1 Introduction

Sycorax (1997 U2) is the brightest of the five irregular satellites of Uranus recently discovered by Gladman et al. (1998, 2000). It is a very small object, orbiting far from Uranus on a retrograde orbit (Table 1).

These five satellites are the first irregular satellites detected around Uranus. The so-called irregular satellites of giant planets are satellites located far from the planet on highly inclined elongated orbits, whereas regular satellites orbit closer to the planet on nearly circular orbits, with low inclinations. Retrograde orbits are found only among irregular satellites.

 

 

Table 1: Physical and orbital properties of Sycorax ($R_{\rm U}$ = Uranian radius).

Semi-major axis	481 $R_{\rm U}$
Eccentricity	0.509
Inclination	152.7$^\circ$
Orbital period	1289 days

Many studies have been published about the origin of the irregular satellites of Jupiter and Saturn. It is now generally accepted that temporary captures by giant planets can occur. We know several examples of temporarily captured comets, such as the famous Shoemaker-Levy 9. However, it is much more difficult to get stable orbits, as this requires an efficient way of energy dissipation. Pollack et al. (1979) suggested that some satellites may have been captured by gas drag in the protoplanetary nebula. This process could explain the origin of the irregular satellites of Jupiter and Saturn. According to Pollack et al., such a process must have taken place shortly before the end of planetary formation. Another scenario was proposed by Brunini (1995) who suggested that, during the last stages of accretion, the giant planets could have locked temporary captured planetesimals on stable orbits, by changing the shape of their Hill sphere during the temporary capture. This requires a high accretion rate, as well as a rapid migration of the planet, which could both favour this process of permanent capture.

In the case of Uranus, the large obliquity of its spin axis has also to be taken into account. It has been suggested that a collision could have occurred at the end of Uranus's formation that could explain the large obliquity (see e.g. Parisi & Brunini 1997). The existence of irregular satellites orbiting around Uranus could give us constraints about the collision hypothesis (Brunini et al. 2001).

In addition to dynamical studies of these irregular satellites, the knowledge of their physical and chemical properties could give us information about their origin and evolution. B, R, and I magnitudes have been obtained by Gladman et al. (1998) for the two largest of these satellites, Sycorax and Caliban. Gladman et al. deduced from these measurements that the two objects were red, and suggested that they could come from the Kuiper Belt, since most Trans-Neptunian Objects (TNOs) are known to be very red. In this paper, we present refined photometric measurements of Sycorax in the visible, as well as some complementary photometric and spectroscopic measurements in the near-infrared.