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Planetary rings are observed not only around giant planets 1 , but also around small bodies such as the Centaur Chariklo 2 and the dwarf planet Haumea 3 . Up to now, all known dense rings were located close enough to their parent bodies, being inside the Roche limit, where tidal forces prevent material with reasonable densities from aggregating into a satellite. Here we report observations of an inhomogeneous ring around the trans-Neptunian body (50000) Quaoar. This trans-Neptunian object has an estimated radius 4 of 555 km and possesses a roughly 80-km satellite 5 (Weywot) that orbits at 24 Quaoar radii 6 , 7 . The detected ring orbits at 7.4 radii from the central body, which is well outside Quaoar’s classical Roche limit, thus indicating that this limit does not always determine where ring material can survive. Our local collisional simulations show that elastic collisions, based on laboratory experiments 8 , can maintain a ring far away from the body. Moreover, Quaoar’s ring orbits close to the 1/3 spin–orbit resonance 9 with Quaoar, a property shared by Chariklo’s 2 , 10 , 11 and Haumea’s 3 rings, suggesting that this resonance plays a key role in ring confinement for small bodies. The authors report observations of a dense and inhomogeneous ring at a surprisingly large distance from the trans-Neptunian body Quaoar.

Observation of stellar occultation by objects of the Solar System is a powerful technique that allows measurements of size and shape of the small bodies with accuracies in the order of the kilometre. In addition, the occultation star probes the surroundings of the object, allowing the study of putative rings/debris or atmosphere around it. Since 2009, more than 60 events by trans-Neptunian and Centaur objects have been detected, involving more than 34 different bodies. Some remarkable results were achieved, such as the discovery of rings around Chariklo and Haumea, or the high albedo of Eris, the lack of global atmosphere around Makemake and the discovery of the double shape of 2014 MU69, among others. After the release of Gaia catalogues, predictions became more accurate, leading to an increasing number of successful observations of occultation events. To keep track of the results achieved with this technique, we created a database to gather all the detected events worldwide. The database is presented as an electronic table (http://occultations.ct.utfpr.edu.br/), where the main information obtained from any occultation by small outer solar system objects are listed. The structure and term definitions used in the database are presented here, as well as some simple statistics that can be done with the available results.

Charon among the stars Stellar occultations, when a Solar System object passes between us and a star and blocks its light, are eagerly awaited by astronomers as they provide a chance to make measurements that are not normally possible. It had been 25 years since a solitary observation of a stellar occultation by Pluto's moon Charon. But on 11 July 2005 another occurred and this time observatories across South America were ideally placed to track it. The resulting haul of data has been used to obtain an accurate measure of Charon's radius, of close to 605 km, and to establish an upper limit (a rather low one) on the density of its atmosphere. Visit tinyurl.com/9c56s for a QuickTime movie of the event. Pluto and its satellite, Charon (discovered in 1978; ref. 1 ), appear to form a double planet, rather than a hierarchical planet/satellite couple. Charon is about half Pluto's size and about one-eighth its mass. The precise radii of Pluto and Charon have remained uncertain, leading to large uncertainties on their densities 2 . Although stellar occultations by Charon are in principle a powerful way of measuring its size, they are rare, as the satellite subtends less than 0.3 microradians (0.06 arcsec) on the sky. One occultation (in 1980) yielded a lower limit of 600 km for the satellite's radius 3 , which was later refined to 601.5 km (ref. 4 ). Here we report observations from a multi-station stellar occultation by Charon, which we use to derive a radius, R C = 603.6 ± 1.4 km (1 σ ), and a density of ρ = 1.71 ± 0.08 g cm -3 . This occultation also provides upper limits of 110 and 15 (3 σ ) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.

The stellar occultation technique is a powerful tool to study distant small solar system bodies. Currently, around 2 500 trans-neptunian objects (TNOs) and Centaurs are known. With the astrometry from Gaia and large surveys like the Large Synoptic Survey Telescope (LSST), accurate predictions of occultation events will be available to tens of thousands of TNOs and Centaurs and boost the knowledge of the outer solar system.

