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 constrainedtheir geometric albedos, p, and effective radii, R, with the Standard ThermalModel for slow rotators. These are the first such measurements of Asbolus; wefind R=33 km +/- 2 km and p=0.12 +/- 0.03. This albedo is higher than all ofthose confidently known for active cometary nuclei. The thermal inertia iscomparable 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 ofnear-Earth asteroids. For Chiron, we find R=74 km +/- 4 km and p=0.17 +/- 0.02.While this albedo is consistent with the established value, previous radiometryby others implied a larger radius. This effect may be partially due to avarying infrared dust coma but all datasets have too low signal to be sure.Four Centaur albedos (out of about 30 objects) are now known. They show adiversity greater than that of the active comets, to which they areevolutionarily linked.Comment: Appearing in Feb 2002 Astronomical Journal, volume 123; 15 pages of text plus 2 figures (1 color); AASTeX format for the origina