Thermal infrared spectrophotometry of the near-Earth asteroids 2100 Ra-Shalom and 1991 EE

Thermal infrared observations of the near-Earth asteroids 2100 Ra-Shalom and 1991 EE are presented and interpreted with the aid of published asteroid thermal models. The broad wavelength range covered allows the best fitting of a family of model spectra to be selected, eliminating the ambiguity associated with deriving model parameters from observations at just one or two thermal wavelengths. Neither the "standard" thermal model nor the "fast rotating" model provide good fits to the thermal spectra. Much improved fits are obtained with a modified standard thermal model with values of the model "beaming" parameter, eta, significantly higher than normally assumed in the standard thermal model. The results for the Aten asteroid 2100 Ra-Shalom provide convincing confirmation that this object has an unusually high surface thermal inertia. The resulting optical albedos (on the H, G magnitude system) and effective diameters are 0.13 +/- 0.04 and 2.48 +/- 0.35 km for 2100 Ra-Shalom (near lightcurve maximum) and 0.30 +/- 0.10 and 1.01 +/- 0.15 km for 1991 EE (referred to the lightcurve mean magnitude). The derived size for 2100 Ra-Shalom is consistent with previous estimates but the moderate albedo, coupled with infrared colors more typical of S-types, leads us to question its classification as C-type. These are the first size and albedo determinations for the Apollo asteroid 1991 EE. The near infrared colors and albedo of 1991 EE suggest an S-type classification.