The Age of β Pictoris

Abstract

We have reanalyzed data for the proposed moving group associated with β Pictoris in order to determine if the group (or part of it) is real and, if so, to derive an improved age estimate for β Pic. By using new, more accurate proper motions from PPM and Hipparcos and a few new radial velocities, we conclude that on kinematic grounds, most of the proposed members of the moving group are not, in fact, associated with β Pic. However, two M dwarfs—or three, actually, since one of them is a nearly equal mass binary—have space motions that coincide with that of β Pic to within 1 km s^-1 with small error bars. Based on a color-magnitude diagram derived from accurate photometry and Hipparcos parallaxes, these two possible proper-motion companions to β Pic are very young; we derive an age of ~20 Myr by comparison with theoretical tracks from F. D'Antona & I. Mazzitelli. In fact, the proposed β 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 activities 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 β Pic by chance is quite small, and therefore we believe that β Pic and the two M dwarfs (GL 799 and GL 803) were formed together. The estimated age for β Pic is then 20 ± 10 Myr, where the uncertainty in the age arises primarily from possible errors in the pre-main-sequence isochrones and in the conversion from color to effective temperature. This young age for β Pic supports the contention that the IR excess for the Vega-like stars is age dependent.