XTAT: A New Multilevel-Multiline Polarized Radiative Transfer Code with PRD

International audienceThis work is intended to the interpretation of the so-called "Second Solar Spectrum" (Stenflo 1996), which is the spectrum of the linear polarization formed by scattering and observed close to the solar internal limb. The lines are also optically thick, and the problem is to solve in a coherent manner, the statistical equilibrium of the atomic density matrix and the polarized radiative transfer in the atmosphere. Following Belluzzi & Landi Degl'Innocenti (2009), 30 % of the solar visible line linear polarization profiles display the M-type shape typical of coherent scattering effect in the far wings. A new theory including both coherent (Rayleigh) and resonant scatterings was developed by Bommier (1997a,b). Raman scattering was later added (Bommier 1999, SPW2). In this theory, which is straightly derived from the Schrödinger equation for the atomic density matrix, the radiative line broadening appears as a non-Markovian process of atom-photon interaction. The collisional broadening is included. The Rayleigh (Raman) scattering appears as an additional term in the emissivity from the fourth order of the atom-photon interaction perturbation development. The development is pursued and finally summed up, leading to a non-perturbative final result. In this formalism, the use of redistribution functions is avoided. The published formalism was limited to the two-level atom without lower level alignment. But most of the solar lines are more complex. We will present how the theory has to be complemented for multi-level atom modeling, including lower level alignment. The role of the collisions as balancing coherent and resonant scatterings is fully taken into account. Progress report will be given about the development of a new code for the numerical iterative solution of the statistical equilibrium and polarized radiative transfer equations, for multi-level atoms and their multi-line spectrum. Fine and hyperfine structures, and Hanle, Kemp (Kemp et al. 1984), Zeeman, incomplete Paschen-Back effects, are included. Doppler redistribution is fully taken into account, by solving the statistical equilibrium for each velocity class of the atoms. In the presentation, a particular attention will be devoted to the depolarizing collisions and their rate estimation