3 Results and discussion

These new photometric results complete BRI data previously obtained by Gladman et al. (1998). Sycorax spectrum has a strong red slope in the visible range: this is roughly the same behaviour as TNOs and Centaurs. Figure 2 shows a color plot (V-R vs. V-I) of Sycorax compared to other small bodies (TNOs, Centaurs, cometary nuclei, Trojans, and irregular Jovian satellites).

Figure 2: V-R vs. V-I color plot. This is a comparison of different types of small bodies in the Solar System: Trans-Neptunian Objects (Barucci et al. 1999; Barucci et al. 2000; Doressoundiram et al. 2001), Cometary nuclei (Thomas & Keller 1989; Luu 1993; Lamy et al. 2001), Trojans (Degewij & Van Houten 1979), Centaurs (Davies 2001), Irregular jovian satellites (Luu 1991), Sycorax (this work). The solar colors are shown for comparison (Hardorp 1980; Hartmann et al. 1982).

Sycorax colors are more similar to those of TNOs, Centaurs, and cometary nuclei, than to those of Trojans, and irregular Jovian satellites. Such a common behaviour among Centaurs, comets, and TNOs is not surprising as Centaurs and short-period comets are believed to come from the Kuiper Belt. Sycorax colors seem to be intermediate between those of neutral and very red TNOs and Centaurs. The visible and near-infrared colors of Sycorax are given in Table 5.

We have also compared the visible colors of Sycorax with those of regular satellites of Uranus: Karkoschka (1997) found that the major satellites of Uranus have visible colors that range from slightly bluer than the F-type asteroids (negative gradient) to slightly redder than the D-type asteroids. So the major regular satellites of Uranus appear to be bluer than Sycorax. JHK measurements of some regular satellites have recently been obtained by Trilling & Brown (2000): several appear to have flat spectra in the near-infrared range, as seems to be the case for Sycorax and many other objects among TNOs and Centaurs, in this spectral range.

Sycorax spectral behaviour from the visible to the near-infrared ranges is much more unusual. The reflectivity increases in the visible range, whereas it strongly decreases from 0.8 to 1.2 $\mu$m, and seems to be flat from 1.2 to 2 $\mu$m. Such a spectral behaviour is very different from that of TNOs and Centaurs: none of the spectra already obtained show such a strong difference between the visible and near-infrared range (see Fig. 3).

Figure 3: Reflectivity of Sycorax, compared to those of some TNOs and Centaurs (from Jewitt & Luu 1998). Solar contribution has been removed using the solar colors (Hardorp 1980; Hartmann et al. 1982). The reflectivity has been computed using the different estimates of the Jmagnitude, in order to minimize the rotational effect on the reflectivity curve. Sycorax spectrum is very different from those of TNOs and Centaurs, especially around 0.8-1.2 $\mu$m. The reflectivity has been normalized to 1 at 0.55 $\mu$m.

 

 

Table 5: Colors of Sycorax. The J-H and the J-Ks colors have been computed using the VLT photometry (May 1999), whereas the other ones have been computed using the TNG photometry (Aug. 2000). The average of the Vmeasurements, as well as the absolute H_V magnitude are also given. Solar colors: B-V=0.76, V-R=0.36, V-I=0.69, V-J=1.08, J-H=0.29, $J-K{\rm s}=0.35$ (Hardorp 1980; Hartmann et al. 1982).

B-V	=	$0.71 \pm 0.10$
V-R	=	$0.52 \pm 0.07$	$\overline{V}=20.75 \pm 0.06$
V-I	=	$1.05 \pm 0.06$
V-J	=	$1.12 \pm 0.16$	$H_V = 7.83 \pm 0.06$
J-H	=	$0.25 \pm 0.11$
J-Ks	=	$0.46 \pm 0.16$

Since the whole data set has not been obtained simultaneously, a possible rotational effect could affect the reflectivity curve. However, we first assumed that no strong rotational effect affects the reflectivity curve, and we tried to investigate the materials which may contribute to the spectrum of Sycorax. We ran a radiative transfer model (Douté & Schmitt 1998), considering simple geographical mixtures of organics (kerogen, Titan tholin, amorphous carbon), minerals (pyroxene and olivine), and water ice (Grundy & Schmitt 1998). Titan tholins (Khare et al. 1984) are complex organic solids that have already been used to explain the red slope observed on the spectrum of Centaur 5145 Pholus, from 0.4 to 1 $\mu$m (see Cruikshank et al. 1998). Kerogen-like compounds (Clark et al. 1993) constitute most of the organic matter of carbonaceous meteorites (these compounds are also called macromolecular carbon). They have been suggested by Gradie & Veverka (1980) to explain spectra of D-type asteroids. Amorphous carbon (Zubko et al. 1996) is a dark featureless compound often included in models of dark solar system objects. Figure 4 shows different models that we have investigated.

Figure 4: Some attempts in modelling the spectrum of Sycorax. None of these models have been fully satisfactory in reproducing the overall shape of the spectrum. Especially, we could not reproduce both the high reflectivity level at 0.8 $\mu$m and the low level at 1.25 $\mu$m. The corresponding albedo has been indicated for each model.

The upper figure shows two models that could both fit the overall behaviour around 2 $\mu$m. But we could not find any mixture of simple organic compounds and water ice that could fit both the spectrum around 0.8 $\mu$m and that around 1.25 $\mu$m. We have also investigated mixtures including silicates to check if they could better fit the spectrum. The mixture pyroxene + olivine could fit the spectrum beyond 1.2 $\mu$m, but does not fit at all the visible part of the spectrum.

The first attempt in modelling Sycorax spectrum did not therefore allow us to find a mixture that could fit the overall shape of the spectrum. To fit both the high reflectivity at 0.8 $\mu$m and the low value at 1.25 $\mu$m is not obvious using simple mixtures.

The whole data set presented here was not obtained simultaneously (see Tables 2 and 4), so these observations could correspond to different areas on Sycorax surface, and we have to consider a possible rotational effect that could affect the reflectivity curve. In particular, the JHKs photometry (VLT, May 1999) was not performed simultaneously (3 hours between the J and the HKs measurements).

The TNG BVRIJ photometry was obtained over a period of 1 hour: a V-B-V-R-V-I-V sequence was carried out during 25 min, and the J measurement was performed 32 min later. Small variations of the V magnitude appear in the measurements, but they are not significant since they are within the error bars. A strong variation of the magnitude within the next 32 min would introduce an error in the computed V-J color. If the V magnitude strongly increases within the 32 min, the V-J color index would increase as well, and the relative reflectivity in the J band may increase enough to equal that in the I band. So the model tholin + water ice (see Fig. 4) could fit the spectrum. Such a strong variation in the magnitude could be explained by different effects: a strong change in the albedo over the surface, an elongated shape for the satellite, ... But for the moment, we are not able to say if such a change really occurred.