Full non-LTE multi-level radiative transfer

I. An atom with three bound infinitely sharp levels

¹ Université de Toulouse, Observatoire Midi-Pyrénées, Cnrs, Cnes, Irap, Toulouse, France
² Cnrs, Institut de Recherche en Astrophysique et Planétologie, 14 av. E. Belin, 31400 Toulouse, France
³ Indian Institute of Astrophysics, Koramangala, Bengaluru 560034, India

^★ Corresponding author: tristan.lagache@univ-tlse3.fr

Received: 2 April 2025
Accepted: 20 May 2025

Abstract

Context. The standard nonlocal thermodynamic equilibrium (non-LTE) multi-level radiative transfer problem only takes into account the deviation of the radiation field and atomic populations from their equilibrium distribution.

Aims. We aim to show how to solve for the full non-LTE (FNLTE) multi-level radiative transfer problem, also accounting for deviation of the velocity distribution of the massive particles from Maxwellian. We considered, as a first step, a three-level atom with zero natural broadening.

Methods. In this work, we present a new numerical scheme. Its initialisation relies on the classic, multi-level approximate Λ-iteration (MALI) method for the standard non-LTE problem. The radiative transfer equations, the kinetic equilibrium equations for atomic populations, and the Boltzmann equations for the velocity distribution functions were simultaneously iterated in order to obtain self-consistent particle distributions. During the process, the observer’s frame absorption and emission profiles were re-computed at every iterative step by convolving the atomic frame quantities with the relevant velocity distribution function.

Results. We validate our numerical strategy by comparing our results with the standard non-LTE solutions in the limit of a two-level atom with Hummer’s partial redistribution in frequency, and with a three-level atom with complete redistribution. In this work, we considered the so-called cross-redistribution problem. We then show new FNLTE results for a simple three-level atom while evaluating the assumptions made for the emission and absorption profiles of the standard non-LTE problem with partial and cross-redistribution.