Atomic line radiative transfer with MCFOST I. Code description and benchmarking

TL;DR

This paper introduces MCFOST-art, a new 3D non-LTE radiative transfer code for atomic systems, demonstrating its accuracy through benchmarking against established codes in stellar atmosphere modeling.

Contribution

The paper presents MCFOST-art, a versatile non-LTE radiative transfer solver integrated with MCFOST, with comprehensive benchmarking showing high accuracy in modeling stellar environments.

Findings

Results agree within a few percent across codes.

Sub-percent agreement between MCFOST-art and RH.

Minor discrepancies due to opacity treatment differences.

Abstract

Aims. We present MCFOST-art, a new non-local thermodynamic equilibrium radiative transfer solver for multilevel atomic systems. The code is embedded in the 3D radiative transfer code MCFOST and is compatible with most of the MCFOST modules. The code is versatile and designed to model the close environment of stars in 3D. Methods. The code solves for the statistical equilibrium and radiative transfer equations using the Multilevel Accelerated Lambda Iteration (MALI) method. We tested MCFOST-art on spherically symmetric models of stellar photospheres as well as on a standard model of the solar atmosphere. We computed atomic level populations and outgoing fluxes and compared these values with the results of the TURBOspectrum and RH codes. Calculations including expansion and rotation of the atmosphere were also performed. We tested both the pure local thermodynamic equilibrium and the out-of-equilibrium problems. Results. In all cases, the results from all codes agree within a few percent at all wavelengths and reach the sub-percent level between RH and MCFOST-art. We still note a few marginal discrepancies between MCFOST-art and TURBOspectrum as a result of different treatments of background opacities at some critical wavelength ranges.