Recherche d'un âge cinématique de l'association β Pictoris
Young moving groups (YMGs) are associations of young stars, mainly M dwarfs, that share a common kinematics, located in the solar neighbourhood and formed at the same epoch by the collapse of a molecular cloud that is unique to every association. They are ideal laboratories for the study of the last steps of stellar formation and the direct imaging of exoplanets. The β Pictoris moving group (βPMG) is one of the youngest and closest YMGs, although its age remains uncertain: ages found using isochrones or the lithium depletion boundary (LDB; 21 − 24 Myr) are inconsistent with traceback ages (11 − 13 Myr), found by tracing back the trajectories of member stars up to the epoch when the size of the YMG was minimal, which coincides with the epoch of stellar formation. This study presents a new numerical method, called Traceback, capable of ﬁnding a kinematic age for a YMG by traceback with data from the Gaia Data Release 2 (DR2) catalog. Its maximal theoretical precision is calculated and two important biases are characterized: a bias due to measurement errors (∼ −4.5 Myr) and a ∆v_r, grav = 0.5 km/s bias on radial velocity (v_r ; vitesse radiale) measurements due to gravitational redshift (∼ −1.8 Myr). When this method is applied to a sample of 46 validated members of βPMG, a corrected kinematic age of 17 − 19 Myr, in agreement with isochrones and LDB ages, is found by minimizing the median absolute deviation (MAD), a measure of the size of the association that is robust against outliers in the data, and the X-U covariance between the X positions and the U velocities of member stars. Furthermore, the use of a minimum spanning tree (MST) is shown not to be advantageous.
Young moving groups Stellar age Kinematics Traceback β Pictoris moving group