Visible and near-infrared spectroscopy of Centaurs and TNOs: update of the ESO Large Program

In the last two years, an ESO Large Program: "Physical studies of Transneptunian objects (TNOs) and Centaurs" was accepted and executed at Cerro Paranal (Chile). The main goal of this project was to investigate the surface properties of these icy bodies through photometric and spectroscopic observations. In this work we present the results obtained for the brightest objects by spectroscopy at the Very Large Telescope (VLT-Chile). We obtained visible spectra for 23 objects, and for 13 of them we obtained near-infrared spectra in the range 1.1-2.45 micron. The visible spectra show a gradient ranging from 10 up to 56%/100nm. In the near-infrared region the spectra also show different behaviour, some are featureless, a few show weak water ice absorption. We discuss the limits of our detection of ices based on a comparison with some new laboratory experiments developed at Catania Observatory. Radiative transfer models have been computed using geographical mixtures of organics, minerals and ices in order to constrain the surface composition. Organic compounds such as kerogen or tholins, are in general necessary to reproduce the red visible slope. The limit of the presented models will be discussed particularly in connection with i) the limited number of materials for which reliable optical constants are available and ii) the dependence on many unknown physical parameters such as grain size, albedo, porosity, etc.