Left panel: colour–magnitude diagram of cluster member candidates on the basis of their proper motions. A 4 Myr isochrone with solar metallicity, combined using a Padova MS-isochrone and a Pisa-FRANEC PMS-isochrone, shifted to a distance of 8 kpc and a foreground extinction of AKs = 2.35 mag, is shown for reference. Stars, with fluxes exceeding the linearity limit of the detector are drawn as crosses in all figures throughout this paper. A two-step colour-cut was applied for stars with H > 14 mag to remove field stars with similar proper motions as the cluster from the cluster sample (see Sect. 6 for details). The vertical short-dashed lines mark the first colour-cut, while in the second step of the colour-cut highly reddened objects to the right of the second isochrone (AKs = 2.89 mag, long-dashed line) are removed. The dots represent the sample of cluster stars after the colour-cut, stars rejected based on their colour are drawn as triangles. Spectroscopically identified field supergiants from the LHO catalogue (Liermann et al. 2009) are marked with diamonds and are removed from the final cluster sample. Right panel: colour–magnitude diagram of stars classified as belonging to the field according to their proper motion (dots) and of stars removed from the member sample based on their colour or known spectral type (triangles). One star, classified as belonging to the field by its proper motion, has an O-star as (ambiguous) counterpart in the LHO catalogue (see Sect. 6) and is marked with a circle. The tilted dotted line is the line of reddening according to the extinction law by Nishiyama et al. (2009) running through the population of red clump stars from the Galactic bulge.
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