Fast approximation of angle-dependent partial redistribution in moving atmospheres

TL;DR

This paper introduces a fast, approximate method for calculating angle-dependent partial redistribution effects in radiative transfer, significantly reducing computational effort while maintaining accuracy in spectral line modeling.

Contribution

The authors develop a novel approach combining angle-averaged redistribution with wavelength transforms to efficiently approximate angle-dependent PRD effects in moving atmospheres.

Findings

The new method closely matches true angle-dependent PRD line profiles.

It significantly reduces computational time compared to traditional angle-dependent calculations.

The approach is effective for modeling spectral lines like Mg II k and Ly alpha in dynamic atmospheres.

Abstract

Radiative transfer modeling of spectral lines including partial redistribution (PRD) effects requires the evaluation of the ratio of the emission to the absorption profile. This quantity requires a large amount of computational work if one employs the angle-dependent redistribution function, which prohibits its use in 3D radiative transfer computations with model atmospheres containing velocity fields. We aim to provide a method to compute the emission to absorption profile ratio that requires less computational work but retains the effect of angle-dependent scattering in the resulting line profiles. We present a method to compute the profile ratio that employs the angle-averaged redistribution function and wavelength transforms to and from the rest frame of the scattering particles. We compare the emergent line profiles of the \MgII\,k and \Lyalpha\ lines computed with angle-dependent PRD, angle-averaged PRD and our new method in two representative test atmospheres. The new method yields a good approximation of true angle-dependent profile ratio and the resulting emergent line profiles while keeping the computational speed and simplicity of angle-averaged PRD theory.