Orbital Characterization of a Newly Discovered Small Satellite Around Quaoar

TL;DR

The study reports the discovery of a faint satellite near Quaoar's rings via stellar occultation and performs a Bayesian orbit analysis under a circular Keplerian model, yielding $a=5838^{+512}_{-326}$ km and $P_{ m orb}=3.6^{+0.5}_{-0.3}$ days (with an eccentric-fit alternative of $a=5910^{+702}_{-472}$ km and $P_{ m orb}=3.7^{+0.7}_{-0.4}$ days); the satellite sits near a $5:3$ mean-motion resonance with Quaoar's outer ring, Q1R. Recovery via occultations would require hundreds of observing stations, and JWST/NIRCam imaging shows no convincing detection due to PSF-model limitations and faintness, though future 30-meter class telescopes should detect it. The findings support a formation scenario in which Quaoar's rings and satellites originated from a broad collisional disk, motivating further occultation campaigns and advanced hydrodynamical/tidal modeling to elucidate the coupled ring-satellite evolution around Quaoar.

Abstract

Recent observations of a stellar occultation have revealed the presence of a previously undiscovered small satellite around Quaoar. Orbiting near Quaoar's unusual ring system, this new satellite has the potential to provide significant insights into the formation and evolution of Quaoar and its ring system. In this letter, we characterize the orbit of this newly discovered satellite, finding that it is likely on a $3.6^{+0.5}_{-0.3}$-day orbit, plausibly placing it near a 5:3 mean motion resonance with Quaoar's outermost known ring. Examining the possibility of observing the newly discovered satellite with further stellar occultations, we estimate that $\sim$hundreds of observing stations are required for recovery, since phase information about its orbit was rapidly lost after the lone detection. We also attempted to recover the satellite in JWST NIRCam imaging of Quaoar, but find no convincing detection. This non-detection is limited by the accuracy of the available NIRCam PSF models, as well as the satellite's extreme faintness and close-in orbital separation. Therefore, current-generation telescopes will likely struggle to directly image this new satellite, but near-future 30-meter-class telescopes should prove capable of detecting it. Discovery of such a satellite provides evidence that the rings around Quaoar may have been part of an initially broad collisional disk that has evolved considerably since its formation. To further explore this hypothesis, we encourage follow-up observations of the rings and satellites with stellar occultations and direct imaging, as well as updated hydrodynamical, collisional, and tidal modeling of the system.

Figures (5)

• Figure 1: The inclination of the local Laplace plane as a function of the inner satellite's semi-major axis and Weywot's mass. Dotted lines show Weywot's expected mass from different densities based on Weywot's diameter of $\sim$165 km (Fernández-Valenzuela et al., in prep.).
• Figure 2: The evolution of the new satellite's on-sky probability density after discovery with high densities shown by brighter color. The 2-d histogram is log-scaled and normalized at each individual epoch to better show the evolution of the distribution. After detection, Keplerian shear rapidly broadens the probability distribution until the distribution is roughly uniform in phase after 1-2 months.
• Figure 3: Similar to Figure \ref{['fig:evolution']}, but for an orbit fit where eccentricity was allowed to be a free parameter. Aside from a slight expansion of the probability distribution away from the intial detection, the main effect is to eliminate the pinch at the top left of the probability distribution.
• Figure 4: Probability of recovering the new satellite as a function of number of observing stations spaced at minimum a satellite diameter ($D$) apart. Solid lines show the probability when stations are placed in regions of highest probability first (in descending order) while dotted lines show the probability with random site selection (although with at least $D$ km apart). For $D =$ 30 km---the lower limit on the satellite's size---a campaign must have at least 98 observing stations to have a $>$ 80% chance of recovery, assuming ideal station placement and perfect recovery if in the shadow path.
• Figure 5: PSF fitting residuals of JWST images of Quaoar. Black circles show a 0.21$"$ radius (7 NIRCam pixels), within which the newly discovered satellite should be. The expected surface brightness of the satellite in the NIRCam images (assuming $V=28$ mag) is $\sim$1 MJy sr$^{-1}$. No clear detection is apparent, with PSF subtraction residuals dominating the region in which we expect the satellite to be. Without improvements to the NIRCam PSF model, or high quality empirical PSFs, recovery of the satellite will be $\sim$impossible.