Interconnection between chromospheric activity, rotation, and mass. The colour-activity diagram (a) connects the colour-period diagram (b) with the rotation-activity diagram (c). a: chromospheric emission ratio log against intrinsic (V − K_s)₀ colour for members of NGC 3532. Member stars with measured rotation periods are shown with filled symbols and those without photometric rotation periods with open symbols. Likely binaries are indicated with squares, whereas single stars are marked with circles. The majority of the data points delineate very clear sequences, indicating the cohesion of chromospheric activity evolution within the coeval stars of an open cluster. The colour coding of the different groups is indicated in the legend on the lower right and is the same in all three panels. The thin lines indicate the sequences described in the text. The stars emboxed in red and black squares are the low-activity and high-activity stars, respectively, shown in Fig. 2 (top to bottom in Fig. 2 corresponds to left to right in this figure). The bluest of these stars has roughly the Solar colour. Stars marked by diamonds are outliers (discussed in the text). b: colour-period diagram of NGC 3532, with groups of stars distinguished by colour. This diagram is nearly a mirror image of the colour-activity diagram, with higher-activity stars being clearly mapped one-to-one onto the faster rotators. c: corresponding rotation-activity diagram, showing the chromospheric emission ratio against the Rossby number Ro. The rotation-activity diagram is remarkably clean despite the diversity in chromospheric activity.