[Paper Review] Line formation in solar granulation: IV. [O I], OI and OH lines and the photospheric O abundance

M. Asplund, N. Grevesse+3 more|ArXiv.org|Dec 11, 2003

Stellar, planetary, and galactic studies63 references513 citations

TL;DR

This study determines the solar photospheric oxygen abundance using a 3D time-dependent hydrodynamical solar atmosphere model and non-LTE radiative transfer for [O I], O I, and OH lines. It finds log ε(O) = 8.66 ± 0.05, resolving long-standing discrepancies between different oxygen indicators and supporting a lower solar metallicity (Z = 0.0126), though this challenges helioseismic models due to sound speed mismatches.

ABSTRACT

The solar photospheric oxygen abundance has been determined from [OI], OI, OH vibration-rotation and OH pure rotation lines by means of a realistic time-dependent, 3D, hydrodynamical model of the solar atmosphere. In the case of the OI lines, 3D non-LTE calculations have been performed, revealing significant departures from LTE as a result of photon losses in the lines. We derive a solar oxygen abundance of log O = 8.66 +/- 0.05. All oxygen diagnostics yield highly consistent abundances, in sharp contrast with the results of classical 1D model atmospheres. This low value is in good agreement with measurements of the local interstellar medium and nearby B stars. This low abundance is also supported by the excellent correspondence between lines of very different line formation sensitivities, and between the observed and predicted line shapes and center-to-limb variations. Together with the corresponding down-ward revisions of the solar carbon, nitrogen and neon abundances, the resulting significant decrease in solar metal mass fraction to Z = 0.0126 can, however, potentially spoil the impressive agreement between predicted and observed sound speed in the solar interior determined from helioseismology.

Motivation & Objective

• To resolve the long-standing discrepancy between oxygen abundances derived from different spectral lines in the solar photosphere.
• To determine the true solar photospheric oxygen abundance using realistic 3D time-dependent hydrodynamical models of the solar atmosphere.
• To assess the impact of non-LTE effects and 3D atmospheric structure on oxygen line formation, particularly for [O I], O I, and OH lines.
• To evaluate the consistency of oxygen abundance determinations across multiple diagnostics with varying formation sensitivities.
• To examine the implications of a revised solar oxygen abundance for solar interior models, especially helioseismology.

Proposed method

• Employed a 3D time-dependent hydrodynamical model of the solar atmosphere to simulate realistic granulation and convective motions.
• Perfomed 3D non-LTE radiative transfer calculations for O I lines, accounting for photon losses and departure from local thermodynamic equilibrium.
• Analyzed [O I] 630.0 nm, O I 777.4 nm triplet, and OH vibration-rotation and pure rotation lines using synthetic spectra.
• Compared synthetic line profiles and center-to-limb variations with high-resolution observations to validate the model.
• Used consistent atomic data, including updated transition probabilities, and accounted for blending with Ni I in the [O I] line.
• Evaluated the consistency of results across different diagnostics and assessed the impact on solar metallicity and helioseismic models.

Experimental results

Research questions

• RQ1 What is the true solar photospheric oxygen abundance when accounting for 3D atmospheric structure and non-LTE effects?
• RQ2 Why do different oxygen indicators—[O I], O I, and OH lines—yield inconsistent abundances in 1D models?
• RQ3 How do departures from LTE affect the formation of O I lines in the solar photosphere?
• RQ4 To what extent do 3D hydrodynamical models improve the agreement between observed and predicted line profiles and center-to-limb variations?
• RQ5 What are the implications of a lower oxygen abundance for solar interior structure and helioseismic models?

Key findings

• The solar oxygen abundance is determined to be log ε(O) = 8.66 ± 0.05 using a 3D time-dependent hydrodynamical model and 3D non-LTE calculations.
• All oxygen diagnostics—[O I], O I, and OH lines—yield highly consistent abundances when using 3D models, resolving prior discrepancies seen in 1D analyses.
• Non-LTE effects in O I lines are significant due to photon losses, and 3D non-LTE calculations are essential for accurate abundance determination.
• The revised oxygen abundance leads to a lower solar metallicity of Z = 0.0126, a significant downward revision from the previous value of Z = 0.0194.
• The new abundance is in excellent agreement with measurements of the local interstellar medium and nearby B stars, resolving the previous overabundance paradox.
• The lower metallicity challenges helioseismic models, as the predicted sound speed no longer matches observations as well, suggesting a possible mismatch between photospheric and interior abundances or inaccuracies in opacity treatments.