K_S versus J–K_S diagram for the initial sample. For comparison, the C-AGBs (blue), O-AGBs (pink), x-AGBs (green) and RSGs (red) regions defined by the theoretical J–K_S color cuts (Cioni et al. 2006; Boyer et al. 2011), and 1268 massive stars (M≥ 8 M_⊙) in the LMC from Bonanos et al. (2009) (cyan) are also shown in the diagram. Four horizontal dotted lines indicate the theoretical luminosity limits of 8, 10, 25 and 30 M_⊙. The vertical dashed-dotted lines indicate the red boundary of carbon-rich stars as J–K_S = 1.6 mag suggested by Hughes & Wood (1990) and a slightly strict blue boundary of J–K_S = 0.8 mag. The 25%, 50% and 75% density contour levels are given by the orange, yellow and green lines, respectively (same below). From the diagram, it can be seen that our sample agrees well with the luminosity limits and most targets are located in a region of 6.5 < K_S < 11.0 mag and 0.8 < J–K_S < 1.6 mag, while about 71% of RSGs candidates are inside the defined RSGs region. See text for details.