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Astron. Astrophys. 338, 340-352 (1998)


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Asteroids as far-infrared photometric standards for ISOPHOT *

T.G. Müller 1, 3 and J.S.V. Lagerros 2

1 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
2 Astronomiska observatoriet, Box 515, S-75120 Uppsala, Sweden
3 ISO Science Operations Centre, Astrophysics Division, Space Science Department of ESA, Villafranca, P.O. Box 50727, E-28080 Madrid, Spain

Received 11 February 1998 / Accepted 22 June 1998

Abstract

Asteroids are used as far-IR calibrators for the imaging photopolarimeter ISOPHOT on board the Infrared Space Observatory ISO. For the 10 selected objects we compiled a large uniform database 1 of 678 individual observations, ranging from 7-[FORMULA].

Applying a new thermophysical model to the observations we derived thermal properties of the selected asteroids, based on spin-vector solutions, direct size measurements and the HG-magnitude system.

Our investigations indicate very rough surfaces, reflected in the beaming effect, and very low levels of heat conduction, expressed in thermal inertias between 5 and [FORMULA]. Due to scattering processes in the porous regolith, the emissivity varies significantly with wavelength. In case of Vesta we find emissivities as low as 0.6 in the far-IR / submillimetre region.

By entering the combined results into the thermophysical model we defined new photometric standards for the far-IR. The absolute accuracy for thermal flux or lightcurve predictions is 5-[FORMULA] for the first category objects and 10-[FORMULA] for the secondaries. The methods and procedures discussed here are included in the first update of the ISOPHOT calibration in 1998.

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

* The ISOPHOT data are based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

Send offprint requests to: T.G. Müller, (tmueller@mpia-hd.mpg.de)

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998

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