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

Abstract

With an estimated diameter in the 320 to 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 $D$ $\lesssim$ 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. 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 ($D$ $\lesssim$ 200 km) bodies. 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 (ADAM) algorithm to reconstruct the 3D-shape model of Interamnia and to determine its spin state. Interamnia's volume-equivalent diameter of 332 $\pm$ 6 km implies a bulk density of $\rho$=1.98 $\pm$ 0.68 gcm${}^{-3}$ , 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 2 $\sigma$ level. 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 ($D$ $\lesssim$ 300km) less massive ($m$ $\lesssim$ 3x10${}^{19}$ kg) bodies.

Accepted for publication in Astronomy and Astrophysics

References in corpus (12)

• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• Main Belt Asteroids with WISE/NEOWISE: Near-Infrared Albedos
• Asteroids' physical models from combined dense and sparse photometry and scaling of the YORP effect by the observed obliquity distribution
• Different origins or different evolutions? Decoding the spectral diversity among C-type asteroids
• ADAM: a general method for using various data types in asteroid reconstruction
• 3D shape of asteroid (6)~Hebe from VLT/SPHERE imaging: Implications for the origin of ordinary H chondrites
• Adaptive optics and lightcurve data of asteroids: twenty shape models and information content analysis
• Against the biases in spins and shapes of asteroids
• The homogeneous internal structure of CM-like asteroid (41) Daphne
• Closing the gap between Earth-based and interplanetary mission observations: Vesta seen by VLT/SPHERE
• The shape of (7) Iris as evidence of an ancient large impact?
• Asteroid mass estimation using Markov-chain Monte Carlo

Cited by in corpus (1)

• Binary asteroid (31) Euphrosyne: Ice-rich and nearly spherical