The Age of β Pictoris

Abstract

We have reanalyzed data for the proposed moving group associated with beta Pic in order to determine if the group (or part of it) is real, and, if so, to derive an improved age estimate for beta Pic. By using new, more accurate proper motions from PPM and Hipparcos and a few new radial velocities, we conclude that on kinematic grounds, two M dwarfs have space motions that coincide with that of beta Pic to within 1 km/s with small error bars. Based on a CM diagram derived from accurate photometry and Hipparcos parallaxes, these two possible proper-motion companions to beta Pic are very young; we derive an age of $\sim$20 Myr by comparison to theoretical tracks from D'Antona & Mazzitelli. In fact, the proposed beta Pic companions comprise two of the three youngest M dwarfs in the sample of 160 dM stars for which we have data. The chromospheric and coronal activity of these two stars also confirm that they are quite young. We argue that the probability that two of the three youngest nearby M dwarfs would accurately share the space motion of beta Pic by chance is quite small, and therefore we believe that beta Pic and the two M dwarfs (GL 799 and GL 803) were formed together. The estimated age for beta Pic is then 20$\pm$10 Myr.