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

Abstract

Context. 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 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 (ADAM) 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 2 σ level. Conclusions. The rather regular shape of Interamnia implies that the size/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 ≤300km) less massive (m ≤3x10 19 kg) bodies.

Vesta and Ceres

“…Adding to that, a nonperfect correction of the atmospheric turbulence may lead to a global absence of contrast due to the topography in the deconvolved VLT/SPHERE images of Ceres. The same phenomenon is also observed in the case of other C-complex asteroids observed by VLT/SPHERE, such as (10) Hygiea (Vernazza et al, 2020) and ( 704) Interamnia (Hanus et al, 2020).…”

The Nature of Low-albedo Small Bodies from 3 μm Spectroscopy: One Group that Formed within the Ammonia Snow Line and One that Formed beyond It

“…As noted in Section 4.1, there is scant but nonzero evidence for low-albedo objects to show both ST and NST spectra on their surfaces, at least at the large scales that are detectable from Earth. The best case for such an object is Interamnia, a large body that has been interpreted as similar to Ceres and Hygiea in terms of density and lack of surface craters (Hanuš et al 2020). However, as shown in Figure 10, the case for variation on Interamnia is far from obvious.…”

“…Licandro et al 2011;Takir & Emery 2012;Hargrove et al 2015;. Other large asteroids, including Ceres, are additionally suspected of having an icy nature based on shape (McCord & Sotin 2005;Hanuš et al 2020;Yang et al 2020;Vernazza et al 2020Vernazza et al , 2021 or orbital hydrogen (Prettyman et al 2017) measurements. The ways in which the physical properties of icy asteroids might affect their representation in the meteorite and interplanetary dust collections were considered by Rivkin et al (2014Rivkin et al ( , 2015a, with Vernazza et al (2015Vernazza et al ( , 2017Vernazza et al ( , 2021 addressing the problem via adaptive optics shape modeling.…”

Probabilistic modeling of asteroid diameters from Gaia DR2 errors

“…We fed the model with the 11,095 Gaia asteroids from Gaia EDR2 for which previous diameter measurements are available from the literature. The list of diameters was compiled from the works of Drummond (2000); Hanuš et al (2013); Nugent et al (2016); Viikinkoski et al (2017); Alí-Lagoa et al (2018); Sanchez et al (2019); Hanuš et al (2020); Herald et al (2020);Podlewska-Gaca et al (2020). Once the relation between the asteroid diameter and σI G , ĪG and σRA is learned via the logistic BART model, we applied it to the list of 2,886 Gaia DR2 asteroids for which we did not find any diameter information.…”