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A&A 468, 797-801 (2007)
DOI: 10.1051/0004-6361:20066507

A generalized $\sqrt{\epsilon}$-law

The role of unphysical source terms in resonance line polarization transfer and its importance as an additional test of NLTE radiative transfer codes
J. Stepán1, 2 and V. Bommier2

1  Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 Ondrejov, Czech Republic
    e-mail: stepan@asu.cas.cz
2  LERMA, Observatoire de Paris-Meudon, CNRS UMR 8112, 5 place Jules Janssen, 92195 Meudon Cedex, France
    e-mail: [jiri.stepan;V.Bommier]@obspm.fr

(Received 4 October 2006 / Accepted 26 March 2007)

Abstract
Context.A derivation of a generalized $\sqrt{\epsilon}$-law for nonthermal collisional rates of excitation by charged perturbers is presented.
Aims.Aim of this paper is to find a more general analytical expression for a surface value of the source function which can be used as an additional tool for verification of the non-LTE radiative transfer codes.
Methods.Under the impact approximation hypothesis, static, one-dimensional, plane-parallel atmosphere, constant magnetic field of arbitrary strength and direction, two-level atom model with unpolarized lower level and stimulated emission neglected, we introduce the unphysical terms into the equations of statistical equilibrium and solve the appropriate non-LTE integral equations.
Results.We derive a new analytical condition for the surface values of the source function components expressed on the basis of irreducible spherical tensors.


Key words: line: formation -- polarization -- radiative transfer



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