[论文解读] The photospheric solar oxygen project: III. Investigation of the centre-to-limb variation of the 630nm [OI]-NiI blend

E. Caffau, H.‐G. Ludwig+6 more|arXiv (Cornell University)|Jun 2, 2015

Solar and Space Plasma Dynamics参考文献 38被引用 14

一句话总结

本研究利用三维流体动力学模型，通过分析太阳光谱中630 nm [O I]-Ni I混合线的中心到边缘变化，分离氧与镍的贡献。结果表明，当镍的丰度较低（A(Ni) ≈ 6.1）时，不同边缘角下的氧丰度（A(O) = 8.73 ± 0.05）保持一致，支持太阳氧丰度偏低的结论，尽管与636 nm线仍存在0.06 dex的持续差异。

ABSTRACT

The solar photospheric abundance of oxygen is still a matter of debate. For about ten years some determinations have favoured a low oxygen abundance which is at variance with the value inferred by helioseismology. Among the oxygen abundance indicators, the forbidden line at 630nm has often been considered the most reliable even though it is blended with a NiI line. In Papers I and Paper II of this series we reported a discrepancy in the oxygen abundance derived from the 630nm and the subordinate [OI] line at 636nm in dwarf stars, including the Sun. Here we analyse several, in part new, solar observations of the the centre-to-limb variation of the spectral region including the blend at 630nm in order to separate the individual contributions of oxygen and nickel. We analyse intensity spectra observed at different limb angles in comparison with line formation computations performed on a CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. The oxygen abundances obtained from the forbidden line at different limb angles are inconsistent if the commonly adopted nickel abundance of 6.25 is assumed in our local thermodynamic equilibrium computations. With a slightly lower nickel abundance, A(Ni)~6.1, we obtain consistent fits indicating an oxygen abundance of A(O)=8.73+/-0.05. At this value the discrepancy with the subordinate oxygen line remains. The derived value of the oxygen abundance supports the notion of a rather low oxygen abundance in the solar hotosphere. However, it is disconcerting that the forbidden oxygen lines at 630 and 636nm give noticeably different results, and that the nickel abundance derived here from the 630nm blend is lower than expected from other nickel lines.

研究动机与目标

• 通过分析630 nm混合线的中心到边缘变化，解决由630 nm [O I]和636 nm [O I]线推导出的太阳氧丰度之间的差异。
• 利用高分辨率太阳观测，确定630 nm谱线混合中氧和镍的独立贡献。
• 通过测试不同边缘角下的一致性，评估630 nm禁戒线作为太阳氧丰度示踪指标的可靠性。
• 探究630 nm与636 nm线之间氧丰度不一致是否源于镍丰度假设错误或模型局限性。
• 评估3D CO5BOLD太阳大气模型在再现太阳圆面各处观测轮廓方面的表现。

提出的方法

• 使用VTT、SST、THEMIS和Hinode等仪器，在多个边缘角（μ）获取高分辨率太阳强度光谱。
• 将观测到的谱线轮廓与基于3D CO5BOLD流体动力学太阳大气模型计算的合成轮廓进行比较。
• 采用局部热动平衡（LTE）辐射转移计算，通过调整镍丰度（A(Ni)）来拟合观测到的混合线。
• 应用蒙特卡洛模拟，估算由630 nm线推导出的氧丰度的不确定性。
• 调整Ni I线的振子强度（log gf），使其与观测到的等效宽度和轮廓形状相匹配。
• 评估合成线加宽效应，特别是高分辨率VTT数据中的效应，以评估模型速度场的准确性。

实验结果

研究问题

• RQ1 630 nm [O I]-Ni I混合线的中心到边缘变化是否足以可靠地分离氧与镍的贡献？
• RQ2 为实现不同边缘角下氧丰度测量的一致性，需要多高的镍丰度？
• RQ3 尽管两者均为禁戒跃迁，为何由630 nm线推导出的氧丰度与636 nm线不同？
• RQ4 3D流体动力学模型在多大程度上能再现观测到的谱线轮廓，尤其是在高光谱分辨率下？
• RQ5 为何合成轮廓的展宽现象比高分辨率观测更明显，尤其是在边缘区域？

主要发现

• 只有当假设镍丰度较低（A(Ni) ≈ 6.1）而非标准值6.25时，由630 nm [O I]线推导出的氧丰度在不同边缘角下才保持一致。
• 推导出的氧丰度为A(O) = 8.73 ± 0.05，支持光球层中太阳氧丰度偏低的结论。
• 3D模型生成的合成谱线轮廓系统性地比观测值更宽，尤其是在高分辨率VTT数据中，表明模型可能高估了速度场。
• 在低边缘角（μ < 0.2）时，模型对观测光谱的拟合效果较差，可能是因为忽略了太阳的球面形状和色球层效应。
• 在修正镍丰度后，由630 nm和636 nm线推导出的氧丰度差异减小至0.06 dex，但仍未得到解释。
• Ni I线对630 nm混合线的贡献最佳拟合对应的log gf值为4.00，表明其实际丰度低于先前假设的水平。