H − K_s vs. K_s color–magnitude diagram of W3 Main. The gray data points represent the sources detected in H and K_s, while the black data points are the sources detected in JHK_s. The very red sources are not detected anymore in the J-band due to their high-extinction which affects the detection in the J-band the strongest. Only sources with magnitude errors less than 0.1 mag are plotted. Overplotted is the extinction vector for A_Ks = 1 mag from the extinction law of Nishiyama et al. (2009). The dashed line shows the combined main sequence and PMS isochrone of 2 Myr for a distance of 2 kpc. Above 5 M_⊙, the main sequence models of Brott et al. (2011) with v_rot = 0 are used, while for lower masses the models have been calculated by Tognelli et al. (2011).