[Paper Review] 3D non-LTE line formation in the solar photosphere and the solar oxygen abundance

D. Kiselman, Åke Nordlund|arXiv (Cornell University)|May 8, 1995

Atmospheric and Environmental Gas Dynamics4 citations

TL;DR

This paper investigates 3D non-LTE line formation in the solar photosphere using hydrodynamic models to reassess the solar oxygen abundance. By modeling the O I 777 nm triplet and OH/[O I] lines beyond 1D LTE approximations, it finds that 3D non-LTE effects reduce, but do not eliminate, the discrepancy between oxygen abundances derived from different spectral lines, suggesting potential resolution within measurement uncertainties of equivalent widths and f-values.

ABSTRACT

We study the formation of O I and OH spectral lines in three-dimensional hydrodynamic models of the solar photosphere. The line source function of the O I 777 nm triplet is allowed to depart from local thermodynamic equilibrium (LTE), within the two-level-atom approximation. Comparison with results from 1D models show that the 3D models alleviate, but do not remove, the discrepancy between the oxygen abundances reported from non-LTE work on the 777 nm triplet and from the [O I] 630 nm and OH lines. Results for the latter two could imply that the solar oxygen abundance is below 8.8 (lg(H) = 12). If this is confirmed, the discrepancy between theory and observation for the 777 nm triplet lines might fall within the range of errors in equivalent width measurements and f-values. The line source function of the 777 nm triplet in the 1.5D approximation is shown to differ insignificantly from the full 3D non-LTE result.

Motivation & Objective

• To resolve the long-standing discrepancy in solar oxygen abundance derived from different spectral lines.
• To investigate the impact of 3D hydrodynamic structure and non-LTE effects on line formation in the solar photosphere.
• To test whether 3D non-LTE modeling can reconcile oxygen abundances from the O I 777 nm triplet with those from [O I] 630 nm and OH lines.
• To evaluate the validity of the 1.5D approximation in modeling 3D non-LTE line formation.

Proposed method

• 3D hydrodynamic simulations of the solar photosphere are used to model atmospheric structure, replacing 1D assumptions.
• Non-LTE radiative transfer is applied to the O I 777 nm triplet using a two-level-atom approximation for the source function.
• Line formation is computed in 3D, accounting for spatially varying temperature, density, and velocity fields.
• Results are compared with 1D LTE and non-LTE models to assess the impact of 3D effects and non-LTE departures.
• The 1.5D approximation is tested by comparing its results to the full 3D non-LTE solution.
• Equivalent widths and line profiles are synthesized and compared with observations to infer oxygen abundance.

Experimental results

Research questions

• RQ1 To what extent do 3D non-LTE effects reduce the discrepancy between oxygen abundances derived from the O I 777 nm triplet and other lines?
• RQ2 How do 3D hydrodynamic structures influence the non-LTE level populations and line formation in the solar photosphere?
• RQ3 Does the 1.5D approximation accurately reproduce the full 3D non-LTE line formation for the O I 777 nm triplet?
• RQ4 Can the observed oxygen abundance discrepancy be attributed to uncertainties in equivalent width measurements or oscillator strengths rather than physical inconsistencies?
• RQ5 What is the implied solar oxygen abundance when consistent 3D non-LTE modeling is applied to multiple lines?

Key findings

• The 3D non-LTE modeling reduces, but does not fully eliminate, the discrepancy between oxygen abundances derived from the O I 777 nm triplet and from [O I] 630 nm and OH lines.
• The results for the [O I] 630 nm and OH lines suggest a solar oxygen abundance below lg(H) = 8.8, indicating a potentially lower abundance than previously thought.
• The 1.5D approximation produces results for the O I 777 nm triplet that are nearly indistinguishable from the full 3D non-LTE solution, validating its use as a computationally efficient alternative.
• The remaining discrepancy in oxygen abundance may be reconciled within the uncertainties of equivalent width measurements and oscillator strength values.
• Non-LTE effects in 3D models significantly alter line formation compared to 1D LTE, particularly in the upper photosphere where temperature and velocity gradients are steepest.