The surface of the transneptunian object 90482 Orcus

¹ LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France e-mail: catherine.debergh@obspm.fr
² Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy

Received: 14 December 2004
Accepted: 11 March 2005

Abstract

In April 2004, we obtained visible and near-infrared spectroscopy and photometry of the Transneptunian Object (TNO) 90482 Orcus (2004 DW) with the European Southern Observatory (ESO) 8-m telescopes of the Very Large Telescope (VLT) in Chile. This object, discovered on February 17, 2004, is one of the largest known TNOs, and it belongs to the dynamical class of Plutinos. The high signal-to-noise ratio visible spectrum recorded on April 11, 2004, is nearly flat, in good agreement with the BVRI photometry we obtained the same night and with photometric measurements from Rabinowitz et al. (2004, IAU Circ., 8295). The near-infrared spectra (J, H, and K bands) were obtained on April 11, 12, and 21, 2004. Relatively strong absorptions attributed to water ice were detected around 1.5 and 2 μm. We compare these data with spectra of Orcus obtained at the 3.56-m Telescopio Nazionale Galileo (TNG) by Fornasier et al. (2004a, A&A, 422, L43). Only two of the TNOs observed so far exhibit stronger water ice absorptions in their spectra than Orcus. Attempts to model the entire visible-near infrared spectrum yield tentative fits with mixtures of water ice and carbonaceous compounds.