Hunter-plot showing projected rotational velocity against nitrogen enhancement. Our population synthesis is shown as a density plot in the background. The color coding corresponds to the number of stars per pixel. Overplotted are data from Hunter et al. (2009a), with surface gravities as indicated by the color (see figure key). Single stars are plotted as circles, radial velocity variables as triangles. Evolutionary tracks of 13 M_⊙, corresponding to the average mass of the sample stars, are shown with their surface gravity coded by the same colors as the observations. The velocities of the tracks have been multiplied by π/4, to account for the average projection effect. The cross in the lower right corner shows the typical error on the observations.