Low Albedos Among Extinct Comet Candidates

We present radiometric effective radii and visual geometric albedos for sixasteroids in comet-like orbits. Our sample has three of the four knownretrograde asteroids (1999 LE31, 2000 DG8, and 2000 HE46) and three objects((18916) 2000 OG44, 2000 PG3, and 2000 SB1) on prograde but highly ellipticalorbits. These measurements more than double the number of known albedos forasteroids with a Tisserand invariant in the cometary regime. We find that allsix of our objects, and nine of the ten now known, have albedos that are as lowas those of active cometary nuclei, which is consistent with their supposedevolutionary connection to that group. This albedo distribution is distinctfrom that of the whole near-Earth and unusual asteroid population, and thestrong correlation between Tisserand invariant and albedo suggests there is asignificant cometary contribution to this asteroid population.Comment: 12 pages + 1 figure, AASTeX v5.0 preprint format, accepted to appear in The Astrophysical Journal (Letters