Principal component analysis for the abundances of stars in the APO-K2 catalogue and BH2*. Blue and green points correspond, respectively, to the α-poor and α-rich populations – generally identified by thin and thick disc components. Red points identify transition stars not unambiguously classified in the two previous groups (see Schonhut-Stasik et al. 2024, for details about the classification). Larger symbols identify young α-rich stars (see text). The position of BH2* is marked by a purple square.