Letter to the Editor

Thermal rotational lightcurve of dwarf-planet (1) Ceres at 235 GHz with the Submillimeter Array

¹ Harvard-Smithsonian Center for Astrophysics, Cambridge MA-02138, USA e-mail: amoullet@cfa.harvard.edu
² LESIA - Observatoire de Paris, 5 place J. Janssen, 92195 Meudon Cedex, France
³ Université Paris 7 Denis-Diderot, 5 rue Thomas Mann, 75205 Paris Cedex, France

Received: 14 April 2010
Accepted: 21 May 2010

Abstract

Context. Previously published measurements of the millimeter-wave thermal rotational lightcurve of dwarf-planet (1) Ceres show incompatible results, proposing peak-to-peak lightcurve amplitudes during the ~9 h rotation period of either 4% or 50%, the latter being difficult to explain physically.

Aims. Better calibrated measurements are necessary to firmly assess the behavior of Ceres' thermal lightcurve, and to relate possible brightness temperature variations to the distribution of local surface properties such as bolometric albedo and emissivity.

Methods. One partial lightcurve of 6.5 h was obtained with the Submillimeter Array (Hawaii) in subcompact configuration at 235 GHz in January 2009, providing better absolute and relative calibration than the previously used single-dish facilities.

Results. The observed disk-averaged lightcurve is compatible with no variation over the measurement window, and has an upper limit of 3% on its amplitude.

Conclusions. The results obtained rule out the possibility of extreme brightness temperature variations, and the upper limit on the lightcurve amplitude could be physically realized by a combination of albedo distribution and realistic ground emissivity variations.