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arXiv 1203.3124astro-ph.SR
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NGC 6752红巨星的钡丰度非局部热力学平衡与三维效应

Barium abundance in red giants of NGC 6752. Non-local thermodynamic equilibrium and three-dimensional effects

V. Dobrovolskas, A. Kučinskas, S. M. Andrievsky, S. A. Korotin, T. V. Mishenina, P. Bonifacio, H. -G. Ludwig, E. Caffau

备注：
Minor typos corrected. Accepted for publication in A&A (9 pages, 3 figures, 6 tables)
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摘要：

（节略）目的：我们研究了银河系球状星团NGC 6752中红巨星大气的非定域热力学平衡（NLTE）偏离和均匀性对Ba Ⅱ谱线形成的影响。研究方法：一维（1D）的本地热力学平衡（LTE）和一维NLTE钡丰度推导使用经典的一维ATLAS恒星模型大气。通过使用3D-1D丰度校正调整它们的1D LTE丰度，获得了较低RGB上的8颗红巨星的三维（3D）LTE丰度，即，利用三维流体动力学（CO 5 BOLD）和经典一维（LHD）恒星模型大气从同一条谱线获得的丰度之间的差异。结果：20颗巨星的平均1D钡铁丰度比为<[Ba/Fe]>{1DNLTE} = 0.05 \pm 0.06（stat.）\pm 0.08（系统）。8颗巨星的3D-1D丰度修正值很小（~+0.05 dex），因此对20颗巨星的平均1DNLTE钡铁丰度比只作了很小的修正，<[Ba/Fe]>_{3D+NLTE} = 0.10 \pm 0.06（stat.）\pm 0.10（系统）。单个星团恒星之间的固有丰度分布很小，可以用丰度测定的不确定性来解释。结论：偏离LTE在这里研究的红巨星大气中钡线的形成中起着重要的作用。三维流体动力学效应的作用也不应该被忽视，即使获得的3D-1D丰度校正很小。这一结果是对3D和1D模型大气中水平温度波动和平均温度分布之间差异的个体贡献进行细微微调的结果：由于大小相似且符号相反，它们的贡献几乎相互抵消。

英文摘要：

(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 <[Ba/Fe]>_{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, <[Ba/Fe]>_{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.

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