# Non-LTE line formation of Fe in late-type stars IV: Modelling of the solar centre-to-limb variation in 3D

## TL;DR

This study employs advanced 3D NLTE modeling with an extensive Fe atom model to accurately reproduce solar spectral line variations across the disk, refining the solar iron abundance measurement.

## Contribution

It introduces a comprehensive 3D NLTE model for Fe line formation using new atomic data, reducing free parameters and improving agreement with solar observations.

## Key findings

- 3D NLTE modeling accurately reproduces solar line centre-to-limb variation.
- The inferred solar iron abundance is log(eps(Fe))=7.48±0.04.
- Highlights need for improved electron collision cross-sections.

## Abstract

Our ability to model the shapes and strengths of iron lines in the solar spectrum is a critical test of the accuracy of the solar iron abundance, which sets the absolute zero-point of all stellar metallicities. We use an extensive 463-level Fe atom with new photoionisation cross-sections for FeI as well as quantum mechanical calculations of collisional excitation and charge transfer with neutral hydrogen; the latter effectively remove a free parameter that has hampered all previous line formation studies of Fe in non-local thermodynamic equilibrium (NLTE). For the first time, we use realistic 3D NLTE calculations of Fe for a quantitative comparison to solar observations. We confront our theoretical line profiles with observations taken at different viewing angles across the solar disk with the Swedish 1-m Solar Telescope. We find that 3D modelling well reproduces the observed centre-to-limb behaviour of spectral lines overall, but highlight aspects that may require further work, especially cross-sections for inelastic collisions with electrons. Our inferred solar iron abundance is log(eps(Fe))=7.48+-0.04.