The period distribution of unevolved close binary systems

Abstract

Period distributions have been examined for various spectral types of about 600 (eclipsing and spectroscopic) close binaries, which are likely to be substantially unevolved. The comparison with the previous corresponding analyses of extensive (but heterogeneous) binary samples allows a clarification of the extent of the evolutionary and observational selection effects. Remarkably, this analysis reveals a great deficiency of short period binaries (with periods corresponding to case A mass transfer) in the whole spectral range. For the late spectral types, this result may be connected with postformation angular momentum loss caused by stellar wind magnetic braking; at least for the late B and A spectral range, a ready interpretation of this finding is that close binaries of corresponding periods and spectral types are rarely formed.