K_S vs. J − K_S CMDs showing RSG and AGB candidates. Left panel: targets from the SMC source catalog are shown as gray dots with contours indicating the number density (same below), while the 1242 RSGs candidates are shown as solid circles. The O-AGB (pluses), C-AGB (crosses), x-AGB (asterisks), and RSG populations defined by theoretical J − K_S color cuts are separated by K0, K1, K2, and K_R lines (solid lines), respectively (same below). The regions of BSG, YSG, and RSG populations defined by the MIST models are separated by the dashed lines. The optical and MIR spectroscopic RSGs are shown as open diamonds and open squares, respectively, and only represent about 25% of the whole RSG population. A reddening vector of A_KS = 0.1 mag is shown as a reference. Two targets both fainter than the K_S-TRGB (K_S ≈ 12.7 mag; dotted line) and IRAC1-TRGB (IRAC1 ≈ 12.6 mag) are excluded from the RSG sample, and are marked as big open circles. One heavily obscured target is shown as big cross (same below). The zoomed-out region is indicated by dash-doted lines. Right panel: zoomed-out region of 8.0 ≤ K_S ≤ 12.0 mag and 0.8 ≤ J − K_S ≤ 1.5 mag, where the distribution of the optical spectroscopic RSGs at the red end follows almost exactly the MIST tracks. See text for details.