Abundances in Ap Stars; Result of Magnetic Binary Evolution
Late Stages of Stellar Evolution
There is an old theory of Fowler et al. (1965) that the peculiar A stars are in the postgiant phase of stellar evolution. This theory tried to explain the anomalous abundances observed in Ap stars but data available today do not confirm it. Another theory concerns the binary nature of Ap stars. It was suggested by Renson and elaborated by van den Heuvel (1968). The latter believed all the peculiar and metallic-line A stars to be evolved binaries. The statistical analysis of mass functions for
... own spectroscopic binaries amongst Am stars developed by myself (Drobyshevski, 1973) shows indeed that about two thirds are evolved binaries. As for peculiar A stars, the situation is complicated as few are known to have companions. These are the stars having strong magnetic fields and it is natural to suppose that the magnetic field must play a role in the evolution of the binary system. Resnikov and myself have made approximate calculations of close binary evolution taking into account the influence of the magnetic field on exchange and loss of mass and angular momentum. The main emphasis was on the realization that the field became turbulent through convective motions in the envelope of the primary com ponent -the red giant. The coherent field has a small influence on the process under consideration because of the small value of its flux through any cross-section of the star as compared with the turbulent field flux Tayler (ed.J, Late Stages of Stellar Evolution, 206-207. All Rights Reserved.