The early A type stars - Refined MK classification, confrontation with Stroemgren photometry, and the effects of rotation

Abstract

The MK classification system for the early A-type stars is refined, and a parallel system of standards for the broad-lined stars is introduced. With this improved system, stars may be classified with significantly greater precision than before. It is shown that spectral types in this system are not systematically affected by rotational line broadening. A total of 372 early A-type stars are classified, and a confrontation of these spectral types with Stroemgren photometry reveals a number of systematic photometric effects of rotation. In particular, high v sin i stars are systematically redder than low v sin i stars of the same spectral type, and the beta index is weakened by rotation. It is concluded that precise spectral classification in conjunction with Stroemgren and H-beta photometry can potentially provide a valuable check and input to the theory of the atmospheres of rotating stars.