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R. G. Walker (Vanguard Research, Inc.), M. Cohen (Vanguard Research, Inc,)
There is a significant literature on optical/near-infrared spectral studies that sense reflected sunlight from the surface of the asteroid, revealing broad spectral features suggestive of families of materials. However, very little work has been done in the thermal infrared spectrum that directly probes the materials emitting on the surfaces of asteroids. We have searched for and extracted 534 thermal spectra of 245 asteroids from the original Dutch (Groningen) archive of spectra observed by the IRAS Low Resolution Spectrometer (LRS). To enhance weak spectral features, our intent was to remove the thermal continuum by dividing the observed LRS spectrum by the spectral energy distribution predicted by the IRAS Standard Thermal Model. Our expectation was that an emissivity spectrum with mean slope of zero would result. We found only 1 asteroid (344 Desiderata) for which this was true. For all other individual asteroids and co-added classes of asteroids the IRAS STM produced spectra that were too blue, that is, representative of a hotter surface temperature distribution than is consistent with the observations. Thermal models such as those proposed for the near earth asteroids (NEATM) by Harris (1998) with the "beaming parameter" in the range of 1.0 to 1.2 best represent the observed LRS spectral shapes. To investigate this further, we solved for the "beaming parameter" using the 12 and 25 micron IRAS broadband observations in the IMPS Catalog. We find, using 1803 sightings of 391 asteroids with both the 12 and 25 micron fluxes observed at SNR greater than 25, that the mean "beaming parameter" is 0.98 with a one sigma uncertainty of 0.08.
This work was supported by NASA ADP Contract NASW-99025.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.