The internal kinematics of local young stellar associations

Context. The local (<200 pc away) young (<50 Myr old) stellar associations (LYSAs) provide fundamental evidence for the study of the star formation process in the local neighbourhood. Aims. We explore robust statistical correlations in the internal kinematics of LYSAs and of these with age. Methods. We analysed a public dataset containing the linear velocity field parameters and expansion ages of 18 LYSAs. We identified the most robust correlations using frequentist and Bayesian methods. Results. Of the 45 correlations studied, we identified only four that satisfied both frequentist and Bayesian criteria, and all four are related to radial motions in the Galactic Z direction. We hypothesise several origins for these four correlations and identify the gravitational potential of the Galactic disk as the most likely driving element. It imprinted the observed motions in the parent molecular clouds, and once the stars were formed, it also damped these motions on a timescale shorter than the LYSAs’ ages. Conclusions. The internal kinematics of LYSAs contain fundamental information about the star formation process that is not fully addressed by star formation theories, in particular rotation and shear. Although the Galactic potential appears to be the driving force of these correlations, we urge the theoretical community to provide predictions about the internal motions of expansion, rotation, and shear of stellar associations.