LATEST RIO-GROUP PUBLICATIONS

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The Rio Group

Since 2005, an international team has been dedicated to the study of distant bodies in the Solar System through stellar occultations. It is led by Dr. Bruno Sicardy, researcher of the Paris-Meudon Observatory, and includes researchers from the Institute of Astrophysics of Andalucia, led by Dr. José Luiz Ortiz and also researchers from Brazil, led by Dr. Roberto Vieira Martins from National Observatory in Rio de Janeiro. The Brazilian team has an infrastructure of LIneA and the support of INCT of e-Universo, and has researchers and students from National Observatoy (ON/MCTIC: Dr. Roberto Vieira Martins, Dr. Julio IB Camargo, Dr. Gustavo Benedetti Rossi), Valongo Observatory (OV/UFRJ: Prof. Dr. Marcelo Assafin, Dr. Altair Gomes Junior) and Federal University of Technology - Paraná (UTFPR/Curitiba: Prof. Dr. Felipe Braga Ri

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Articles

The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation

Haumea—one of the four known trans-Neptunian dwarf planets—is a very elongated and rapidly rotating body1,2,3. In contrast to other dwarf planets4,5,6, its size, shape, albedo and density are not well constrained. The Centaur Chariklo was the first body other than a giant planet known to have a ring system7, and the Centaur Chiron was later found to possess something similar to Chariklo’s rings8,9. Here we report observations from multiple Earth-based observatories of Haumea passing in front of a distant star (a multi-chord stellar occultation). Secondary events observed around the main body of Haumea are consistent with the presence of a ring with an opacity of 0.5, width of 70 kilometres and radius of about 2,287 kilometres. The ring is coplanar with both Haumea’s equator and the orbit of its satellite Hi’iaka. The radius of the ring places it close to the 3:1 mean-motion resonance with Haumea’s spin period—that is, Haumea rotates three times on its axis in the time that a ring particle completes one revolution. The occultation by the main body provides an instantaneous elliptical projected shape with axes of about 1,704 kilometres and 1,138 kilometres. Combined with rotational light curves, the occultation constrains the three-dimensional orientation of Haumea and its triaxial shape, which is inconsistent with a homogeneous body in hydrostatic equilibrium. Haumea’s largest axis is at least 2,322 kilometres, larger than previously thought, implying an upper limit for its density of 1,885 kilograms per cubic metre and a geometric albedo of 0.51, both smaller than previous estimates1,10,11. In addition, this estimate of the density of Haumea is closer to that of Pluto than are previous estimates, in line with expectations. No global nitrogen- or methane-dominated atmosphere was detected.

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Articles

A ring system detected around the Centaur (10199) Chariklo

Hitherto, rings have been found exclusively around the four giant planets in the Solar System1. Rings are natural laboratories in which to study dynamical processes analogous to those that take place during the formation of planetary systems and galaxies. Their presence also tells us about the origin and evolution of the body they encircle. Here we report observations of a multichord stellar occultation that revealed the presence of a ring system around (10199) Chariklo, which is a Centaur—that is, one of a class of small objects orbiting primarily between Jupiter and Neptune—with an equivalent radius of 124??9?kilometres (ref. 2). There are two dense rings, with respective widths of about 7 and 3 kilometres, optical depths of 0.4 and 0.06, and orbital radii of 391 and 405 kilometres. The present orientation of the ring is consistent with an edge-on geometry in 2008, which provides a simple explanation for the dimming3 of the Chariklo system between 1997 and 2008, and for the gradual disappearance of ice and other absorption features in its spectrum over the same period4,5. This implies that the rings are partly composed of water ice. They may be the remnants of a debris disk, possibly confined by embedded, kilometre-sized satellites.

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