Letter to the Editor

Surface characterization of 28978 Ixion (2001 KX) ^*

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² European Southern Observatory, Alonso de Cordova 3107, Santiago de Chile
³ Observatory, PO Box 14, 00014 University of Helsinki, Finland
⁴ Observatoire Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁵ Department of Astronomy, 211 SSRB, University of Florida, Gainesville, FL 32611-2055, USA
⁶ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Corresponding author: H. Boehnhardt, hboehnha@mpia-hd.mpg.de

Received: 21 November 2003
Accepted: 7 January 2004

Abstract

The Plutino 28978 Ixion, one of the largest objects in the Kuiper Belt, is measured by R filter photometry and linear polarimetry over the phase angle range as well as by BVRI colours and visible spectroscopy. Ixion is a medium red object with spectral slope of 17.7%/100 nm (400-900 nm). While its opposition phase curve surge (0.2 mag/deg) is within the typical range found for other solar system objects, it displays unusually high negative polarization (min. -1.3%). Comparison with model spectra and numerical modeling of the opposition brightening and polarization effects suggest an areal mixture of absorbing (dark) and icy (bright) compounds on its surface: the mixing ratio is about 6:1 for dark and bright material of 0.21 and 0.79 single-scattering albedo and of 250 and 33 dimensionless mean free path (length multiplied by wave number), respectively.