[Paper Review] Line formation in solar granulation VI. [C I], C I, CH and C2 lines and the photospheric C abundance

M. Asplund, N. Grevesse+3 more|ArXiv.org|Oct 28, 2004

Solar and Space Plasma Dynamics3 references123 citations

TL;DR

This study determines the solar photospheric carbon abundance using a 3D hydrodynamical solar model and non-LTE corrections, analyzing [C i], C i, CH, and C₂ lines. It finds a significantly lower abundance of log ε_C = 8.39 ± 0.05, resolving long-standing discrepancies among diagnostics and revising previous 1D-based estimates downward by 0.17 dex.

ABSTRACT

The solar photospheric carbon abundance has been determined from [C I], C I, CH vibration-rotation, CH A-X electronic and C2 Swan electronic lines by means of a time-dependent, 3D, hydrodynamical model of the solar atmosphere. Departures from LTE have been considered for the C I lines. These turned out to be of increasing importance for stronger lines and are crucial to remove a trend in LTE abundances with the strengths of the lines. Very gratifying agreement is found among all the atomic and molecular abundance diagnostics in spite of their widely different line formation sensitivities. The mean of the solar carbon abundance based on the four primary abundance indicators ([C I], C I, CH vibration-rotation, C_2 Swan) is log C = 8.39 +/- 0.05, including our best estimate of possible systematic errors. Consistent results also come from the CH electronic lines, which we have relegated to a supporting role due to their sensitivity to the line broadening. The new 3D based solar C abundance is significantly lower than previously estimated in studies using 1D model atmospheres.

Motivation & Objective

• To resolve discrepancies in solar carbon abundance estimates derived from different atomic and molecular lines.
• To determine the photospheric carbon abundance using a state-of-the-art 3D hydrodynamical solar atmosphere model.
• To account for non-LTE effects in C i lines and assess their impact on abundance determinations.
• To evaluate the consistency of multiple abundance indicators—[C i], C i, CH vibration-rotation, C₂ Swan, and CH electronic transitions—under 3D modeling.
• To reconcile the solar carbon abundance with independent measurements in B stars, the interstellar medium, and solar wind, and to assess implications for C/O ratios.

Proposed method

• Employed a time-dependent, 3D hydrodynamical model of the solar photosphere to simulate realistic atmospheric structure and dynamics.
• Applied non-LTE radiative transfer calculations to C i lines, revealing significant departures from LTE, especially in stronger lines.
• Used high-precision atomic and molecular data, including transition probabilities, excitation potentials, and dissociation energies from VALD and NIST databases.
• Conducted detailed spectral synthesis and χ² fitting of observed line profiles for [C i] 872.7 nm, C i, CH vibration-rotation, C₂ Swan, and CH A-X transitions.
• Accounted for collisional broadening using Anstee & O’Mara (1995) and Barklem et al. (1998) tables, with Unsöld’s formula as fallback for lines outside tabulated ranges.
• Compared results across diagnostics to test consistency and identify systematic errors, with the 3D model enabling agreement across widely different line formation sensitivities.

Experimental results

Research questions

• RQ1 What is the solar photospheric carbon abundance when derived from multiple atomic and molecular lines using a 3D hydrodynamical model?
• RQ2 How do non-LTE effects in C i lines influence the derived carbon abundance, and are they essential for consistency?
• RQ3 To what extent do different abundance indicators—[C i], C i, CH, C₂—agree when analyzed under 3D modeling and non-LTE conditions?
• RQ4 How does the new 3D-based carbon abundance compare with previous 1D-based estimates and independent measurements in B stars and the interstellar medium?
• RQ5 What is the impact of the new carbon abundance on the solar C/O ratio, and does it align with measurements from solar wind and flares?

Key findings

• The solar carbon abundance is determined as log ε_C = 8.39 ± 0.05, with the uncertainty including systematic errors, based on a mean of four primary diagnostics: [C i], C i, CH vibration-rotation, and C₂ Swan lines.
• Non-LTE effects in C i lines are significant and increase with line strength, and their inclusion is crucial to eliminate trends in LTE abundances with line strength.
• Excellent agreement is found among all primary abundance indicators in the 3D model, despite their widely different temperature and pressure sensitivities, validating the consistency of the result.
• The 3D model resolves the long-standing discrepancy between atomic and molecular line abundances seen in 1D models, where molecular lines yielded systematically higher abundances.
• The new abundance is significantly lower than previous 1D-based estimates: -0.17 dex lower than Anders & Grevesse (1989) and -0.28 dex lower than Lambert (1978).
• The revised solar C/O ratio is 0.54 ± 0.09, in excellent agreement with measurements from solar flares (0.54 ± 0.04) and solar wind (0.47 ± 0.01), supporting the new low carbon abundance.