Observation of an eclipse of U-3 Titania by U-2 Umbriel on December 8, 2007 with ESO-VLT

Context. Equinox occurred on Uranus in 2007, allowing unique observations of mutual events of the satellites that occur every 42 years. On December 8, 2007, we observed an eclipse of Titania U-3 by Umbriel U-2. Aims. Our goal was to record an observation of very high accuracy in order to evaluate the quality of the available dynamical models of the motion of the satellites. Aims. Our goal was to record an observation of very high accuracy in order to evaluate the quality of the available dynamical models of the motion of the satellites. Methods. Such an observation is challenging because of the faintness of the satellites, the vicinity of the bright planet Uranus, and the small amplitude of the magnitude drop observable during the eclipse. We recorded the event in K-band, using the ESO Very Large Telescope in Chile equipped with the NACO adaptive optics camera. Results. High signal/noise ratio images were obtained for the event making possible the determination of relative positions of the involved satellites. Comparing our results with theoretical models of the satellites motions, we obtain a valuable assessment of the accuracy of those models. Conclusions. Such observations provide important constraints on the orbits of the satellites. We discuss what is needed to improve existing dynamical models of the Uranian satellite system.