Abstract
The importance, for the study of the magnetic Ap stars, of a proper treatment of the transfer of polarized radiation in Stark broadened hydrogen lines in the presence of a magnetic field is stressed. It is shown that in the physical conditions prevailing in the atmospheric layers of magnetic Ap stars where the near wings of the hydrogen Balmer lines are formed, the linear Stark effect due to the charged particles; the Stark effect produced by the Lorentz field resulting from the thermal motions of the hydrogen atoms in the magnetic field; and the Zeeman effect have the same order of magnitude. Since the hydrogen lines are so strong in Ap stars (and thus strongly influence the radiative transfer throughout the stellar atmosphere), and since they are used as a major diagnostic tool (to determine the surface gravity and the longitudinal magnetic field), it is essential to achieve a good understanding of how they are formed. A formalism to address the abovementioned transfer problem is presented. It is a generalization of the unified theory of the Stark broadening of hydrogen lines developed by Cooper, Smith, and Vidal, which accounts for the effect of the magnetic field and which treats the various profiles that are the elements of the Müller matrix appearing in the equation of transfer of the Stokes vector representing the polarized light beam. Two regimes of approximation are considered: (i) the perturbing ions are treated as quasistatic and the Lorentz electric field resulting from the motions of the hydrogen atoms in the magnetic field is neglected, or (ii) the Lorentz electric field is accounted for and the ions are treated by the same impact approach as the electrons. The limits of validity of both cases are briefly discussed.
Published Version
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