Letter to the Editor

“TNOs are Cool”: A survey of the trans-Neptunian region ^*

III. Thermophysical properties of 90482 Orcus and 136472 Makemake

¹ Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon UK, OX11 0QX, UK e-mail: tanya.lim@stfc.ac.uk
² The University of Arizona, Tucson AZ 85721, USA
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁴ Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d'Azur, Laboratoire Cassiopée B.P. 4229, 06304 Nice Cedex 4, France
⁵ Observatoire de Paris, Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique (LESIA), 5 Place Jules Janssen, 92195 Meudon Cedex, France
⁶ Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, PO Box 67, Hungary
⁷ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany
⁸ Laboratoire d'Astrophysique de Marseille, CNRS & Université de Provence, 38 Rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁹ Instituto de Astrofísica de Andalucía (CSIC) C/ Camino Bajo de Huétor, 50, 18008 Granada, Spain
¹⁰ Observatoire de Paris, Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique (LESIA), University of Paris 7 “Denis Diderot”, 4 Rue Elsa Morante, 75205 Paris Cedex, France
¹¹ Deutsches Zentrum für Luft- und Raumfahrt, Berlin-Adlershof, Rutherfordstraße 2, 12489 Berlin-Adlershof, Germany
¹² Department of Physics and Astronomy, Science Laboratories, University of Durham, South Road, Durham, DH1 3LE, UK
¹³ Newton Fellow of the Royal Society, Astrophysics Research Centre, Physics Building, Queen's University, Belfast, County Antrim, BT7 1NN, UK
¹⁴ Northern Arizona University, Department of Physics & Astronomy, PO Box 6010, Flagstaff, AZ 86011, USA
¹⁵ INAF - Osservatorio Astronomico di Roma, via di Frascati, 33, 00040 Monte Porzio Catone, Italy
¹⁶ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei Müchen, Germany
¹⁷ IMCCE/Observatoire de Paris, CNRS, 77 Av. Denfert-Rochereau, 75014 Paris, France
¹⁸ Herschel Science Centre, European Space Astronomy Centre (ESAC), Camino bajo del Castillo, s/n, Urbanizacion Villafranca del Castillo, Villanueva de la Cañada, 28692 Madrid, Spain
¹⁹ Universität Bern, Hochschulstrasse 4, 3012 Bern, Switzerland

Received: 1 April 2010
Accepted: 12 May 2010

Abstract

Context. The goal of the Herschel open time programme “TNOs are Cool!” is to derive the physical and thermal properties for a large sample of Centaurs, and trans-Neptunian objects (TNOs), including resonant, classical, detached and scattered disk objects.

Aims. Based on observations of two targets we tried (i) to optimise the SPIRE observing technique for faint (close to the background confusion noise), slowly moving targets; (ii) to test different thermal model techniques; (iii) to determine radiometric diameter and albedo values; (iv) to compare with Spitzer results whenever possible.

Methods. We obtained SPIRE photometry on two targets and PACS photometry on one of the targets.

Results. We present results for the two targets, (90482) Orcus and (136472) Makemake, observed with SPIRE and for one of those targets, Makemake, observed with PACS. We adopt p_V = 0.27 and D = 850 km as our best estimate of the albedo and diameter of Orcus using single terrain models. With two-terrain models for Makemake, the bright terrain is fitted by, 0.78 < p_V < 0.90, and the dark terrain 0.02 < p_V < 0.12, giving 1360 < D < 1480 km.

Conclusions. A single terrain model was derived for Orcus through the SPIRE photometry combined with MIPS data. The Makemake data from MIPS, PACS and SPIRE combined are not compatible with a single terrain model, but can be modelled with a two-terrain fit. These science demonstration observations have shown that the scanning technique, which allows us to judge the influence of background structures, has proved to be a good basis for this key programme.