Physical properties of Centaur (60558) 174P/Echeclus from stellar occultations

Abstract

The Centaur (60558) Echeclus was discovered on 2000 March 03, orbiting between the orbits of Jupiter and Uranus. After exhibiting frequent outbursts, it also received a comet designation, 174P. If the ejected material can be a source of debris to form additional structures, studying the surroundings of an active body like Echeclus can provide clues about the formation scenarios of rings, jets, or dusty shells around small bodies. Stellar occultation is a handy technique for this kind of investigation, as it can, from Earth-based observations, detect small structures with low opacity around these objects. Stellar occultation by Echeclus was predicted and observed in 2019, 2020, and 2021. We obtain upper detection limits of rings with widths larger than 0.5 km and optical depth of τ = 0.02. These values are smaller than those of Chariklo's main ring; in other words, a Chariklo-like ring would have been detected. The occultation observed in 2020 provided two positive chords used to derive the triaxial dimensions of Echeclus based on a 3D model and pole orientation available in the literature. We obtained a = 37.0 ± 0.6 km, b = 28.4 ± 0.5 km, and c = 24.9 ± 0.4 km, resulting in an area-equivalent radius of 30.0 ± 0.5 km. Using the projected limb at the occultation epoch and the available absolute magnitude ($\rm {H}_{\rm {v}} = 9.971 \pm 0.031$), we calculate an albedo of pv = 0.050 ± 0.003. Constraints on the object's density and internal friction are also proposed.