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

Asteroids as far-infrared photometric standards for ISOPHOT ^*

T.G. Müller ^{1, 3} and J.S.V. Lagerros <sup>2

1</sup> 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-.

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 . 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- for the first category objects and 10- 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)

This article contains no SIMBAD objects.

Contents

• 1. Introduction
• 2. The thermophysical model
  • 2.1. Shape, size and spin vector
  • 2.2. Albedo and emissivity
  • 2.3. Heat conduction
  • 2.4. The thermal IR beaming
  • 2.5. Multiple scattering
• 3. Properties of the individual asteroids
• 4. Observational photometric data
  • 4.1. IRAS data
  • 4.2. JCMT data
  • 4.3. UKIRT mid-IR N and Q data
  • 4.4. IRTF mid-IR N and Q data
  • 4.5. ISOPHOT
  • 4.6. Other data
• 5. Methods
  • 5.1. Data analysis
  • 5.2. Derivation of thermophysical parameters
    • 5.2.1. Diameter and Albedo
    • 5.2.2. Thermal inertia and beaming
    • 5.2.3. Wavelength dependent emissivity
  • 5.3. Data selection
• 6. Results
  • 6.1. Direct versus radiometric size
  • 6.2. Thermal inertia
  • 6.3. The surface roughness parameters
  • 6.4. Wavelength dependent emissivity
• 7. Discussion
  • 7.1. Accuracy of the individual standards
  • 7.2. General limitations
  • 7.3. Outlook
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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