The solar carbon, nitrogen, and oxygen abundances from a 3D LTE analysis of molecular lines

TL;DR

This study provides a homogeneous 3D LTE analysis of molecular lines to determine solar abundances of C, N, and O, addressing longstanding debates and comparing molecular and atomic line results.

Contribution

It offers a comprehensive 3D LTE analysis of molecular lines for solar C, N, and O abundances, highlighting the agreement and discrepancies with atomic line analyses.

Findings

Carbon abundance: 8.47±0.02 dex

Nitrogen abundance: 7.83 dex (mean estimate)

Oxygen abundance: 8.70±0.04 dex

Abstract

Carbon, nitrogen, and oxygen are the fourth, sixth, and third most abundant elements in the Sun. Their abundances remain hotly debated due to the so-called solar modelling problem that has persisted for almost $20$ years. We revisit this issue by presenting a homogeneous analysis of $408$ molecular lines across $12$ diagnostic groups, observed in the solar intensity spectrum. Using a realistic 3D radiative-hydrodynamic model solar photosphere and LTE (local thermodynamic equilibrium) line formation, we find $\log\epsilon_{C} = 8.47\pm0.02$, $\log\epsilon_{N} = 7.89\pm0.04$, and $\log\epsilon_{O} = 8.70\pm0.04$. The stipulated uncertainties mainly reflect the sensitivity of the results to the model atmosphere; this sensitivity is correlated between the different diagnostic groups, which all agree with the mean result to within $0.03$ dex. For carbon and oxygen, the molecular results are in excellent agreement with our 3D non-LTE analyses of atomic lines. For nitrogen, however, the molecular indicators give a $0.12$ dex larger abundance than the atomic indicators, and our best estimate of the solar nitrogen abundance is given by the mean: $7.83$ dex. The solar oxygen abundance advocated here is close to our earlier determination of $8.69$ dex, and so the present results do not significantly alleviate the solar modelling problem.