The effect of electron scattering redistribution on atomic line polarization

Abstract

The polarization of spectral lines is generated by the scattering of angularly anisotropic incident radiation field on the atoms in the stellar atmosphere. This atomic scattering polarization is modified by frequency non-coherent scattering of line photons on free electrons. With modern spectropolarimeters of high sensitivity, it is possible to detect such changes in the spectral line polarization caused by scattering on electrons. We present new and efficient numerical techniques to solve the problem of line radiative transfer with atomic and electron scattering frequency redistribution in planar media. The evaluation and use of angle-dependent partial frequency redistribution functions (both atomic and electron scattering type) in the transfer equation require a lot of computing effort. In this paper, we apply a decomposition technique to handle this numerically difficult problem. This recently developed technique is applied for the first time to the electron scattering partial redistribution. This decomposition technique allows us to devise fast iterative methods of solving the polarized line transfer equation. An approximate lambda iteration (ALI) method and a method based on Neumann series expansion of the polarized source vector are proposed. We show that these numerical methods can be used to obtain a solution of the problem, when both atomic and electron scattering partial frequency redistribution are considered together. This is in contrast with the classical numerical methods which require a great amount of computing time. We show the importance of electron scattering redistribution in the far wing line polarization, which has practical implications in the analysis of polarized stellar or solar spectra, where non-coherent electron scattering controls the line wing transfer.