The rings of (2060) Chiron: Evidence of an evolving system

TL;DR

The paper investigates the emergence and evolution of a ring system around the small body (2060) Chiron by analyzing a 2023 multi-chord stellar occultation. It contrasts an equatorial-disk representation with a spherical-shell model, deriving three co-planar rings at roughly $r \,\approx\, 273$, 325, and 438 km, plus an outer feature near 1380 km, embedded in a broad disk whose mean ring-plane orientation is $\lambda \\approx\,151°$ and $\beta \\approx\,20°$. The total ring mass is estimated near $m_{ring} \\approx 10^{11}$ kg, with a ring-to-Chiron mass ratio $\\mu \\approx 3.5\\times 10^{-8}$, and the inner rings lie close to the 1/2 and 1/3 spin-orbit resonances at radii $a_{1/2} \\approx 238±45$ km and $a_{1/3} \\approx 312±59$ km. Overall, the findings support ongoing ring formation around Chiron, offering valuable constraints on the processes that generate and confine rings around small bodies and informing models of circum-object disk evolution and episodic activity in the outer Solar System.

Abstract

The centaur (2060) Chiron has long been a candidate for hosting material in orbit, based on occultation and photometric and spectroscopic data. Here, we present a multichord stellar occultation observed on 2023 September 10 UT that reveals new and complex structures surrounding Chiron. High-cadence light curves show multiple secondary events that are best explained (when compared with a multishell interpretation) with a system of three confined rings located at average radii of 273, 325, and 438 km, the outermost of which lies beyond Chiron's Roche limit. The rings appear coplanar, with a mean pole orientation of λ = 151° +/- 4° and \b{eta} = 20° +/- 6°. A broader, disklike structure extends from about 200 to 800 km, and a newly detected faint feature is observed at ~1380 km. Chiron thus appears as the fourth small solar system body known for hosting a ring system. Comparisons with previous occultation events that have occurred since 1994 show that these features are not permanent. With these observations, we may witness for the first time the ongoing formation and evolution of a ring system.

Figures (4)

• Figure 1: Comparison between the equatorial disk and shell models. In the panels, the black curve represents the correspondent model compared to the observed light curve. a) The time-based light curve is projected in the radial distance in the ring plane considering the preferred mean pole orientation. b) The normalized flux as a function as the radial distance in the sky plane, compared with the model based in the consecutive spherical shells. The residuals plus 1 for the equatorial disk (c) and shell (d) models
• Figure 2: Left: Sky-plane projection of the shell model. The color map shows the optical depth $\tau = - \ln(T)/2$ from three Gaussian-distributed spherical shells. The central grey circle (98 km radius) marks Chiron's projected size; black ellipses indicate the confined rings; red circles denote the Roche limits. Right: Sky-plane view of the equatorial disk model. Optical depth $\tau = \tau_{\mathrm{N}}/|\sin(B)|$ is shown. The grey circle (98 km radius) marks Chiron. Black ellipses show the three narrow rings; dashed blue ellipses mark the 1/2 and 1/3 spin-orbit resonances; red ellipses indicate Roche limits. Inset: zoom-in of the innermost region with rings Chi1R (1), Chi2R (2), and Chi3R (3).
• Figure 3: Occultation profile from the PE160 observation on September 10, 2023 event of the normal optical depth as a function of radial distance in the ring plane shows the extent of the broad ring and the arrangement of the dense rings. The Coloured regions indicate the boundaries of the SOR 1/2 resonance (green), SOR 1/3 resonance (red), and the Roche limit (blue). The vertical dotted lines indicates the respective ring center.
• Figure 4: Sky-plane projection of Chiron and its confined rings. The projected ellipsoid and the occulting chords for each event is plotted in turquoise, with their $1\sigma$ uncertainties represented by the red segments at the chord extremities. The dashed blue lines represent the occulting chords as seen from each site. The red segments mark the projected widths of the ring events plus their respective $1\sigma$ error bars. The labels 1, 2, and 3 refers to the Chi1R, Chi2R, and Chi3R rings, respectively.