[论文解读] Observational constraints on the origin of the elements. I. 3D NLTE formation of Mn lines in late-type stars

M. Bergemann, A. J. Gallagher+10 more|arXiv (Cornell University)|May 13, 2019

Stellar, planetary, and galactic studies参考文献 78被引用 25

一句话总结

本研究为晚型星中的锰（Mn）谱线开发了一套3D非局部热动平衡（NLTE）辐射转移模型，结合了改进的原子数据和流体动力学恒星大气模型。结果表明，1D LTE丰度存在显著偏差，3D NLTE修正将太阳Mn丰度提高至5.52±0.03 dex——与陨石值一致，并解决了金属贫乏恒星（如HD 122563）中电离/激发态不平衡的问题。

ABSTRACT

Manganese (Mn) is a key Fe-group elements, commonly employed in stellar population and nucleosynthesis studies to explore the role of SN Ia. We have developed a new non-local thermodynamic equilibrium (NLTE) model of Mn, including new photo-ionisation cross-sections and new transition rates caused by collisions with H and H- atoms. We applied the model in combination with 1-dimensional (1D) LTE model atmospheres and 3D hydrodynamical simulations of stellar convection to quantify the impact of NLTE and convection on the line formation. We show that the effects of NLTE are present in Mn I and, to a lesser degree, in Mn II lines, and these increase with metallicity and with effective temperature of a model. Employing 3D NLTE radiative transfer, we derive new abundance of Mn in the Sun, A(Mn)=5.52 +/- 0.03 dex, consistent with the element abundance in C I meteorites. We also apply our methods to the analysis of three metal-poor benchmark stars. We find that 3D NLTE abundances are significantly higher than 1D LTE. For dwarfs, the differences between 1D NLTE and 3D NLTE abundances are typically within 0.15 dex, however, the effects are much larger in the atmospheres of giants owing to their more vigorous convection. We show that 3D NLTE successfully solves the ionisation and excitation balance for the RGB star HD 122563 that cannot be achieved by 1D LTE or 1D NLTE modelling. For HD 84937 and HD 140283, the ionisation balance is satisfied, however, the resonance Mn I triplet lines still show somewhat lower abundances compared to the high-excitation lines. Our results for the benchmark stars confirm that 1D LTE modelling leads to significant systematic biases in Mn abundances across the full wavelength range from the blue to the IR. We also produce a list of Mn lines that are not significantly biased by 3D and can be reliably, within the 0.1 dex uncertainty, modelled in 1D NLTE.

研究动机与目标

• 为解决在1D LTE模型中忽略非局部热动平衡（NLTE）效应和3D恒星对流所导致的锰（Mn）丰度测定系统性偏差。
• 通过开发结合更新原子数据（包括光致电离截面和与H及H⁻的碰撞速率）的新NLTE模型，提高对晚型星中Mn丰度测量的准确性。
• 量化NLTE与3D流体动力学对流对不同恒星类型和金属丰度下Mn谱线形成联合影响。
• 解决长期存在的金属贫乏基准恒星（特别是HD 122563）中Mn电离与激发平衡问题。
• 识别受3D和NLTE效应影响最小的Mn谱线，使1D NLTE分析可实现更低系统不确定性的可靠丰度测定。

提出的方法

• 开发了一套新的Mn 3D NLTE辐射转移模型，整合了详细的量子力学光致电离截面和与H及H⁻原子的碰撞速率。
• 将该模型应用于1D LTE和1D NLTE模型大气以及恒星对流的3D流体动力学模拟，以分离NLTE与3D动力学的影响。
• 对紫外、光学及近红外（H波段）波段范围内的Mn I和Mn II谱线进行全面的辐射转移计算。
• 利用3D NLTE模型为太阳及三颗金属贫乏基准恒星（HD 84937、HD 140283和HD 122563）推导新的Mn丰度。
• 对比1D LTE、1D NLTE、3D LTE和3D NLTE情景下的结果，量化丰度测定中的系统性偏差。
• 识别出Mn I的特定多重线（9、23、24、32）受3D和NLTE效应影响最小，偏差低于0.1 dex，适用于1D NLTE分析。

实验结果

研究问题

• RQ1 NLTE与3D对流效应如何影响晚型星中Mn I和Mn II谱线的形成？
• RQ2 1D LTE模型在多大程度上系统性地偏差Mn丰度测定结果，特别是在金属贫乏和巨星中？
• RQ3 3D NLTE建模能否解决在金属贫乏恒星（如HD 122563）中长期存在的电离与激发平衡问题？
• RQ4 哪些Mn谱线对3D和NLTE效应具有鲁棒性，可被可靠地用于1D NLTE丰度分析？
• RQ5 当充分考虑3D NLTE效应和更新原子数据时，太阳的真实Mn丰度是多少？

主要发现

• 新计算的3D NLTE太阳Mn丰度为5.52±0.03 dex，与CI陨石丰度5.50±0.03 dex高度一致。
• 1D LTE低估了太阳Mn丰度0.18 dex（5.34±0.04 dex），而1D NLTE低估0.11 dex（5.41±0.05 dex），凸显了采用3D NLTE的必要性。
• 对于金属贫乏巨星如HD 122563，3D NLTE丰度比1D LTE结果高出达1 dex，解决了以往无法解释的电离与激发不平衡问题。
• 在3D NLTE模型中，3488和3497 Å处的Mn II谱线显著强于1D模型，这是由于3D非均匀大气中线致散射增强所致，纠正了1D LTE中对这些谱线的高估。
• 1D LTE中红外H波段Mn I谱线存在0.2–0.35 dex的偏差，但1D NLTE与3D NLTE之间的差异仅为-0.15 dex，表明1D NLTE研究中可统一修正-0.15 dex。
• Mn I的多重线9、23、24和32受3D与NLTE效应影响最小，偏差低于0.1 dex，是进行可靠1D NLTE丰度分析的理想选择。