21 Lutetia and other M-types: Their sizes, albedos, and thermal properties from new IRTF measurements

The M-type taxonomic group, introduced by Tholen (1989), contains asteroids with generally featureless spectra and IRAS albedos of around 0.2. M-type asteroids were originally believed to have a metallic surface composition. However, it now seems that other types of surface composition may also give rise to M-type spectra (e.g. Rivkin et al., 2000). One of the Rosetta fly-by targets, M-type asteroid 21 Lutetia, was recently found by several authors to display spectral features indicative of a more primitive surface composition, which is usually associated with a C-type classification and a low albedo incompatible with the IRAS albedo for Lutetia. We report results from new thermal-infrared observations of Lutetia and other M-type asteroids performed at the NASA IRTF. The radiometric sizes and albedos of the targets have been determined and information on thermal properties such as thermal inertia has been derived from the apparent color temperatures of their thermal continua. One might expect metallic surfaces to have higher thermal conductivities and therefore to be associated with higher values of thermal inertia. The diameter and albedo derived for Lutetia are 98.3 +/- 5.9 km and pv = 0.208 +/- 0.025, in excellent agreement with the IRAS values. A C-type albedo (pv up to 0.1) is therefore ruled out but our results are not indicative of an unusually high thermal inertia. We also report on the analysis of observations of the M-type asteroids (129) Antigone and (785) Zwetana, both of which appear to have patches of very high and low radar albedo (M. Shepard, private communication) consistent with (but not restricted to) a metal and non-metal surface composition, respectively.