Visible spectroscopy of the new ESO large program on trans-Neptunian objects and Centaurs - Part 1

Aims. A second large observational program was started to improve our knowledge of the physical properties of Centaurs and trans-Neptunian objects, TNOs. The program is being executed in the facilities at the Cerro Paranal of the European Southern Observatory, ESO, in Chile. Methods. Visible spectroscopy was carried out using FORS1 at the UT2 of the VLT telescope. We computed the spectral slope, S, for each object, and we searched for the possible existence of weak absorption features. We compared our data with those from the literature. Results. We present spectra for 21 objects, 7 of them with no previously reported spectra. Our sample includes 5 Centaurs, 5 resonant objects, 8 classical TNOs, 2 detached objects, and 1 scattered disk object. There are significant differences between the distribution of spectral slopes for Centaurs and classical TNOs. The data presented here generally agree with previous published data. One exception is 60 558 Echeclus, whose spectral slope is considerably smaller than previous measurements. Another interesting object is 47 932 ( 2000 GN(171)), which does not show evidence of a proposed feature at 0.7 mu m. Most of the spectra are linear and featureless; however, the well-known bands of solid methane are detected in 136 199 Eris spectra and some weak features are detected for a few other objects. The most interesting case is that of 2003 AZ(84), which has a feature at about 0.65 mu m, maybe due to aqueous alteration.