Full non-LTE spectral line formation III. The case of a two-level atom with broadened upper level

TL;DR

This paper develops a comprehensive non-LTE radiation transfer model for a two-level atom with a broadened upper level, providing analytic and numerical solutions, and explores how collisions affect emission profiles and velocity distributions.

Contribution

It introduces an analytic equivalence between full non-LTE source functions and PFR models, and presents an iterative operator splitting method for solving the problem.

Findings

Analytic equivalence of non-LTE source functions with PFR models.

Numerical solution method using operator splitting techniques.

Deviations of velocity distribution functions from equilibrium.

Abstract

In the present paper we consider the full nonlocal thermodynamic equilibrium (non-LTE) radiation transfer problem. This formalism allows us to account for deviation from equilibrium distribution of both the radiation field and the massive particles. In the present study two-level atoms with broadened upper level represent the massive particles. In the absence of velocity-changing collisions, we demonstrate the analytic equivalence of the full non-LTE source function with the corresponding standard non-LTE partial frequency redistribution (PFR) model. We present an iterative method based on operator splitting techniques to numerically solve the problem at hand. We benchmark it against the standard non-LTE transfer problem for a two-level atom with PFR. We illustrate the deviation of the velocity distribution function of excited atoms from the equilibrium distribution. We also discuss the dependence of the emission profile and the velocity distribution function on elastic collisions and velocity-changing collisions.