Letter to the Editor

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

II. The thermal lightcurve of (136108) Haumea

¹ Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon Cedex, France e-mail: emmanuel.lellouch@obspm.fr
² Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, PO Box 67, Hungary
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁴ Université Paris-7 “Denis Diderot”, 4 rue Elsa Morante, 75205 Paris Cedex 13, France
⁵ Laboratoire d'Astrophysique de Marseille, CNRS & Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁶ Newton Fellow of the Royal Society, Astrophysics Research Centre, Physics Building, Queen's University, Belfast, County Antrim, BT7 1NN, UK
⁷ Instituto de Astrofísica de Andalucía (CSIC) C/ Camino Bajo de Huétor 50, 18008 Granada, Spain
⁸ Deutsches Zentrum für Luft- und Raumfahrt, Berlin-Adlershof, Rutherfordstraße 2, 12489 Berlin-Adlershof, Germany
⁹ Space Science and Technology Department, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, Oxon UK, OX11 0QX, UK
¹⁰ Observatoire de la Côte d'Azur, laboratoire Cassiopée BP 4229, 06304 Nice Cedex 4, France
¹¹ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany
¹² Stewart Observatory, The University of Arizona, Tucson AZ 85721, USA
¹³ Northern Arizona University, Department of Physics & Astronomy, PO Box 6010, Flagstaff, AZ 86011, USA
¹⁴ INAF – Osservatorio Astronomico di Roma, via di Frascati, 33, 00040 Monte Porzio Catone, Italy
¹⁵ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei Müchen, Germany
¹⁶ IMCCE, Observatoire de Paris, 77 av. Denfert-Rochereau, 75014 Paris, France
¹⁷ Department of Physics and Astronomy, Science Laboratories, Durham University, South Road, Durham, DH1 3LE, UK
¹⁸ European Space Agency (ESA), 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: 31 March 2010
Accepted: 11 May 2010

Abstract

Thermal emission from Kuiper belt object (136108) Haumea was measured with Herschel–PACS at 100 μm and 160 μm for almost a full rotation period. Observations clearly indicate a 100 μm thermal lightcurve with an amplitude of a factor of ~2, which is positively correlated with the optical lightcurve. This confirms that both are primarily due to shape effects. A 160 μm lightcurve is marginally detected. Radiometric fits of the mean Herschel- and Spitzer- fluxes indicate an equivalent diameter D ~ 1300 km and a geometric albedo p_v ~ 0.70–0.75. These values agree with inferences from the optical lightcurve, supporting the hydrostatic equilibrium hypothesis. The large amplitude of the 100 μm lightcurve suggests that the object has a high projected a/b axis ratio (~1.3) and a low thermal inertia as well as possible variable infrared beaming. This may point to fine regolith on the surface, with a lunar-type photometric behavior. The quality of the thermal data is not sufficient to clearly detect the effects of a surface dark spot.