The structure and kinematics of the newborn stars of Upper Scorpius

OB associations are sparse, gravitationally unbound groups of young stars that share a common origin, which may include dissolution of initially dense and bound clusters (Wright 2020). Most stars form in OB associations, so studying their formation and evolution is key to understanding the origin of the Galactic field population, including the early environments of typical planetary systems. In May 2022 we conducted AAT-HERMES spectroscopic observations of Upper Scorpius, the densest region of the nearest OB association Sco-Cen (Squicciarini et al. 2021). We observed nearly 1800 stars, measuring Equivalent Widths (EW) of Li (6708Å) absorption and Hɑ (6563Å) emission lines to identify Young Stellar Objects (YSOs). We also selected likely YSOs on the basis of Gaia DR3 variability (Marton et al. 2022). We also use Gaia astrometry (Gaia Collaboration et al. 2021a) to investigate the spatial and kinematic structure of this YSO population. We found nearly 407 targets that were confirmed by Lithium or Hɑ EW, which included 256 targets being flagged as YSOs on the basis of Gaia variability. Studying projected YSO density on the sky, we found several dense groups with distinct kinematics, perhaps being the remnant cores of dense clusters that helped form the OB association. We examined the association of OB stars with these groups. We also identified YSOs that have proper motions directed radially away from these groups and assessed their kinematic expansion age. We discuss the implications of these results for the birth of the Sco-Cen OB association.