Does Rotation Alone Determine Whether an A‐Type Star’s Spectrum Is Abnormal or Normal?

As noted by Abt & Morrell, virtually all of the metallic line (Am) and peculiar A (Ap) stars have equatorial rotational velocities less than 120 km s -1 , and most of the normal A0-F0 main-sequence stars have equatorial rotational velocities greater than 120 km s -1 . However, at all spectral types there are some (10%-20%) of the normal stars that have smaller rotational velocities. If this overlap is real, then a star's rotational velocity is insufficient to explain its abnormal or normal spectra. We studied the A5-F0 and A2-A4 stars and found in both cases that there are stars classified as "normal" that have unusually weak Ca II K lines and/or that occur in short-period binaries. Therefore, the overlap seems to be due to undetected marginal abnormal stars. Among the A0-A1 stars we find that our inability to distinguish consistently the class IV from the class V stars can explain the overlap because the class IV stars have lower rotational velocities than class V stars. We conclude from statistical arguments that rotation alone can explain the appearance of an A star as either abnormal or normal.