) , F. Marchis (9) , B. Warner (10) , R. Behrend (11) , V. Asenjo (12) , N. Berger , M. Bronikowska (13) , T. Brothers (8) , S. Charbonnel (14) , C. Colazo (15) , J.-F. Coliac , R. Duffard (16) , A. Jones , A. Leroy (17) , A. Marciniak (6) , R. Melia (15) , D. Molina (18) , J. Nadolny (19) , M. Person (8) , O. Pejcha (20) , H. Riemis (21) , B. Shappee (22) , K. Sobkowiak (6) , F. Soldán (23) , D. Suys (21) , R. Szakats (24) , J. Vantomme (21) , M. Birlan (5) , J. Berthier (5) , P. Bartczak (6) , C. Dumas (25) , G. Dudziński (6) , J. Ďurech (1) , J. Castillo-Rogez (26) , F. Cipriani (27) , R. Fetick (2) , T. Fusco (2) , J. Grice (28) , M. Kaasalainen (29) , A. Kryszczynska (6) , Philippe Lamy (30) , T. Michalowski (6) , P. Michel (31) , T. Santana-Ros (32, 33) , P. Tanga (28) , F. Vachier (5) , A. Vigan (2) , O. Witasse (27) , Bin Yang (34)

1 Astronomical Institute of Charles University
2 LAM - Laboratoire d'Astrophysique de Marseille
3 TUT - Tampere University of Technology [Tampere]
4 STAR - Space Sciences, Technologies and Astrophysics Research Institute
5 IMCCE - Institut de Mécanique Céleste et de Calcul des Ephémérides
6 Astronomical Observatory [Poznan]
7 LAGRANGE - Joseph Louis LAGRANGE
8 EAPS - Department of Earth, Atmospheric and Planetary Sciences [MIT, Cambridge]
9 SETI Institute
10 CS3 - Center for Solar System Studies
11 ObsGE - Observatoire Astronomique de l'Université de Genève
12 Asociación Astronómica Astro Henares
13 Institute of Geology [Poznan]
14 Observatoire de Durtal
15 OAC - Observatorio Astronómico de Córdoba
16 IAA - Instituto de Astrofísica de Andalucía
17 L'Uranoscope de l'Ile de France
18 Anunaki Observatory
19 Departamento de Astrofísica [La laguna]
20 UK - Univerzita Karlova [Praha, Česká republika] = Charles University [Prague, Czech Republic]
21 Geneva Observatory
22 Institute for Astronomy [Honolulu]
23 Observatorio Amanecer de Arrakis
24 Konkoly Observatory
25 Thirty Meter Telescope Observatory
26 JPL - Jet Propulsion Laboratory
27 ESTEC - European Space Research and Technology Centre
28 OCA - Observatoire de la Côte d'Azur
29 Department of Mathematics [Tampere]
30 PLANETO - LATMOS
31 Observatoire de la Côte d'Azur
32 DFESTS - Departamento de Fisica, Ingenieria de Sistemas y Teoria de la Señal [Alicante]
33 ICCUB - Institut de Ciencies del Cosmos
34 ESO - European Southern Observatory [Santiago]

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.

Domaines: Planète et Univers [physics]

J. Hanuš, P. Vernazza, M. Viikinkoski, M. Ferrais, N. Rambaux, et al.. (704) Interamnia: a transitional object between a dwarf planet and a typical irregular-shaped minor body. Astronomy and Astrophysics - A&A, 2020, 633, A65 (17 p.). ⟨10.1051/0004-6361/201936639⟩. ⟨insu-02441792⟩