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Register a new userOn the mass and origin of Chariklos rings
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Authors
Margaret Pan
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Observations in 2013 and 2014 of the Centaur 10199 Chariklo and its ring system consistently indicated that the radial width of the inner, more massive ring varies with longitude. That strongly suggests that this ring has a finite eccentricity despite the fast differential precession that Chariklos large quadrupole moment should induce. If the inferred apse alignment is maintained by the rings self-gravity, as it is for the Uranian rings, we estimate a ring mass of a few times 10^16 g and a typical particle size of a few meters. These imply a short collisional spreading time of ~10^5 years, somewhat shorter than the typical Centaur dynamical lifetime of a few Myrs and much shorter than the age of the solar system. In light of this time constraint, we evaluate previously suggested ring formation pathways including collisional ejection and satellite disruption. We also investigate in detail a contrasting formation mechanism, the lofting of dust particles off Chariklos surface into orbit via outflows of sublimating CO and/or N_2 triggered after Chariklo was scattered inward by giant planets. This latter scenario predicts that rings should be common among 100-km class Centaurs but rare among Kuiper belt objects and smaller Centaurs. It also predicts that Centaurs should show seasonal variations in cometary activity with activity maxima occurring shortly after equinox.
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Two narrow and dense rings (called C1R and C2R) were discovered around the Centaur object (10199) Chariklo during a stellar occultation observed on 2013 June 3. Following this discovery, we planned observations of several occultations by Chariklos system in order to better characterize the physical properties of the ring and main body. Here, we use 12 successful occulations by Chariklo observed between 2014 and 2016. They provide ring profiles (physical width, opacity, edge structure) and constraints on the radii and pole position. Our new observations are currently consistent with the circular ring solution and pole position, to within the $pm 3.3$ km formal uncertainty for the ring radii derived by Braga-Ribas et al. The six resolved C1R profiles reveal significant width variations from $sim 5$ to 7.5 km. The width of the fainter ring C2R is less constrained, and may vary between 0.1 and 1 km. The inner and outer edges of C1R are consistent with infinitely sharp boundaries, with typical upper limits of one kilometer for the transition zone between the ring and empty space. No constraint on the sharpness of C2Rs edges is available. A 1$sigma$ upper limit of $sim 20$ m is derived for the equivalent width of narrow (physical width <4 km) rings up to distances of 12,000 km, counted in the ring plane.
The Centaur (10199) Chariklo has the first rings system discovered around a small object. It was first observed using stellar occultation in 2013. Stellar occultations allow the determination of sizes and shapes with kilometre accuracy and obtain characteristics of the occulting object and its vicinity. Using stellar occultations observed between 2017 and 2020, we aim at constraining Chariklos and its rings physical parameters. We also determine the rings structure, and obtain precise astrometrical positions of Chariklo. We predicted and organised several observational campaigns of stellar occultations by Chariklo. Occultation light curves were measured from the data sets, from which ingress and egress times, and rings width and opacity were obtained. These measurements, combined with results from previous works, allow us to obtain significant constraints on Chariklos shape and rings structure. We characterise Chariklos ring system (C1R and C2R), and obtain radii and pole orientations that are consistent with, but more accurate than, results from previous occultations. We confirmed the detection of W-shaped structures within C1R and an evident variation of 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 ring particles size larger than a few microns. The C1R ring eccentricity is found to be smaller than 0.022 (3-sigma), and its width variations may indicate an eccentricity higher than 0.005. We fit a tri-axial shape to Chariklos detections over eleven occultations and determine that Chariklo is consistent with an ellipsoid with semi-axes of 143.8, 135.2 and 99.1 km. Ultimately, we provided seven astrometric positions at a milliarcseconds accuracy level, based on Gaia EDR3, and use it to improve Chariklos ephemeris.
Transit and radial velocity observations indicate a dearth of sub-Jupiter--mass planets on short-period orbits, outlined roughly by two oppositely sloped lines in the period--mass plane. We interpret this feature in terms of high-eccentricity migration of planets that arrive in the vicinity of the Roche limit, where their orbits are tidally circularized, long after the dispersal of their natal disk. We demonstrate that the two distinct segments of the boundary are a direct consequence of the different slopes of the empirical mass--radius relation for small and large planets, and show that this relation also fixes the mass coordinate of the intersection point. The period coordinate of this point, as well as the detailed shape of the lower boundary, can be reproduced with a plausible choice of a key parameter in the underlying migration model. The detailed shape of the upper boundary, on the other hand, is determined by the post-circularization tidal exchange of angular momentum with the star and can be reproduced with a stellar tidal quality factor $Q^prime_*sim10^6$.
High spatial resolution observations of protoplanetary disks (PPDs) by ALMA have revealed many details that are providing interesting constraints on the disk physics as well as dust dynamics, both of which are essential for understanding planet formation. We carry out high-resolution, 2D global hydrodynamic simulations, including the effects of dust feedback, to study the stability of dusty rings. When the ring edges are relatively sharp and the dust surface density becomes comparable to the gas surface density, we find that dust feedback enhances the radial gradients of both the azimuthal velocity profile and the potential vorticity profile at the ring edges. This eventually leads to instabilities on meso-scales (spatial scales of several disk scale heights), causing dusty rings to be populated with many compact regions with highly concentrated dust densities on meso-scales. We also produce synthetic dust emission images using our simulation results and discuss the comparison between simulations and observations.
As differentiated planetesimals cool, their cores can solidify from the outside-in, as evidenced by paleomagnetic measurements and cooling rate estimates of iron meteorites. The details of outside-in solidification and fate of residual core melt are poorly understood. For a core primarily composed of Fe and Ni alloyed with lighter constituent elements, like sulfur, such inward core growth would likely be achieved by growth of solid FeNi dendrites. Growth of FeNi dendrites results in interconnected pockets of residual melt that become progressively enriched in sulfur up to a eutectic composition of 31 wt percent sulfur as FeNi continues to solidify. Here we show that regions of residual sulfur-enriched FeNi melt in the core attain sufficient excess pressures to propagate via dikes into the mantle. Thus, core material will intrude into the overlying rocky mantle or possibly even erupt onto the plantesimals surface. We refer to these processes collectively as ferrovolcanism. Our calculation show that ferrovolcanic surface eruptions are more likely on bodies with mantles less than 50 km thick. We show that intrusive ferromagmatism can produce pallasites, an enigmatic class of meteorites composed of olivine crystals entrained in a matrix of FeNi metal. Ferrovolcanic eruptions may explain the observations that Psyche has a bulk density inconsistent with iron metorites yet shows evidence of a metallic surface composition.
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