[Paper Review] Line formation in solar granulation VII. CO lines and the solar C and O isotopic abundances

Patrick Scott, M. Asplund+2 more|CERN Bulletin|May 4, 2006

Solar and Space Plasma Dynamics62 references48 citations

TL;DR

This study uses a 3D radiative-hydrodynamic solar atmosphere model to analyze CO spectral lines from the ATMOS space mission, determining solar carbon and oxygen isotopic abundances with high precision. It finds a solar carbon abundance of log ε(C) = 8.39 ± 0.05 and isotopic ratios of 12C/13C = 86.8+3.9−3.7 and 16O/18O = 479+29−28, suggesting a cool COmosphere above the photosphere and challenging current solar system formation models.

ABSTRACT

CO spectral line formation in the Sun has long been a source of consternation for solar physicists, as have the elemental abundances it seems to imply. We modelled solar CO line formation using a realistic, ab initio, time-dependent 3D radiative-hydrodynamic model atmosphere. Results were compared with observations from the space-based ATMOS experiment. We employed weak 12C16O, 13C16O and 12C18O lines from the fundamental and first overtone bands to determine the solar carbon abundance, as well as the 12C/13C and 16O/18O isotopic ratios. A weighted carbon abundance of log epsilonC = 8.39 +-0.05 was found. We note with satisfaction that the derived abundance is identical to our recent 3D determination based on CI, [C I], C2 and CH lines. Identical calculations were carried out using 1D models, but only the 3D model was able to produce abundance agreement between different CO lines and the other atomic and molecular diagnostics. Solar 12C/13C and 16O/18O ratios were measured as 86.8+3.9-3.7 (delta13C = 30+46-44) and 479+29-28 (delta18O = 41+67-59), respectively. These values may require current theories of solar system formation to be revised. Excellent agreement was seen between observed and predicted weak CO line shapes, without invoking micro- or macroturbulence. Agreement breaks down for the strongest CO lines however, which are formed in very high atmospheric layers. The simplest explanation is that temperatures are overestimated in the highest layers of the 3D simulation. Thus, our analysis supports the presence of a COmosphere above the traditional photospheric temperature minimum, with an average temperature of less than 4000K. The shortcoming of the model atmosphere is not surprising, given that it was never intended to properly describe such high layers.

Motivation & Objective

• To determine the solar carbon abundance using weak CO lines from the ATMOS space mission.
• To measure the 12C/13C and 16O/18O isotopic ratios using CO spectral lines in the solar infrared spectrum.
• To test whether 3D radiative-hydrodynamic models can reproduce observed CO line profiles without invoking micro- or macroturbulence.
• To investigate discrepancies between observed and predicted strongest CO lines, indicating potential model limitations in high atmospheric layers.
• To assess the implications of the results for solar atmosphere structure and solar system formation theories, particularly the CO self-shielding hypothesis.

Proposed method

• Employed a realistic, ab initio, time-dependent 3D radiative-hydrodynamic model atmosphere to simulate solar granulation and CO line formation.
• Used weak 12C16O, 13C16O, and 12C18O lines from the fundamental (Δv=1) and first overtone (Δv=2) bands to derive elemental abundances.
• Compared synthetic line profiles with observed space-based spectra from the ATMOS experiment to validate model predictions.
• Conducted parallel calculations using 1D model atmospheres to compare the performance of 3D vs. 1D approaches.
• Analyzed line bisectors and strengths to assess temperature structure and non-LTE effects in high atmospheric layers.
• Evaluated the consistency of derived abundances across multiple diagnostics (C i, [C i], C2, CH, and CO lines) to validate results.

Experimental results

Research questions

• RQ1 Can 3D radiative-hydrodynamic models accurately reproduce observed CO line profiles in the solar spectrum without assuming micro- or macroturbulence?
• RQ2 What is the solar carbon abundance derived from CO lines, and how does it compare with results from other atomic and molecular diagnostics?
• RQ3 What are the precise values of the 12C/13C and 16O/18O isotopic ratios in the Sun, and how do they compare with predictions from solar system formation models?
• RQ4 Why do the strongest CO lines show discrepancies between observed and predicted strengths, and what does this imply about the temperature structure in the upper solar atmosphere?
• RQ5 Does the model support the existence of a cool COmosphere above the photosphere, and what are its average temperature and implications?

Key findings

• A weighted solar carbon abundance of log ε(C) = 8.39 ± 0.05 was derived from CO lines, in excellent agreement with 3D results from C i, [C i], C2, and CH lines.
• The 12C/13C isotopic ratio was measured as 86.8+3.9−3.7 (δ13C = 30+46−44), indicating a higher ratio than previously assumed.
• The 16O/18O isotopic ratio was determined to be 479+29−28 (δ18O = 41+67−59), suggesting a need to revise current solar system formation theories.
• The 3D model successfully reproduced weak CO line shapes without requiring micro- or macroturbulence, while 1D models failed to achieve consistent agreement across diagnostics.
• Discrepancies in strongest CO lines suggest that temperatures in the highest atmospheric layers are overestimated in the 3D model, implying a cool COmosphere with an average temperature below 4000 K.
• The model's shortcomings in high layers are expected, as it was not designed to accurately describe such regions, highlighting the need for improved modeling of the upper chromosphere.