The age of AB Doradus

We have derived a new age estimate for the nearby young star AB Dor and have investigated the resulting implications for testing theoretical evolutionary models with the data reported by Close and coworkers for the low-mass companion AB Dor C. Using color-magnitude diagrams, we find that the AB Dor moving group is roughly coeval with the Pleiades (tau = 100-125 Myr) and is clearly older than IC 2391 (tau = 35-50 Myr). In fact, based on a comparison of the kinematics of AB Dor and the Pleiades, we suggest that the stars identified by Zuckerman and coworkers as members of a moving group with AB Dor are remnants of the large-scale star-formation event that formed the Pleiades. Using the age of tau = 50(-20)(+50) Myr adopted by Close, the luminosity predicted by the models of Chabrier and Baraffe for AB Dor C is larger than the value reported by Close but is still within the quoted uncertainties. Meanwhile, the agreement is good when our age estimate for AB Dor C is adopted. Thus, we find no evidence in the data presented by Close for AB Dor C to suggest that previous studies using the models of Chabrier and Baraffe and bolometric luminosity as the mass indicator have significantly underestimated the masses of young low-mass stars and brown dwarfs.