Asteroids as Far‐Infrared Photometric Standards

Abstract

With the successful launch of the Infrared Space Observatory (ISO) in 1995 November, it has been necessary to establish a set of celestial standards for the photometric calibration of the instruments. In support of the imaging photopolarimeter ISOPHOT, a sample of 10 asteroids, which cover the flux range between 5 Jy (bright standard stars) and 1000 Jy (Uranus and Neptune) at wavelengths between 50 and 200 mm, has been investigated. Targeted ground-based mid-infrared and submillimeter observations, as well as spaceborne Infrared Astronomical Satellite (IRAS) and ISO measurements have been used to test the reliability and accuracy of several thermal flux models. A new approach for the thermophysical description of minor planets (J. S. V. Lagerros, AA A&A, 315, 625 [1996b]), including an ellipsoidal shape, thermal behavior, and surface roughness effects, led to the best agreement between observed and predicted brightness. The necessary input parameters for an optimized far-infrared application have been derived from the observational results. The uncertainties of the absolute calibration depend on the individual object and the wavelength interval. In the far-infrared ISOPHOT wavelength range (50–200 mm), an accuracy between 5% and 20% can be achieved. First efforts to use the asteroids as photometric standards in the far-infrared (ISO) and submillimeter (Heinrich-Hertz-Telescope and James Clerk Maxwell Telescope) have begun already. Future application for airborne (SOFIA) and spaceborne (FIRST and Space Infrared Telescope Facility [SIRTF]) experiments can be expected.