Astrometria de Urano e de seus satélites principais: 18 anos de observações no OPD/LNA

Abstract

Most of our current knowledge of the Solar System relies on ground-based observations involving techniques like photometry and astrometry, but also Lunar Laser Ranging (LLR) and Very Large Baseline Interferometry (VLBI). In particular, through techniques as VLBI, radar and LLR the modern ephemerides of the four inner planets, the Sun and the Moon are consistent with the current reference frame adopted by IAU ( International Astronomical Union ), the second realization of the International Celestial Reference Frame (ICRF2), to 1 mas (milliarsec or 0 .001). Such an accuracy, however, is not verified to the external planets. Their ephemerides are not as well determinated and their uncertainty may reach 200 mas. In addition, the determination of their orbits is highly dependent on direct imaging at optical wavelengths. In the context of a long term collaboration among researchers from the Observat´orio Nacional - ON/MCTI and the Observat´orio do Valongo - OV/UFRJ, Solar System observations have been carried out for many years at the Observat´orio do Pico dos Dias - Laborat´orio Nacional de Astrof´ısica (OPD/LNA). As far as Uranus is concerned, images have been acquired with the Perkin-Elmer (1,60 m) as well as the Boller & Chivens (0,60 m) telescopes between 1992 and 2009. We present here a survey of this material, as well as procedures for the precise determination of the positions of Uranus as obtained from its main satellites (Miranda, Ariel, Umbriel, Titania and Oberon). As the image of Uranus is almost always overexposed, the satellites that lie closer to the planet have the determination of their photocenter affected by the scattered light of the central body. For this reason, we applied the procedure of digital coronagraphy, which helps to attenuate the influence of the scattered light from Uranus, leaving the image with a flatter sky background. Although some of the observations were already presented in publications (most of the observations from the Perkin-Elmer telescope), the contribution brought by this work comes mainly from the availability of denser catalogs and with a more precise astrometry (UCAC2, 2MASS) in addition to new reduction techniques. The positions obtained with the Boller & Chivens telescope has never been published nor reduced. In particular, positions of Uranus were obtained with accuracies within the range 35 to 40 mas for the Perkin-Elmer (1,60 m) telescope and accuracies within the range 50 to 60 mas for the Boller & Chivens (0,60 m) telescope, obtained from the positions of the satellites. It represents a gain factor of about 2 when compared to the ones found in the literature, mainly the results from the Perkin-Elmer telescope and also a significant material to improve the ephemeris of the planet.