Arguments Indicating that Ap and Am Stars are Evolved Spectroscopic Binaries

Abstract

Roughly three types of evolution of spectroscopic binaries may occur:Case (a): the primary fills its Roche limit already before the end of core hydrogen burning;Case (b): the primary fills its Roche limit after core hydrogen exhaustion but before the onset of helium burning, andCase (c): the primary does not fill its Roche limit before the end of core helium burning.Since long-period variables of Population I as well as Population II may reach radii of 10 AU, case (c) will occur in systems with periods up to 100 years. Since about 50% of the A and B stars are spectroscopic binaries, many evolved spectroscopic binaries are expected to occur in the galactic system. Computations show that, using the mass-ratio distribution of unevolved systems derived by Kuiper, the number of systems with one evolved component is expected to be at least as large as the number of unevolved systems, in the spectral regions A and B.