101 Trojans: a tale of period bimodality, binaries, and extremely slow rotators from K2 photometry

TL;DR

This study analyzes the rotation periods, binary nature, and densities of Jovian Trojan asteroids using K2 photometry, revealing a period bimodality, a significant binary fraction, and low densities supporting an outer Solar System origin.

Contribution

It provides new photometric data for 45 Trojans, extends the sample to 101, and identifies a period bimodality, binary candidates, and low-density characteristics.

Findings

Bimodal distribution of rotation periods with a cutoff at ~100 hr.

Approximately 25% are slow rotators likely due to binarity.

Low densities suggest compositions similar to cometary and TNO populations.

Abstract

Various properties of Jovian trojan asteroids such as composition, rotation periods, and photometric amplitudes, or the rate of binarity in the population can provide information and constraints on the evolution of the group and of the Solar System itself. Here we present new photometric properties of 45 Jovian trojans from the K2 mission of the Kepler space telescope, and present phase-folded light curves for 44 targets, including (11351) Leucus, one of the targets of the Lucy mission. We extend our sample to 101 asteroids with previous K2 Trojan measurements, then compare their combined amplitude- and frequency distributions to other ground-based and space data. We show that there is a dichotomy in the periods of Trojans with a separation at $\sim 100$ hr. We find that 25% of the sample are slow rotators (P$\geq$30 hr), which excess can be attributed to binary objects. We also show that 32 systems can be classified as potential detached binary systems. Finally, we calculate density and rotation constraints for the asteroids. Both the spin barrier and fits to strengthless ellipsoid models indicate low densities and thus compositions similar to cometary and TNO populations throughout the sample. This supports the scenario of outer Solar System origin for Jovian trojans.