A numerical approach for modelling the polarisation signals of strong resonance lines with partial frequency redistribution. Numerical applications to two-term atoms and plane-parallel atmospheres

Fabio Riva; Gioele Janett; Luca Belluzzi; Tanausú del Pino Alemán; Ernest Alsina Ballester; Javier Trujillo Bueno; Pietro Benedusi; Simone Riva; Rolf Krause

doi:10.1051/0004-6361/202453107

arXiv · 2505.20968

A numerical approach for modelling the polarisation signals of strong resonance lines with partial frequency redistribution. Numerical applications to two-term atoms and plane-parallel atmospheres

Fabio Riva·Gioele Janett·Luca Belluzzi·Tanausú del Pino Alemán·Ernest Alsina Ballester·Javier Trujillo Bueno·Pietro Benedusi·Simone Riva·Rolf Krause

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Abstract

Aims. The main goal of this paper is to present an accurate and efficient numerical strategy for solving the radiative transfer problem for polarised radiation in strong resonance lines forming out of local thermodynamic equilibrium, taking angle-dependent (AD) partial frequency redistribution (PRD) effects and J-state interference into account. We consider the polarisation produced both by the Zeeman effect and by the scattering of anisotropic radiation, along with its sensitivity to the Hanle and magneto-optical effects. Methods. We introduce a formalism that allows treating both a two-level and a two-term atom in the presence of arbitrary magnetic and bulk velocity fields. The problem is formulated by treating the population of the lower level/term as a fixed input parameter. This approach makes the problem linear with respect to the radiation field, enabling the application of efficient matrix-free preconditioned iterative methods for its solution. Additionally, the computation of the scattering emissivity in the comoving frame, together with a careful choice of the angular and spectral quadrature nodes, allow us to speed up the calculations by reducing the number of evaluations of the redistribution functions. Results. The proposed solution strategy is applied to synthesise the Stokes profiles of the Mg ii h&k doublet and the H i Ly-α line in 1D semi-empirical models. The results demonstrate that the method is both fast and accurate. A comparison with calculations from HanleRT-TIC displays an overall good agreement, thereby validating our solution strategy. Moreover, for the wavelength-integrated polarisation profiles of the H i Ly-α line, we find an excellent agreement between the results obtained including PRD effects in their general AD description and those obtained considering the angle-averaged simplifying approximation.

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Fabio Riva, Gioele Janett, Luca Belluzzi, Tanausú del Pino Alemán, Ernest Alsina Ballester, Javier Trujillo Bueno, Pietro Benedusi, Simone Riva, Rolf Krause. 2025-05-27. A numerical approach for modelling the polarisation signals of strong resonance lines with partial frequency redistribution. Numerical applications to two-term atoms and plane-parallel atmospheres. https://doi.org/10.1051/0004-6361%2F202453107

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