Thermal Properties of Centaurs Asbolus and Chiron

We have measured the mid-infrared thermal continua from two Centaurs, inactive (8405) Asbolus and active 95P = (2060) Chiron, and have constrained their geometric albedos, p, and effective radii, R, with the standard thermal model for slow rotators. These are the first such measurements of Asbolus; we find R = 33 ± 2 km and p = 0.12 ± 0.03. This albedo is higher than all of those confidently known for active cometary nuclei. The thermal inertia is comparable to or lower than those of main-belt asteroids, the Moon, and Chiron; lower than those of the icy Galilean satellites; and much lower than those of near-Earth asteroids. For Chiron, we find R = 74 ± 4 km and p = 0.17 ± 0.02. While this albedo is consistent with the established value, previous radiometry by others implied a larger radius. This discrepancy may be partially due to a varying infrared dust coma, but all data sets have too low signal to be sure. Four Centaur albedos (out of about 30 objects) are now known. They show a diversity greater than that of the active comets, to which they are evolutionarily linked.