Composition of KBO (50000) Quaoar^*

¹ NASA Ames Research Center, Mail Stop 245-6, Moffett Field, CA 94035, USA e-mail: [Cristina.M.DalleOre;Dale.P.Cruikshank;Rachel.M.Mastrapa]@nasa.gov
² SETI Institute, 515 N. Whisman Rd., Mountain View, CA 94043 CA, USA
³ LESIA, Observatoire de Paris, 92195 Meudon Pricipal Cedex, France e-mail: [Antonella.Barucci;Catherine.deBergh;Sonia.Fornasier;Alvaro.Alvarez;Davide.Perna]@obspm.fr
⁴ Earth and Planetary Sciences Dept, University of Tennessee, Knoxville, TN 37996, USA e-mail: JEmery2@utk.edu
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA e-mail: Frederic.Merlin@obspm.fr
⁶ Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ 86011, USA e-mail: David.Trilling@nau.edu
⁷ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Roma, Italy e-mail: Dotto@mporzio.astro.it
⁸ Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy

Received: 30 January 2009
Accepted: 7 April 2009

Abstract

Aims. The objective of this work is to investigate the physical properties of objects beyond Neptune – the new frontiers of the Solar System – and in particular to study the surface composition of (50 000) Quaoar, a classical Transneptunian (or Kuiper Belt) object. Because of its distance from the Sun, Quaoar is expected to have preserved, to a degree, its original composition. Our goals are to determine to what degree this is true and to shed light on the chemical evolution of this icy body.

Methods. We present new near-infrared (3.6 and 4.5 μm) photometric data obtained with the Spitzer Space Telescope. These data complement high resolution, low signal-to-noise spectroscopic and photometric data obtained in the visible and near-infrared (0.4–2.3 μm) at VLT-ESO and provide an excellent set of constraints in the model calculation process. We perform spectral modeling of the entire wavelength range – from 0.3 to 4.5 μm by means of a code based on the Shkuratov radiative transfer formulation of the slab model. We also attempt to determine the temperature of H₂O ice making use of the crystalline feature at 1.65 μm.

Results. We present a model confirming previous results regarding the presence of crystalline H₂O and CH₄ ice, as well as C₂H₆ and organic materials, on the surface of this distant icy body. We attempt a measurement of the temperature and find that stronger constraints on the composition are needed to obtain a precise determination.

Conclusions. Model fits indicate that N₂ may be a significant component, along with a component that is bright at m, which we suggest at this time could be amorphous H₂O ice in tiny grains or thin grain coatings. Irradiated crystalline H₂O could be the source of small-grained amorphous H₂O ice. The albedo and composition of Quaoar, in particular the presence of N₂, if confirmed, make this TNO quite similar to Triton and Pluto.