On the age of the β Pictoris moving group

Abstract

Binks & Jeffries and Malo et al. have recently reported Li depletion boundary (LDB) ages for the β Pictoris moving group (BPMG) which are twice as old as the oft-cited kinematic age of ∼12 Myr.In this study, we present (1) a new evaluation of the internal kinematics of the BPMG using the revised Hipparcos astrometry and best available published radial velocities, and assess whether a useful kinematic age can be derived, and (2) derive an isochronal age based on the placement of the A-, F-, and G-type stars in the colour-magnitude diagram (CMD).We explore the kinematics of the BPMG looking at velocity trends along Galactic axes, and conducting traceback analyses assuming linear trajectories, epicyclic orbit approximation, and orbit integration using a realistic gravitational potential.None of the methodologies yield a kinematic age with small uncertainties using modern velocity data.Expansion in the Galactic X and Y directions is significant only at the 1.7σ and 2.7σ levels, and together yields an overall kinematic age with a wide range ( 13-58 Myr; 95 per cent CL).The A-type members are all on the zero-age main sequence, suggestive of an age of >20 Myr, and the loci of the CMD positions for the late-F-and G-type pre-main-sequence BPMG members have a median isochronal age of 22 Myr (± 3 Myr statistical, ±1 Myr systematic) when considering four sets of modern theoretical isochrones.The results from recent LDB and isochronal age analyses are now in agreement with a median BPMG age of 23 ± 3 Myr (overall 1σ uncertainty, including ±2 Myr statistical and ±2 Myr systematic uncertainties).