Dynamical environment and stability around Centaur (2060) Chiron

TL;DR

This work assesses the dynamical stability of material around Centaur (2060) Chiron, which hosts a broad disk and multiple rings, by modeling Chiron as a triaxial body and performing test-particle integrations with REBOUND using a non-spherical gravity field. The authors map stability and spin–orbit resonance structure via Poincaré surfaces of section, elucidating how equatorial ellipticity $C_{22}$ and mass $M$ shape the inner chaotic region (up to ~260 km) and the extended stable disk beyond it. They find that for nominal parameters the inner disk is short-lived (months to a year) unless replenishment occurs, while outer regions can remain stable for decades; resonance locations such as $1:3$, $1:5$, and $2:5$ show bifurcations that could explain ring gaps and asymmetries. The study proposes two viable scenarios to reconcile the observed disk with stability: either a small $C_{22} o0.012$ or a relatively low mass ${M \\lesssim 2 imes10^{18}}$ kg, while acknowledging limitations from neglected ring self-gravity and collisional processes. Overall, the results highlight the critical role of Chiron's figure in shaping its circum-body environment and motivate future occultations and shape refinements to better link rings/disk features to specific resonances.

Abstract

A recent stellar occultation revealed that the Centaur (2060) Chiron hosts a broad disk extending beyond ~200 km from its centre, embedding three ring-like structures (Chi1R, Chi2R, and Chi3R), while a tenuous outer ring (Chi4R) lies beyond the Roche limit. Here, we present a first dynamical assessment of the system's stability through numerical simulations of test particles, accounting for Chiron's triaxial figure. For an equatorial ellipticity of C22~0.02, as inferred from the most recent shape estimates, our simulations reveal a chaotic inner zone extending to ~260 km, where particle lifetimes reach up to a year, while particles beyond ~260 km can remain stable for at least a decade. These results suggest that the innermost portion of the disk is ephemeral and can only persist if continuously replenished. For lower ellipticity values (C22<0.012), however, the entire disk is located within the stable region, regardless of Chiron's mass. Under the physical parameters currently available in the literature, Chi2R is possibly linked to the 1:3 spin-orbit resonance, while Chi1R cannot be linked to the 1:2 resonance, as previously proposed, since this resonance is unstable. Instead, Chi1R and Chi3R may be associated with the 2:5 and 1:5 spin-orbit resonances, respectively. Both the 1:3 and 1:5 resonances are bifurcated, generating chaotic zones that may explain the gap in Chi2R and the longitudinal asymmetry observed in Chi3R.

Figures (5)

• Figure 1: Upper panel shows the ring plane pole-on view of the material around Chiron, with the grayscale proportional to the normal optical depth. The lower panel contains the normal optical depth curve obtained at Observatório do Pico dos Dias and the model fitted, including the ring and the disk material. The blue dotted lines represent the position of Chi1R, Chi2R, and Chi3R. Figure modified from Pereira2025.
• Figure 2: Lifetime of particles in the vicinity of Chiron, assuming the object’s nominal parameters. Each colored pixel corresponds to a particle with a given initial semi-major axis (x-axis) and eccentricity (y-axis), with color indicating its lifetime. Filled black squares mark the maximum eccentricity recorded for particles that begin on Keplerian circular orbits and survive until the end of the simulation, whereas open black squares indicate the last values of the eccentricities registered by the N-body code for particles that are lost through ejection or collision with Chiron. Vertical red lines mark the theoretical locations of spin–orbit resonances, colored curves the resonance centers extracted from Poincaré maps, and shaded regions their widths. The observed ring locations are shown by vertical black lines.
• Figure 3: Selected Poincaré maps showing the stability islands of spin–orbit resonances closest to each ring. The top panel highlights in blue the three islands of the 2:5 SOR, near Chi1R; the middle panel, in shades of green, the double islands of the 1:3 SOR, near Chi2R; and the bottom panel, in shades of purple, the double islands of the 1:5 SOR, near Chi3R. The Jacobi constant associated with each map is indicated in the upper-right corner of the panels.
• Figure 4: Stable region boundary around Chiron as a function of its mass and $C_{22}$, shown by the colormap. Black curves mark the parameter combinations where the boundary coincides with the inner edge of the disk, Chi1R, and Chi2R. The white ‘X’ indicates Chiron’s nominal parameters.
• Figure 5: Theoretical locations of spin–orbit resonances (dashed lines) and stable region boundaries (black curves) for different $C_{22}$ values, as a function of Chiron’s mass. The grey area marks the extent of the detected disk, and the narrow dark-grey bands indicate the rings.