The Early a Stars. I. Rotation and Metallicism.

The problem of classifying main-sequence stars near type AO is reviewed. Attention is drawn to a similar difficulty with regard to Am (metallic-line) stars previously found only as early as AS. Recent detail analyses, using model atmospheres, of two early A stars, Sirius and 68 Tau, have shown abundance anomalies similar to those in Am stars. Since these stars had been classified as normal, it is suggested that other Am stars of early type can be found by improved methods, one of which is discussed here. The line strength of the strong line of ionized scandium at X 4246 relative to the line of ionized strontium at 4215, indicates the presence of an Am star. In normal stars these two lines have the same strength, but in Am stars as well as in Sirius and 68 Tau the scandium line is considerably weaker. This criterion, which is useful for detection of Am stars over a wide range of effective temperature, is clearly discernible only for sharp-line stars and in spectra of relatively high dispersion. A diagram of rotation as a function of B - V color for the brightest stars in the sky indicates many sharp4ine stars near type AO, which have formerly been classified as normal. High-dispersion spectra of most of them show that about one-half have anomalous Sc/Sr line strength. It is probable that they are the heretofore hidden analogues of the later-type Am stars and that the metallic-line phenomena extend into the early A stars. Three hypotheses concerning the origin of the metallic-line stars are reviewed. The main facts necessary for a theoretical explanation are summarized.