[Paper Review] The solar photospheric nitrogen abundance. Analysis of atomic transitions with 3D and 1D model atmospheres

E. Caffau, E. Maiorca+6 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|Mar 19, 2009

Atmospheric Ozone and Climate29 references69 citations

TL;DR

This study determines the solar photospheric nitrogen abundance using atomic spectral lines analyzed with 3D hydrodynamical (CO 5 BOLD) and 1D model atmospheres, accounting for non-local thermodynamic equilibrium (NLTE) and granulation effects. The authors recommend A(N) = 7.86 ± 0.12, which, when combined with updated oxygen and neon abundances, yields a solar metallicity Z = 0.0156 and Z/X = 0.0213—bringing photospheric abundances into better agreement with helioseismic constraints.

ABSTRACT

CONTEXT: In recent years, the solar chemical abundances have been studied in considerable detail because of discrepant values of solar metallicity inferred from different indicators, i.e., on the one hand, the "sub-solar" photospheric abundances resulting from spectroscopic chemical composition analyses with the aid of 3D hydrodynamical models of the solar atmosphere, and, on the other hand, the high metallicity inferred by helioseismology. AIMS: After investigating the solar oxygen abundance using a CO5BOLD 3D hydrodynamical solar model in previous work, we undertake a similar approach studying the solar abundance of nitrogen, since this element accounts for a significant fraction of the overall solar metallicity, Z. METHOD: We used a selection of atomic spectral lines to determine the solar nitrogen abundance, relying mainly on equivalent width measurements in the literature. We investigate the influence on the abundance analysis, of both deviations from local thermodynamic equilibrium ("NLTE effects") and photospheric inhomogeneities ("granulation effects"). RESULTS: We recommend use of a solar nitrogen abundance of A(N)=7.86+-0.12 whose error bar reflects the line-to-line scatter. CONCLUSION: The solar metallicity implied by the CO5BOLD-based nitrogen and oxygen abundances is in the range 0.0145<= Z <= 0.0167. This result is a step towards reconciling photospheric abundances with helioseismic constraints on Z. Our most suitable estimates are Z=0.0156 and Z/X=0.0213.

Motivation & Objective

• To determine the solar photospheric nitrogen abundance using high-resolution spectral lines and advanced 3D model atmospheres.
• To assess the impact of non-LTE (NLTE) effects and photospheric granulation on nitrogen abundance determination.
• To reconcile photospheric abundance determinations with helioseismic constraints on solar metallicity (Z).
• To evaluate the consistency of current solar spectral atlases and identify systematic uncertainties in line measurements.
• To provide a robust estimate of solar metallicity by combining updated N, O, and Ne abundances with established C and Ne values.

Proposed method

• Analysis of atomic nitrogen spectral lines using equivalent width measurements from high-resolution solar spectral atlases.
• Application of the CO 5 BOLD 3D hydrodynamical solar model atmosphere to compute radiative transfer and line formation.
• Incorporation of non-LTE (NLTE) corrections to account for departures from local thermodynamic equilibrium in line formation.
• Comparison of results from 3D and 1D model atmospheres to quantify the impact of atmospheric inhomogeneities (granulation effects).
• Systematic error estimation based on line-to-line scatter in abundance determinations to derive the final uncertainty.
• Combination of the derived nitrogen abundance with literature values for oxygen, neon, and carbon to compute total solar metallicity (Z) and Z/X.

Experimental results

Research questions

• RQ1 What is the solar photospheric nitrogen abundance when analyzed with 3D model atmospheres and NLTE corrections?
• RQ2 How do granulation effects and non-LTE departures influence the derived nitrogen abundance?
• RQ3 To what extent does the updated nitrogen abundance reduce the discrepancy between photospheric and helioseismic metallicity estimates?
• RQ4 How do discrepancies between historical solar spectral atlases affect abundance determinations?
• RQ5 What is the resulting solar metallicity (Z) when combining the new nitrogen abundance with established values for oxygen, neon, and carbon?

Key findings

• The recommended solar nitrogen abundance is A(N) = 7.86 ± 0.12, reflecting line-to-line scatter and accounting for 3D and NLTE effects.
• The 3D model atmosphere reduces the nitrogen abundance compared to 1D models, but the difference is smaller than previously assumed.
• The study identifies significant inconsistencies between historical solar spectral atlases, introducing systematic uncertainties larger than noise levels.
• The derived solar metallicity is Z = 0.0156 and Z/X = 0.0213 when combining the new N and O abundances with Grevesse & Sauval (1998) values for Ne and C.
• This metallicity range (0.0145 ≤ Z ≤ 0.0167) brings photospheric abundances into better agreement with helioseismic constraints, which infer Z ≈ 0.017–0.024.
• The results suggest that the tension between photospheric and helioseismic metallicity estimates is significantly reduced, though not fully resolved.