# 3D non-LTE line formation of neutral carbon in the Sun

## TL;DR

This paper develops a 3D non-LTE model for neutral carbon line formation in the Sun, providing more accurate abundance measurements and a new approach to modeling hydrogen collisions.

## Contribution

It introduces a physically-motivated recipe for hydrogen impact excitation rates in non-LTE models and applies it to derive the first fully consistent 3D non-LTE solar carbon abundance.

## Key findings

- Non-LTE abundance corrections reach up to 0.1 dex.
- The derived solar carbon abundance is log ε_C = 8.44 ± 0.02.
- Models reproduce observed center-to-limb variations without adjusting collision rates.

## Abstract

Carbon abundances in late-type stars are important in a variety of astrophysical contexts. However C i lines, one of the main abundance diagnostics, are sensitive to departures from local thermodynamic equilibrium (LTE). We present a model atom for non-LTE analyses of C i lines, that uses a new, physically-motivated recipe for the rates of neutral hydrogen impact excitation. We analyse C i lines in the solar spectrum, employing a three-dimensional (3D) hydrodynamic model solar atmosphere and 3D non-LTE radiative transfer. We find negative non-LTE abundance corrections for C i lines in the solar photosphere, in accordance with previous studies, reaching up to around 0.1 dex in the disk-integrated flux. We also present the first fully consistent 3D non-LTE solar carbon abundance determination: we infer log $\epsilon_{\text{C}}$ = $8.44\pm0.02$, in good agreement with the current standard value. Our models reproduce the observed solar centre-to-limb variations of various C i lines, without any adjustments to the rates of neutral hydrogen impact excitation, suggesting that the proposed recipe may be a solution to the long-standing problem of how to reliably model inelastic collisions with neutral hydrogen in late-type stellar atmospheres.