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

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 C = 8.47±0.02, log N = 7.89±0.04, and log O = 8.70±0.04. 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.

Observational constraints on the origin of the elements. IV: The standard composition of the Sun

“…Amarsi et al 2018;Bergemann et al 2021). Also, typically very weak atomic and molecular features across the entire range from the optical to mid-IR at ∼ 1.5 µm are used for the solar analysis (such as the lines of OH, CN, CH, and NH, Amarsi et al 2021), which is usually inapplicable for large samples of stars. In addition, full 3D NLTE calculations are computationally prohibitive and are currently not feasible for large stellar samples.…”