A dip in the past: the star formation history of young associations narrated by kinematics

Stellar associations can be discerned as overdensities of sources not only in the physical space but also, more importantly, in the velocity space. The common motion of their members, gradually eroded by the galactic tidal field, is partially reminiscent of the initial kinematic structure. Using recent data from Gaia EDR3, combined with radial velocities from GALAH and APOGEE, we traced back the present positions of stars belonging to the Scorpius-Centaurus association. A significant part of these stars (the "clustered" population) appears composed of many smaller entities, which were in a more compact configuration in the past. The presence of a kinematic duality is reflected into an age spread between this younger clustered population and an older diffuse population, in turn confirmed by a different fraction of disc-bearing stars. Star formation in the region appears to have lasted more than 10 Myr and to have proceeded in small groups that, after a few Myr, dissolve in the field of the older population but retain for some time memory of their initial structure. The consequences of our results will be discussed here, and a comparison with stellar ages inferred through isochrone fitting will shed light on the timescale of gas removal.