Full non-LTE multi-level radiative transfer II. The case of a 5-level Ca ii atom with broadened excited levels

TL;DR

This paper extends the full non-LTE radiative transfer formalism to multi-level atoms with broadened excited levels, introducing an efficient iterative solution method and analyzing Ca II spectral lines.

Contribution

It develops a new numerical approach for FNLTE in multi-level atoms with broadened levels and applies it to Ca II lines, showing standard NLTE suffices in this case.

Findings

Standard NLTE with partial redistribution is sufficient for Ca II lines.

The new iterative method efficiently solves the coupled kinetic equations.

FNLTE effects are negligible for Ca II H & K and infrared triplet lines.

Abstract

The so-called full non-local thermodynamic equilibrium (FNLTE) radiative transfer problem allows us to take into account not only deviations of the radiation field from the Planckian but also deviations of the densities and velocity distributions of massive particles from Maxwell-Boltzmann statistics. This article discusses the extension of this formalism to physically realistic multi-level atoms, including natural broadening of the excited levels. In practice, we must solve self-consistently a coupled set of kinetic equations and determine, for each line, an emission and absorption profile by convolving a non-Lorentzian atomic profile with a non-Maxwellian velocity distribution at each iteration. To solve this numerically challenging problem, we have developed a new efficient iterative method based on well-known approximate operator techniques. After validating our numerical strategy, we present the results obtained for the H & K lines and the infrared triplet of the Ca II. Under the conditions studied, for this particular atomic model and for a simplified atmosphere, we find that the standard NLTE with partial redistribution is sufficient to describe the formation of Ca II spectral lines. The more exact treatment of FNLTE is unnecessary in the case of Ca II H & K, and infrared triplet lines, even when accounting for velocity-changing collisions.