EDP Sciences
Free Access
Volume 418, Number 1, April IV 2004
Page(s) 347 - 356
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361:20040025
Published online 02 April 2004

A&A 418, 347-356 (2004)
DOI: 10.1051/0004-6361:20040025

65 Cybele in the thermal infrared: Multiple observations and thermophysical analysis

T. G. Müller1 and J. A. D. L. Blommaert2

1  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
    e-mail: tmueller@mpe.mpg.de
2  Katholieke Universiteit Leuven, Instituut voor Sterrenkunde Celestijnenlaan 200B, 3001 Leuven, Belgium
    e-mail: joris.blommaert@ster.kuleuven.ac.be

(Received 19 May 2003 / Accepted 15 January 2004 )

We investigated the physical and thermal properties of 65 Cybele , one of the largest main-belt asteroids. Based on published and recently obtained thermal infrared observations, including ISO measurements, we derived through thermophysical modelling (TPM) a size of $302\times290\times232$ km ( $\pm$4%) and an geometric visible albedo of $0.050\pm0.005$. Our model of a regolith covered surface with low thermal inertia and "default" roughness describes the wavelengths and phase angle dependent thermal effects very well. Before/after opposition effect and beaming behaviour can be explained in that way. We found a constant emissivity of 0.9 at wavelengths up to about 100  $\mu$m and lower values towards the submillimetre range, indicating a grain size distribution dominated by 200  $\mu$m particle sizes. The spectroscopic analysis revealed an emissivity increase between 8.0 and 9.5  $\mu$m. We compared this emissivity behaviour with the Christiansen features of carbonaceous chondrite meteorites, but a conclusive identification was not possible. A comparison between the Standard Thermal Model (STM) and the applied TPM clearly demonstrates the limitations and problems of the STM for the analysis of multi-epoch and -wavelengths observations. While the TPM produced a unique diameter/albedo solution, the calculated STM values varied by $\pm$30% and showed clear trends with wavelength and phase angle. 65 Cybele can be considered as a nice textbook case for the thermophysical analysis of combined optical and thermal infrared observations.

Key words: minor planets, asteroids -- radiation mechanisms: thermal -- infrared: solar system

Offprint request: T. G. Müller, tmueller@mpe.mpg.de

© ESO 2004

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