CMD of NGC 1783 with the bona fide stars color-coded according to their best-fit vsini values. The box highlights the eMSTO region, which is shown as a zoomed-in view in the right panel. The blue and red star symbols mark the slow-rotating target #44909 and the fast-rotating target #43977, respectively, for which the spectral fitting results are shown in Fig. 8, and their spectra are compared in Fig. 9. The dashed violet lines are the fiducial lines adopted to verticalize the color distribution.