PRAIA - Package for the Reduction of Astronomical Images Automatically - is a suite of photometric and astrometric tasks designed to cope with huge amounts of heterogeneous observations with fast processing, no human intervention, minimum parametrization and yet maximum possible accuracy and precision. It is the main tool used to analyse astronomical observations by an international collaboration involving Brazilian, French and Spanish researchers under the Lucky Star umbrella for Solar System studies. Here, we focus on the concepts of differential aperture photometry and digital coronagraphy underneath PRAIA, used in the reduction of stellar occultations, rotational light curves, mutual phenomena and natural satellite observations. We highlight novelties developed by us and never before reported in the literature, which significantly enhance the precision and automation of photometry and digital coronagraphy, such as: a) PRAIA's pixelized aperture photometry (PCAP); b) fully automatic object detection and aperture determination (BOIA); c) better astrometry improving the aperture and coronagraphy centre, including the new Photogravity Center Method besides circular and elliptical Gaussian and Lorentzian generalized profiles; d) coronagraphy of faint objects close to bright ones and vice-versa; e) use of elliptical rings for the coronagraphy of elongated profiles; f) refined quartile ring statistics; g) multiprocessing image capabilities for faster computation speed. We give examples showing the photometry performance, discuss the advantages of PRAIA over other popular packages, and point out the uniqueness of its digital coronagraphy in comparison with other coronagraphy tools. Besides Solar System works, PRAIA can also be used in the differential photometry and digital coronagraphy of any astrophysical observations. PRAIA codes are publicly available at: https://ov.ufrj.br/en/PRAIA/.

PRAIA - Package for the Reduction of Astronomical Images Automatically - is a suite of astrometric and photometric tasks designed to cope with huge amounts of heterogeneous observations with fast processing, no human intervention, minimum parametrization and yet maximum possible accuracy and precision. It is the main tool used to analyse astronomical observations by an international collaboration involving Brazilian, French and Spanish researchers under the Lucky Star umbrella for Solar System studies. In this paper, we focus on the astrometric concepts underneath PRAIA, used in reference system works, natural satellite and NEA astrometry for dynamical and ephemeris studies, and lately for the precise prediction of stellar occultations by planetary satellites, dwarf-planets, TNOs, Centaurs and Trojan asteroids. We highlight novelties developed by us and never reported before in the literature, which significantly enhance astrometry precision and automation. Such as the robust object detection and aperture characterization (BOIA), which explains the long standing empirical photometry/astrometry axiom that recommends using apertures with 2 - 3 sigma (Gaussian width) radius. We give examples showing the astrometry performance, discuss the advantages of PRAIA over other astrometry packages and comment about future planed astrometry implementations. PRAIA codes and input files are publicly available for the first time at: https://ov.ufrj.br/en/PRAIA/. PRAIA astrometry is useful for Solar System as well as astrophysical observations.

Quasars are the choicest objects to define a quasi-inertial reference frame. At the same time, they are active galactic nuclei powered by a massive black hole. As the astrometric precision of ground-based optical observations approaches the limit set by the forthcoming GAIA mission, astrometric stability can be investigated. Though the optical emission from the core region usually exceeds the other components by a factor of a hundred, the variability of those components must surely imply some measure of variability of the astrometric baricenter. Whether this is confirmed or not, it puts important constraints on the relationship of the quasar's central engine to the surrounding distribution of matter. To investigate the correlation between long-term optical variability and what is dubbed as the “random walk” of the astrometric center, a program is being pursued at the WFI/ESO 2.2m. The sample was selected from quasars known to undergo large-amplitude and long-term optical variations (Smith et al. 1993; Teerikorpi 2000). The observations are typically made every two months. The treatment is differential, comparing the quasar position and brightness against a sample of selected stars for which the average relative distances and magnitudes remain constant. The provisional results for four objects bring strong support to the hypothesis of a relationship between astrometric and photometric variability. A full account is provided by Andrei et al. (2009).

Many small ground-based telescopes (with diameter less than 2m) allow us to perform programs of observations well adapted to astrometric measurements. The improvement of limiting magnitudes thanks to the use of CCD detector and their availability make them very useful for follow-up programs or observations on alert. This communication gives several examples of research carried out by members of the IAU working group “Astrometry by small ground-based telescopes”. We also propose setting up of a network of observers for the Gaia follow-up observations.

The resolution of pairs of objects closer than the scale of seeing, and of difference of magnitude larger than ten percent is unreliable by direct imaging. The resulting image FWHM differs from a true PSF by no more than four percent. Yet, the peak of the associated Gaussian is shifted to a larger proportion. The main results are the description of the FWHM and peak location shifts as function of the seeing scale, the centers separation, and of the magnitudes difference. Analytically, the estimators of variation were the resulting Gaussian amplitude, mean value, and standard deviation. The later is shown to be the most reliable estimator.