A Study of Uranus' Irregular Satellites

PARISI, M. G.; MARIS, M.; CARRARO, G.; BRUNINI, A.

PUCON, CHILE

Simposio; XI IAU Regional Latin American Meeting of Astronomy; 2005

IAU

Very recently, rich systems of irregular satellites of the giant planets have been discovered. Their physical and dynamical properties provide a window on processes operating in the young Solar System. In particular for Uranus they may witness the mechanism leading to the peculiar tilt of its rotation axis, which is usually attributed to a giant tangential impact that suffered Uranus with another protoplanet at the end of its formation. We have carried out the first time-dependent BVRI photometrical study of the three Uranian irregulars, Prospero, Stephano and Setebos and new observations of Sycorax at ESO using FORS2 at VLT-UT1 in two consecutive nights in July 2005 (Parisi et al. 2006 submitted to Icarus, Maris et al. 2006, submitted to ApJ Letters). We find very strong evidences of the extremely irregular structure of these objects in the variation of ? 1.5 mag of the R light curve. Considerable variations were found in all the bands. These results allow us to cast light on the formation (capture) mechanism of these objects, suggesting that these satellites should be fragments of one or more collisional interactions between planetesimals passing near the planet or between a planetesimal and a preexisting satellite, the so called ``break-up´´ processes. These results imply not only a break-up origin for these objects but also that a large impact as the cause of Uranus obliquity cannot be ruled out.