Helium settling and mass loss in magnetic Ap stars -: I.: The chemical stratification

The chemical stratification is likely to play a predominant role in the excitation mechanism of high order modes in rapidly oscillating Ap stars. In this paper we discuss some processes (convection, He settling, mass loss and turbulent mixing) which may strongly affect the chemical composition of Ap stars. We discuss their interplay with a dipolar magnetic field in both magnetic polar and equatorial regions, where the magnetic lines are respectively mainly radially and horizontally oriented. We also present stellar evolutionary computations for magnetic Ap stars. As the magnetic geometry strongly breaks the sphericity of the star, we modeled and computed the polar and equatorial regions separately. We examine the effects of various chemical transport processes on the chemical stratification of both regions. In a forthcoming paper, we will study the stability of these models against pulsations.