[论文解读] Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. III. Line formation in the atmospheres of giants located close to the base of RGB

V. Dobrovolskas, Kučinskas, A.+6 more|arXiv (Cornell University)|Oct 29, 2013

Stellar, planetary, and galactic studies参考文献 22被引用 26

一句话总结

本研究利用3D流体动力学CO5BOLD模型和1D LHD模型大气，探究了在四颗接近红巨星支底端的红巨星（Teff ≈ 5000 K，log g = 2.5，[M/H] = 0.0，-1.0，-2.0，-3.0）中，对流如何影响谱线形成。研究发现，由于水平温度涨落，低电离势和低至中等激发势的中性原子的丰度修正在[M/H] = -3.0时可达到-0.8 dex；而分子线（如CO）的修正可达-1.5 dex，凸显了在金属贫乏恒星丰度研究中使用3D模型的迫切需求。

ABSTRACT

We utilize state-of-the-art 3D hydrodynamical and classical 1D stellar model atmospheres to study the influence of convection on the formation properties of various atomic and molecular spectral lines in the atmospheres of four red giant stars, located close to the base of the red giant branch, RGB ($T_{\mathrm eff}\approx5000$ K, $\log g=2.5$), and characterized by four different metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0. The role of convection in the spectral line formation is assessed with the aid of abundance corrections, i.e., the differences in abundances predicted for a given equivalent width of a particular spectral line with the 3D and 1D model atmospheres. We find that for lines of certain neutral atoms the abundance corrections strongly depend both on metallicity of a given model atmosphere and the line excitation potential. While abundance corrections for all lines of both neutral and ionized elements tend to be small at solar metallicity, for lines of neutral elements with low ionization potential and low-to-intermediate $χ$ they quickly increase with decreasing metallicity, reaching in their extremes to -0.6...-0.8 dex. In all such cases the large abundance corrections are due to horizontal temperature fluctuations in the 3D hydrodynamical models. Abundance corrections of molecular lines are very sensitive to metallicity of the underlying model atmosphere and may be larger (in absolute value) than -0.5 dex at [M/H] = -3.0 (-1.5 dex in the case of CO). We also find that an approximate treatment of scattering in the 3D model calculations leads to the abundance corrections that are altered by less than ~0.1 dex, both for atomic and molecular (CO) lines, with respect to the model where scattering is treated as true absorption throughout the entire atmosphere, with the largest differences for the resonance and low-excitation lines.

研究动机与目标

• 评估3D流体动力学对接近红巨星支底端的红巨星谱线形成的影响。
• 量化由对流引起的温度涨落以及3D与1D模型之间大气结构差异对原子和分子谱线形成的影响。
• 评估丰度修正对金属量、激发势和电离势的敏感性，覆盖一系列恒星参数。
• 考察散射处理方式和混合长度参数（αMLT）对3D模型中丰度修正的影响。
• 确定丰度修正在400–1600 nm波长范围内是否显著依赖于波长。

提出的方法

• 使用CO5BOLD代码计算了3D流体动力学模型大气，模拟了恒星大气中随时间演化的对流。
• 使用LHD代码生成了1D模型大气，输入参数（Teff，log g，[M/H]，成分，状态方程，消光截面）与3D模型完全一致，以便直接比较。
• 丰度修正计算为同一谱线等效宽度下，3D与1D模型所推导出的丰度之差。
• 在两种模型中使用相同的谱线列表和消光截面处理方法求解辐射转移，包括将散射视为真实吸收与在光学薄区域忽略散射的对比。
• 测试了在1D模型中改变混合长度参数（αMLT）的影响，以评估其对丰度修正的影响。
• 在400–1600 nm波长范围内分析谱线形成，以评估修正的波长依赖性。

实验结果

研究问题

• RQ1 3D流体动力学效应（特别是水平温度涨落）如何改变接近红巨星支底端的红巨星谱线形成？
• RQ2 与1D模型相比，3D模型中原子和分子谱线的丰度修正在多大程度上依赖于金属量和激发势？
• RQ3 在外层大气中忽略散射处理对3D模型大气中结果丰度修正的影响有多大？
• RQ4 1D模型中混合长度参数（αMLT）的选择对丰度修正的敏感性如何，特别是对高激发线而言？
• RQ5 在这些模型大气中，丰度修正在400–1600 nm波长范围内是否表现出显著的波长依赖性？

主要发现

• 对于电离势较低的中性原子（如Mg i，Ti i，Fe i，Ni i），在[M/H] = -3.0和低至中等激发势（χ）条件下，丰度修正可达-0.8 dex，主要归因于3D模型中的水平温度涨落。
• 对于电离态元素以及电离势较高（Eion ≥ 10 eV）的中性原子，所有金属量和激发势下，丰度修正均保持较小（≤ ±0.1 dex）。
• 分子线（尤其是CO）在[M/H] = -3.0时表现出高达-1.5 dex的丰度修正，表明3D模型中对金属量高度敏感。
• 丰度修正在400–1600 nm波长范围内变化极小，表明对流效应在整个宽光谱窗口内均具有一致重要性。
• 在外层大气中忽略散射对丰度修正的影响小于0.1 dex，最大影响出现在中性原子和CO的共振线及低激发线。
• 对混合长度参数（αMLT）的敏感性对弱线和低激发线（χ < 6 eV）较小，但对高激发线（如χ = 10 eV的Fe ii）可达到最高0.1 dex，尤其在低金属量时更为显著。