Close-to-fission dumbbell Jupiter-Trojan (17365) Thymbraeus

TL;DR

This study characterizes the Jupiter Trojan (17365) Thymbraeus, showing it is likely a bilobated dumbbell-shaped asteroid with low density, close to fission, and not a binary system, using ground-based lightcurve observations and equilibrium modeling.

Contribution

It provides the first detailed shape and density analysis of Thymbraeus, suggesting it is a bilobated object rather than a binary, and discusses its proximity to fission conditions.

Findings

Thymbraeus is likely a bilobated dumbbell-shaped asteroid.

Its density is approximately 830 kg/m^3, consistent with other Trojans.

The asteroid's rotation is near the fission threshold.

Abstract

Every population of small bodies in the Solar system contains a sizable fraction of multiple systems. Among these, the Jupiter Trojans have the lowest number of known binary systems and the least characterized. We aim at characterizing the reported binary system (17365) Thymbraeus, one of the only seven multiple systems known among Jupiter Trojans. We conducted light curves observing campaigns in 2013, 2015, and 2021 with ground-based telescopes. We model these lightcurves using dumbbell equilibrium figures. We show that Thymbraeus is unlikely a binary system. Its light curves are fully consistent with a bilobated shape: a dumbbell equilibrium figure. We determine a low density of 830 +/- 50 kg.m-3 , consistent with the reported density of other Jupiter Trojan asteroids and small Kuiper-belt objects. The angular velocity of Thymbraeus is close to fission. If separated, its components would become a similarly-sized double asteroid such as the other Jupiter Trojan (617) Patroclus.