Posição de objetos centauros e transnetunianos através de ocultações estelares na era Gaia

Abstract

Most of the small bodies of the outer Solar System, with the exception of Pluto, were discovered after 1992. It is believed that due to their distance to the Sun, these objects can have information about the Solar System’s formation and dynamical evolution. Therefore, a detailed study of these bodies can contribute significantly to the Solar System’s evolution theory. However, the direct observation of light reflected from the surface of the body allow us determine astrometric position with large error bars. A diameter of 1000 km, at distance of 45 UA from Earth results on angular size of 30 milliseconds of arc on sky plane. So, these bodies appears in the images as dim points of light. Although it’s possible to determine their diameters from termal images, the uncertainties are around 20% of body’s size. An efficient observational alternative has been used on the study of this class of objects, the stellar occultation. The technique consists on the photometric observation of a star, while an object transits in front of it, on the tangent plane of sky. The study of temporal variation of the star brightness allows the accurate characterization the object. With uncertainties of the order of a few kilometers, the information obtained from a stellar occultation allows us to build the object instantaneous profile. The object’s diameter, shape and relative center can be calculated from this profile. We can also detect or discover structures, such as rings, around the main body. This work analyzes 25 stellar occultations by Centaurs and Transneptunian objects, detected between 2009 and 2017. The data set involves events analyzed for the first time here, and events that had results published in the literature. Using the tangent plane on the celestial sphere, and with an observer in Earth’s center, the object’s center was determined, relative to the occulted star. Star positions were taken from the Gaia catalogue, which have uncertainties of the order of a kilometer at the object’s distance, therefore, the calculated center error bars are very precise. This central position is compared with ephemeris in the closest approach instant. Usually, the measured and calculated positions are different allowing us to derive an offset. This offset is used to correct the body ephemeris. Therefore, we obtain the object’s celestial coordinates at the moment of closest approach. These astrometric positions are used to propagate the object’s ephemeris in time. Although careful astrometric measurements are used to calculate the ephemeris, stellar occultation provides high precision positions, allowing improvements on the calculated ephemerid. Better ephemeris allows better predictions for future events, and so the correct places over Earth where they can be observed, contributing to the characterization of this class of small Solar System objects.