[论文解读] Barium abundance in red giants of NGC 6752. Non-local thermodynamic equilibrium and three-dimensional effects

V. Dobrovolskas, A. Kučinskas+6 more|arXiv (Cornell University)|Mar 14, 2012

Stellar, planetary, and galactic studies参考文献 44被引用 10

一句话总结

本研究调查了非局部热动平衡（NLTE）和三维（3D）大气效应在球状星团NGC 6752中20颗红巨星的钡（Ba II）谱线形成中的影响。使用一维局部热动平衡（1D LTE）、一维非局部热动平衡（1D NLTE）和三维局部热动平衡（3D LTE）模型大气，研究发现NLTE效应显著降低了[Ba/Fe]的平均值，从1D LTE的0.24 dex降至1D NLTE的0.05 dex；而3D修正值较小（+0.05 dex），由于相反贡献的抵消，最终得到的3D+NLTE [Ba/Fe]为0.10 dex，星族间无内在丰度弥散。

ABSTRACT

(Abridged) Aims: We study the effects related to departures from non-local thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red giant stars in Galactic globular cluster NGC 6752, to assess their influence on the formation of Ba II lines. Methods: One-dimensional (1D) local thermodynamic equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D ATLAS stellar model atmospheres. The three-dimensional (3D) LTE abundances were obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE abundances using 3D-1D abundance corrections, i.e., the differences between the abundances obtained from the same spectral line using the 3D hydrodynamical (CO5BOLD) and classical 1D (LHD) stellar model atmospheres. Results: The mean 1D barium-to-iron abundance ratios derived for 20 giants are _{1D NLTE} = 0.05 \pm0.06 (stat.) \pm0.08 (sys.). The 3D-1D abundance correction obtained for 8 giants is small (~+0.05 dex), thus leads to only minor adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for the 20 giants, _{3D+NLTE} = 0.10 \pm0.06(stat.) \pm0.10(sys.). The intrinsic abundance spread between the individual cluster stars is small and can be explained in terms of uncertainties in the abundance determinations. Conclusions: Deviations from LTE play an important role in the formation of barium lines in the atmospheres of red giants studied here. The role of 3D hydrodynamical effects should not be dismissed either, even if the obtained 3D-1D abundance corrections are small. This result is a consequence of subtle fine-tuning of individual contributions from horizontal temperature fluctuations and differences between the average temperature profiles in the 3D and 1D model atmospheres: owing to the comparable size and opposite sign, their contributions nearly cancel each other.

研究动机与目标

• 评估非局部热动平衡（NLTE）和三维（3D）大气效应对NGC 6752红巨星钡丰度测定的影响。
• 确定该星团红巨星分支恒星中是否存在内在的星族间钡丰度差异。
• 量化Ba II谱线的3D–1D丰度修正值，并评估其在不同大气模型背景下的重要性。
• 使用一致的恒星参数和原子数据，比较1D LTE、1D NLTE和3D LTE的钡丰度。
• 检查3D–1D修正值对1D模型中微湍流速度的敏感性。

提出的方法

• 利用NGC 6752中20颗红巨星的经典ATLAS9模型大气，推导1D LTE和1D NLTE钡丰度。
• 通过应用从流体动力学CO 5 BOLD和1D LHD模型大气导出的3D–1D丰度修正值，获得8颗低红巨星分支恒星的3D LTE丰度。
• 在3D和1D模型中使用相同的恒星参数、消光系数和状态方程，以严格差分方式计算3D–1D修正值。
• 使用同一观测数据集中的高分辨率光谱数据，确保所有丰度测定的一致性。
• 通过改变1D模型中的微湍流速度进行敏感性测试，评估其对3D–1D修正值的影响。
• 将1D NLTE和3D–1D修正值结合，推导最终的3D+NLTE [Ba/Fe]丰度，同时考虑统计和系统误差。

实验结果

研究问题

• RQ1 NLTE效应对NGC 6752红巨星中推导的钡对铁丰度比有何影响？
• RQ2 Ba II谱线在这些恒星中的3D–1D丰度修正值的大小和符号是什么？
• RQ3 3D效应是否由于温度非均匀性和平均温度分布差异的相反贡献而相互抵消？
• RQ4 NGC 6752红巨星中是否存在内在的星族间钡丰度弥散？
• RQ5 3D–1D修正值对1D模型中微湍流速度选择的敏感性如何？

主要发现

• 20颗红巨星的平均1D LTE [Ba/Fe]丰度比为0.24 ± 0.05（统计）± 0.08（系统）dex。
• 应用NLTE修正后，平均1D NLTE [Ba/Fe]比值降至0.05 ± 0.06（统计）± 0.08（系统）dex。
• 8颗低红巨星分支恒星的3D–1D丰度修正值较小，平均为+0.05 dex，个体修正值范围在–0.03至+0.02 dex之间。
• 最终的3D+NLTE [Ba/Fe]丰度比为0.10 ± 0.08（统计）± 0.10（系统）dex，仅略高于1D NLTE值。
• 较小的净3D修正值源于相反贡献的近似抵消：Δ₃D–⟨₃D⟩ ≈ –0.1 dex 和 Δ⟨₃D⟩–₁D ≈ +0.1 dex，具体取决于谱线强度。
• 在20颗红巨星中未检测到[Ba/Fe]的内在丰度弥散，其变化与测量不确定性一致。