Clustering of Ori OB1a. Top panels: initial heuristic clusters and bottom panels show the final clustering. Left: star map of the association. Only stars between −3.5 <  μ_αcos(δ) <  3.5 mas yr⁻¹, −4.5 <  μ_δ <  4 mas yr⁻¹, and 1.9 <  ϖ <  3.5 mas are plotted. Yellow circles mark associations in Kharchenko et al. (2013). The circle marked as ASCC 21a was added by us. Blue filled circles mark fields observed in the GALAH survey. Middle: proper motion plane. Only stars inside the green polygon from the left-hand panel are plotted. Red and blue polygons mark two distinct proper motion groups that we use in heuristic clustering. Top-right: distribution of stars’ parallaxes inside the green polygon (grey line). Bottom-right: distribution of stars’ radial velocities inside the green polygon (grey line). Only a fraction of stars have radial velocity measured in Gaia DR2. For stars observed in the GALAH and APOGEE surveys, we use the radial velocities measured by them. Distributions for individual groups are shown with the matching colours.