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R.G. Walker (MIRA)
I have applied a broadband variant of the Near Earth Asteroids Thermal Model proposed by A. Harris (1998) to the observed IRAS asteroid 12 and 25 micron fluxes (IMPS database). The method allows separation of the three variables: the thermal beaming parameter, the wavelength independent emissivity, and the albedo, and permits 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 emissivity = 1.0 are compared to 54 occultation diameters and yield an average infrared emissivity = 0.793 with a 1 sigma uncertainty of 0.007. Using this value of emissivity, the diameters, albedos, and thermal beaming parameters are derived for each asteroid using all of the observations of 694 asteroids with multiple observations at SNR > 10 in the 12 and 25 micron bands of IRAS. The mean value of the beaming parameter = 1.067 with a 1 sigma uncertainty of 0.087 for asteroids of all classes represented in the dataset. There was no significant difference of the beaming parameter between classes C, S, F, M, K, and D. The resulting diameters are on average only about 1% larger than the IMPS diameters, however, the derived albedos are 12% smaller.
The paper presents the methods used to obtain the above results, as well as the new diameters, albedos, and beaming parameters for 694 asteroids. These data will be online at the MIRA website, http://www.mira.org.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.