The <i>Gaia</i> DR2 census of the Scorpius OB2 association based on kinematic modeling

Context. The Scorpius Centaurus OB association (Sco OB2) is the nearest massive star forming region, and provides a valuable opportunity to study the outcome and progress of the star formation process in detail. Sco OB2 hosts a (pre-)main-sequence population comprising stars that were born about ~5 to 20 million years ago. Given their close distance (100–150 pc), they span an enormous area (285° ≤ l ≤ 360°) on the sky. Historically the association has been divided into three subgroups: Upper Scorpius (US), Upper Centaurus Lupus (UCL), and Lower Centaurus Crux (LCC). Aims. We studied the spatial, kinematical, and age structure of the OB association in order to identify subgroups without using arbitrarily defined boundaries. Methods. Based on Gaia DR2 data, we carried out a comprehensive membership analysis applying a linear velocity vector field model for the entire association. We obtained a census where each candidate star was assigned a membership probability by comparing the observed proper motion to the prediction of our kinematic model. Results. Our census includes 5106 members in the mass range from about 5 M ⊙ down to the brown-dwarf regime (&lt;0.08 M ⊙ ); the members with mass &lt;1 M ⊙ are pre-main-sequence stars. We confirm the structured distribution of stars as reported previously, as well as the “new” subgroup Lower Scorpius (LS) centered on V1062 Sco and about 25 pc more distant than the other subgroups in Sco OB2. Our five-dimensional membership analysis excludes the cluster IC2602 (~40 Myr). We determined the age of the individual subgroups, taking into account the interstellar extinction. Conclusions. We identified substructures in Sco OB2 in the spatial, kinematical, and age distribution, without applying arbitrary boundaries. By measuring the radial velocity distribution for 616 members, we found a typical velocity dispersion of a few km s −1 , showing no evidence for expansion of the subgroups. The configuration and age of the subgroups are discussed in terms of recent star formation scenarios proposed for this region.