Optical photometric diagrams for all stars in the region studied. Lines are the same as those in Fig. 4. The three young stellar populations we identified (see Sect. 5.1 for details) have been depicted with different colors: A (blue), B (green), and C (red). Small light gray dots are unclassified stars. Black empty circles indicate the ten selected targets for the spectroscopic analysis (see Sect. 5.1).