ESO Large Program on physical studies of Trans-Neptunian objects and Centaurs: Final results of the visible spectrophotometric observations^*

¹ Astronomy Department of Padova University, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
² LESIA, Observatoire de Paris, 92195 Meudon Pricipal Cedex, France e-mail: [alain.doressoundiram;antonella.barucci;Catherine.deBergh;Audrey.Delsanti]@obspm.fr
³ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: tozzi@arcetri.astro.it
⁴ Max-Planck-Institute for Astronomy, Koenigstuhl 17, 69117 Heidelberg, Germany e-mail: hboehnha@mpia-hd.mpg.de
⁵ Astronomy Technology Centre, Blackford Hill, Edinburgh, EH9 3HJ, Scotland, UK e-mail: J.Davies@roe.ac.uk
⁶ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040, Monte Porzio Catone (Roma), Italy e-mail: dotto@mporzio.astro.it

Corresponding author: S. Fornasier, fornasier@pd.astro.it

Received: 12 February 2004
Accepted: 8 April 2004

Abstract

The Large Program on physical studies of TNOs and Centaurs, started at ESO Cerro Paranal on April 2001, has recently been concluded. This project was devoted to the investigation of the surface properties of these icy bodies through photometric and spectroscopic observations. In this paper we present the latest results on these pristine bodies obtained from the spectrophotometric investigation in the visible range. The newly obtained spectrophotometric data on 3 Centaurs and 5 TNOs, coming from 2 observing runs at the Very Large Telescope (VLT), show a large variety of spectral characteristics, comprising both gray and red objects in the two different populations. A very broad and weak absorption feature, centered around 7000 Å , has been revealed in the spectrum of the gray TNO 2003 AZ84. This absorption is very similar to a feature observed on low albedo main belt asteroids and attributed to the action of the aqueous alteration process on minerals. This process was previously also claimed as the most plausible explanation for some peculiar visible absorption bands observed on 2000 EB173 and 2000 GN171 in the framework of the Large Program (Lazzarin et al. [CITE]; de Bergh et al. [CITE]). This detection seems to reinforce the hypothesis that aqueous alteration might have taken place also at such large heliocentric distances.
We also report the results of a spectroscopic investigation performed outside the Large Program on the very interesting TNO 2000 GN171 during part of its rotational period. This object, previously observed twice in the framework of the Large Program, had shown during the early observations a very peculiar absorption band tentatively attributed to aqueous alteration processes. As this feature was not confirmed in a successive spectrum, we recently repeated the investigations of 2000 GN171, finding out that it has an heterogeneous composition. Finally an analysis of the visible spectral slopes is reported for all the data coming from the Large Program and those available in literature.