IRAS Diameters and Albedos Revisited

Russell G. Walker

Monterey Institute for Research in Astronomy (MIRA)

Abstract. 

A broadband variant of the Near Earth Asteroid Thermal Model proposed by A. Harris (1998) is applied to the observed IRAS asteroid 12 and 25 micron fluxes (IMPS database).  The method allows separation of the three variables: the thermal beaming parameter, h, the wavelength independent emissivity, e, and the albedo, p_v, and permits direct estimation of these parameters.  The key is that the sub-solar temperature on the asteroid surface is uniquely determined for any assumed surface temperature distribution from the ratio of the IRAS 12 and 25 micron fluxes. 

Diameters derived from the model fluxes assuming e = 1.0 are compared to 54 occultation diameters and yield an average infrared emissivity e = 0.793 with a 1s uncertainty of 0.007.   Using this value of e, the diameters, albedos, and thermal beaming parameters are derived for each asteroid using all 3336 observations of 654 asteroids with multiple observations at SNR > 10 in the 12 and 25 micron bands of IRAS.  The mean value of h = 1.067 with a 1s uncertainty of 0.087 for asteroids of all classes represented in the dataset.  There was no significant difference of h between classes C, S, F, M, K, and D.  The resulting diameters are on average only about 1.4% larger than the IMPS diameters, however, the derived albedos average 11% smaller.  

The paper presents the methods used to obtain the above results, as well as the new diameters, albedos, and beaming parameters for 654 asteroids. These data are available here.  The full research paper is available here.