Beyond Point Masses. V. Weywot's Non-Keplerian Orbit

TL;DR

This study analyzes Weywot's orbit around Quaoar, revealing non-Keplerian dynamics, a very low eccentricity, and Quaoar's oblateness, which suggests differentiation and impacts ring stability understanding.

Contribution

It provides the first detailed dynamical model including non-spherical gravity and surface heterogeneity effects, refining orbital parameters and internal structure insights.

Findings

Weywot's orbit deviates from Keplerian due to non-spherical gravity.

Upper limit on Weywot's eccentricity is e<0.02.

Quaoar's oblateness indicates it is differentiated with a density of 1751 kg/m^3.

Abstract

We present a detailed dynamical analysis of the Quaoar-Weywot system based on nearly 20 years of high-precision astrometric data, including new HST observations and stellar occultations. Our study reveals that Weywot's orbit deviates significantly from a purely Keplerian model, requiring the inclusion of Quaoar's non-spherical gravitational field and center-of-body-center-of-light (COB-COL) offsets in our orbit models. We place a robust upper limit on Weywot's orbital eccentricity ($e<0.02$), substantially lower than previous estimates, which has important implications for the strength of mean motion resonances (MMRs) acting on Quaoar's ring system. Under the assumption that Quaoar's rings lie in its equatorial plane, we detect Quaoar's dynamical oblateness, $J_2$, at $\sim$2$\sigma$ confidence. The low $J_2$ value found under that assumption implies Quaoar is differentiated, with a total bulk density of $1751\pm13$ (stat.) kg m$^{-3}$. Additionally, we detect significant COB-COL offsets likely arising from latitudinal albedo variations across Quaoar's surface. These offsets are necessary to achieve a statistically robust orbit fit and highlight the importance of accounting for surface heterogeneity when modeling the orbits of dwarf planet moons. These findings improve our understanding of Quaoar's interior and surface while providing key insights into the stability and confinement mechanisms of its rings.