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(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 B. Yang 34
30 IMPEC - LATMOS
LATMOS - Laboratoire Atmosphères, Milieux, Observations Spatiales
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.
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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, EDP Sciences, 2020, 633, A65 (17 p.). ⟨10.1051/0004-6361/201936639⟩. ⟨insu-02441792⟩

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