Integral-field spectroscopy of (90482) Orcus-Vanth⋆
European Space Astronomy Centre, ESA, PO Box 78, 28691
Villanueva de la Cañada,
2 LESIA, Observatoire de Paris, CNRS, 5 place Jules Janssen, 92190 Meudon, France
3 Université Paris 7 Denis-Diderot, 5 rue Thomas Mann, 75205 Paris Cedex, France
4 IMCCE, Observatoire de Paris, UPMC, CNRS, 77 Av. Denfert Rochereau 75014 Paris, France
5 Department of Earth, Atmospheric, and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
6 Instituto de Astrofísica de Andalucía, CSIC, Apt 3004, 18080 Granada, Spain
7 Queen’s University, Belfast, County Antrim BT7 1 NN, Ireland
8 Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 5, France
9 Institut Universitaire de France, 103 Bld Saint Michel, 75005 Paris, France
10 Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
11 Utah State University, 0300 Old Main Hill, Logan, UT 84322, USA
12 Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse, 85748 Garching, Germany
Accepted: 27 August 2011
Aims. We seek to constrain the surface composition of the trans-Neptunian object (90482) Orcus and its small satellite Vanth, as well as their mass and density.
Methods. We acquired near-infrared spectra (1.4−2.4 μm) of (90482) Orcus and its companion Vanth using the adaptive-optics-fed integral-field spectrograph SINFONI mounted on Yepun/UT4 at the European Southern Observatory Very Large Telescope. We took advantage of a very favorable appulse (separation of only 4″) between Orcus and the UCAC2 29643541 star (mR = 11.6) to use the adaptive optics mode of SINFONI, allowing both components to be spatially resolved and Vanth colors to be extracted independently from Orcus.
Results. The spectrum of Orcus we obtain has the highest signal-to-noise ratio to date, and we confirm the presence of H2O ice in crystalline form, together with the presence of an absorption band at 2.2 μm. We set an upper limit of about 2% to the presence of methane, and 5% for ethane. Since the methane alone cannot account for the 2.2 μm band, the presence of ammonia is suggested to the level of a couple of percent. The colors of Vanth are found to be slightly redder than those of Orcus, but the large measurement uncertainties prevent us from drawing any firm conclusions about the origin of the pair (capture or co-formation). Finally, we reset the orbital phase of Vanth around Orcus, and confirm the orbital parameters derived by Brown and collaborators.
Key words: techniques: high angular resolution / techniques: imaging spectroscopy / Kuiper belt objects: individual: (90482) Orcus / methods: observational
© ESO, 2011