A-type stars rotation and spectral peculiarities

Those abnormalities are generally attributed to diffusion and can occur only with little rotational mixing. However at all types there are overlaps of these distributions, implying that a given intermediate rotational velocity is insufficient to determine whether the star should have a normal or abnormal spectrum. However, we realized that (1) some of our "standards," such as Vega and Alpha Dra, are really abnormal, causing us to classify similar peculiar stars as "normal," (2) many of the "normal" stars near A2 IV have the characteristics of peculiar stars such as low rotational velocities and weak 4481 Mg II and K lines, and (3) the mean rotational velocities of "normal" stars are depressed just at those types where the Ap and Am stars are most frequent.

Therefore we conclude that the overlaps are due to our failure to detect all the abnormal stars and that a specific rotational velocity is probably enough to determine whether a star will have a normal or abnormal spectrum.