Photometry of Irregular Satellites of Uranus and Neptune

We present BVR photometric colors of six Uranian and two Neptunian irregularsatellites, collected using the Magellan Observatory (Las Campanas, Chile) andthe Keck Observatory, (Manua Kea, Hawaii). The colors range from neutral tolight red, and like the Jovian and the Saturnian irregulars (Grav et al. 2003)there is an apparent lack of the extremely red objects found among the Centaursand Kuiper belt objects. The Uranian irregulars can be divided into three possible dynamical families,but the colors collected show that two of these dynamical families, the Calibanand Sycorax-clusters, have heterogeneous colors. Of the third possible family,the 168-degree cluster containing two objects with similar average inclinationsbut quite different average semi-major axis, only one object (U XXI Trinculo)was observed. The heterogeneous colors and the large dispersion of the averageorbital elements leads us to doubt that they are collisional families. We favorsingle captures as a more likely scenario. The two neptunians observed (N IINereid and S/2002 N1) both have very similar neutral, sun-like colors. Togetherwith the high collisional probability between these two objects over the age ofthe solar system (Nesvorny et al. 2003, Holman et al. 2004), this suggests thatS/2002 N1 be a fragment of Nereid, broken loose during a collision or crateringevent with an undetermined impactor.Comment: 13 pages (including 3 figures and 2 tables). Submitted to ApJ Letter