Asteroids as far-infrared photometric standards for ISOPHOT

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 of 678 individual observations, ranging from 7-2 000 microns. 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 25 {J m(-2} s({-0.5} K^{-1})) . 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 5-10 % for the first category objects and 10-20 % for the secondaries. The methods and procedures discussed here are included in the first update of the ISOPHOT calibration in 1998. 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. Table 7 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html