Silicon levitation in chemically peculiar stars and the oblique rotator model

Abstract

The behavior of the radiation force on silicon in chemically peculiar late B stars is investigated. The equation of transfer is solved for the 126 most important lines in the first four ionization stages of silicon, taking into account departures from LTE in Si I. It is shown that the radiation force can maintain within a factor of two the normal silicon abundances observed in nonmagnetic Ap and Bp stars, but that the presence of horizontal magnetic field lines can lead to substantial apparent silicon overabundances in the range of effective temperatures where Si stars are observed. A specific mechanism involving upward diffusion of Si I is outlined for producing overabundances in magnetic stars at places where magnetic field lines are horizontal; with this mechanism the location of silicon-rich spots on a stellar surface depends on the exact magnetic field configuration. The temperature boundaries of Si stars and observations of other elements in such stars are discussed in the framework of the oblique-rotator model.