Refined physical parameters for Chariklo’s body and rings from stellar occultations observed between 2013 and 2020

Abstract

Context. The Centaur (10199) Chariklo has the first ring system discovered around a small object. It was first observed using stellar occultation in 2013. Stellar occultations allow sizes and shapes to be determined with kilometre accuracy, and provide the characteristics of the occulting object and its vicinity. Aims. Using stellar occultations observed between 2017 and 2020, our aim is to constrain the physical parameters of Chariklo and its rings. We also determine the structure of the rings, and obtain precise astrometrical positions of Chariklo. Methods. We predicted and organised several observational campaigns of stellar occultations by Chariklo. Occultation light curves were measured from the datasets, from which ingress and egress times, and the ring widths and opacity values were obtained. These measurements, combined with results from previous works, allow us to obtain significant constraints on Chariklo’s shape and ring structure. Results. We characterise Chariklo’s ring system (C1R and C2R), and obtain radii and pole orientations that are consistent with, but more accurate than, results from previous occultations. We confirm the detection of W-shaped structures within C1R and an evident variation in radial width. The observed width ranges between 4.8 and 9.1 km with a mean value of 6.5 km. One dual observation (visible and red) does not reveal any differences in the C1R opacity profiles, indicating a ring particle size larger than a few microns. The C1R ring eccentricity is found to be smaller than 0.022 (3σ), and its width variations may indicate an eccentricity higher than ~0.005. We fit a tri-axial shape to Chariklo’s detections over 11 occultations, and determine that Chariklo is consistent with an ellipsoid with semi-axes of 143.8−1.5+1.4, 135.2−2.8+1.4, and 99.1−2.7+5.4 km. Ultimately, we provided seven astrometric positions at a milliarcsecond accuracy level, based on Gaia EDR3, and use it to improve Chariklo’s ephemeris.

A dense ring of the trans-Neptunian object Quaoar outside its Roche limit

“…An assessment of the effect of the 1/3 SOR can be achieved through N-body collisional simulations with a ring that completely surrounds Quaoar. Such simulations applied to Chariklo actually show that the 1/3 SOR not only excites the orbital eccentricities of the particles as expected from equation (11), but also leads to a ring confinement near this resonance 27,28 . However, these simulations do not account yet for self-gravity.…”

“…where 'apparent' refers to the quantity measured in the sky plane (see ref 11 and references therein). Knowing the ring pole orientation, these values can be converted to their values normal to the ring plane, p and τ .…”

“…where r i is the observed distance of the i th data point to the ring center, r ′ i is the corresponding distance of the model (the ellipse) and σ i is the uncertainty on the distance associated with the timing uncertainty of the i th data point. Finally, the σ model parameter accounts for extra uncertainties associated with our model 11,36 , stemming in particular from the unknown shape of Quaoar itself, resulting in model bias on its center position. Considering we have a single chord event that we fit an area equivalent circle of radius 555 km, but Quaoar's shape may be a Maclaurin spheroid 4 with an equatorial radius of 569 km and an oblateness of 0.087, this would cause a bias in the central position typically of ∼27 km, the value we choose for the σ model .…”

“…For Chariklo, a 1/3 = 408 ± 20 km (ref. 47 ), while its two rings orbit at 386 km and 400 km from the body 11 . Similarly, for Haumea we have a 1/3 = 2, 285 ± 8 km, while the ring orbits at 2, 287 +75 −45 km (ref.…”

“…Each secondary event provides the radial width W r , a mean normal opacity p and a mean normal optical depth τ of the ring, once projected in the ring plane 10,11 . These quantities also provide the equivalent width (E p = pW r ) and equivalent depth (A τ = τ W r ) of each profile, see Methods and Table 1.…”