We use data from five stellar occultations observed between 2013 and 2016 to constrain Chariklo's size and shape, and the ring reflectivity. We consider four possible models for Chariklo (sphere, Maclaurin spheroid, triaxial ellipsoid, and Jacobi ellipsoid), and we use a Bayesian approach to estimate the corresponding parameters. . Finally, the Jacobi model has semiaxes a=157±4km, b=139±4km, and c=86±1km, and density . Depending on the model, we obtain topographic features of 6-11km, typical of Saturn icy satellites with similar size and density. We constrain Chariklo's geometric albedo between 3.1% (sphere) and 4.9% (ellipsoid), while the ring I/F reflectivity is less constrained between 0.6% (Jacobi) and 8.9% (sphere). The ellipsoid model explains both the optical light curve and the long-term photometry variation of the system, giving a plausible value for the geometric albedo of the ring particles of 10%-15%. The derived mass of Chariklo of 6-8×10 18 kg places the rings close to 3:1 resonance between the ring mean motion and Chariklo's rotation period.
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 Chariklo's 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 ±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 ∼5 to 7.5km. 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 C2R's edges is available. A 1s upper limit of ∼20m is derived for the equivalent width of narrow (physical width 4 < km) rings up to distances of 12,000km, counted in the ring plane.
Pluto and its main satellite, Charon, occulted the same star on 2008 June 22. This event was observed from Australia and La Réunion Island, providing the east and north Charon Plutocentric offset in the sky plane (J2000): X = + 12,070.5 ± 4 km (+ 546.2 ± 0.2 mas), Y = + 4,576.3 ± 24 km (+ 207.1 ± 1.1 mas) at 19:20:33.82 UT on Earth, corresponding to JD 2454640.129964 at Pluto. This yields Charon's true longitude L = 153.483 ± 0.• 071 in the satellite orbital plane (counted from the ascending node on J2000 mean equator) and orbital radius r = 19,564 ± 14 km at that time. We compare this position to that predicted by (1) the orbital solution of Tholen & Buie (the "TB97" solution), (2) the PLU017 Charon ephemeris, and (3) the solution of Tholen et al. (the "T08" solution). We conclude that (1) our result rules out solution TB97, (2) our position agrees with PLU017, with differences of ΔL = + 0.073 ± 0.• 071 in longitude, and Δr = + 0.6 ± 14 km in radius, and (3) while the difference with the T08 ephemeris amounts to only ΔL = 0.033 ± 0.• 071 in longitude, it exhibits a significant radial discrepancy of Δr = 61.3 ± 14 km. We discuss this difference in terms of a possible image scale relative error of 3.35 × 10 −3 in the 2002-2003 Hubble Space Telescope images upon which the T08 solution is mostly based. Rescaling the T08 Charon semi-major axis, a = 19, 570.45 km, to the TB97 value, a = 19636 km, all other orbital elements remaining the same ("T08/TB97" solution), we reconcile our position with the re-scaled solution by better than 12 km (or 0.55 mas) for Charon's position in its orbital plane, thus making T08/TB97 our preferred solution.
In October 2014, several specimens of Gobioninae were caught in the river Mur in Styria (Austria), later described as a new species of the genus Romanogobio. The species Romanogobio skywalkeri is to current knowledge endemic to the river Mur and of high conservation value. This study aims to investigate, for the first time, the microhabitat of R. skywalkeri in the upper river Mur. In winter 2016 and spring 2017 a sampling campaign was carried out to investigate the microhabitat of this species. In total, 60 adult and subadult individuals were caught. The microhabitat of these 60 individuals was assessed regarding water depth, flow velocity (v40 and v0) and substrate composition. R. skywalkeri at this point seems to be a highly rheophilic species with a narrow habitat preference. The preferred habitat found during our study consisted solely of riffle sections with fast flow velocity (0.45–1.3 m/s) and coarse substrate fractions, mostly meso‐ and macrolithal (6.3–40 cm). The current distribution range only covers about 100 km of the Mur between Fisching and Frohnleiten, which makes this endemic species highly vulnerable to habitat loss and alteration by anthropogenic impacts.
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