[论文解读] Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. IV. Oxygen diagnostics in extremely metal-poor red giants with infrared OH lines

V. Dobrovolskas, A. Kučinskas+5 more|arXiv (Cornell University)|Feb 23, 2015

Stellar, planetary, and galactic studies参考文献 40被引用 9

一句话总结

本研究证明，3D流体动力学CO5BOLD模型大气能显著改善对极端金属贫金属红巨星中氧丰度的测定，通过校正红外OH线中的对流效应。应用3D校正后，−0.2至−0.3 dex的校正值使基于OH线的氧丰度与[O I] 630 nm线结果高度一致，解决了长期存在的差异，并验证了在使用3D模型时，红外OH线可作为EMP恒星中可靠的氧丰度示踪指标。

ABSTRACT

Context. Although oxygen is an important tracer of Galactic chemical evolution, measurements of its abundance in the atmospheres of the oldest Galactic stars are still scarce and rather imprecise. At the lowest end of the metallicity scale, oxygen can only be measured in giant stars and in most of cases such measurements rely on a single forbidden [O I] 630 nm line that is very weak and frequently blended with telluric lines. Although molecular OH lines located in the ultraviolet and infrared could also be used for the diagnostics, oxygen abundances obtained from the OH lines and the [O I] 630 nm line are usually discrepant to a level of ~0.3-0.4 dex. Aims. We study the influence of convection on the formation of the infrared (IR) OH lines and the forbidden [O I] 630 nm line in the atmospheres of extremely metal-poor (EMP) red giant stars. Methods. We used high-resolution and high signal-to-noise ratio spectra of four EMP red giant stars obtained with the VLT CRIRES spectrograph. For each EMP star, 4-14 IR OH vibrational-rotational lines located in the spectral range of 1514-1548 and 1595-1632 nm were used to determine oxygen abundances by employing standard 1D LTE abundance analysis methodology. We then corrected the 1D LTE abundances obtained from each individual OH line for the 3D hydrodynamical effects. Results. We find that the influence of convection on the formation of [O I] 630 nm line in the atmospheres of EMP giants studied here is minor, which leads to very small 3D-1D abundance corrections (< -0.01 dex). On the contrary, IR OH lines are strongly affected by convection and thus the abundance corrections for these lines are significant, 3D-1D ~ -0.2 ... -0.3 dex. These abundance corrections do indeed bring the 1D LTE oxygen abundances of EMP red giants obtained using IR OH lines into better agreement with those determined from the [O I] 630 nm line.

研究动机与目标

• 解决在极端金属贫金属（EMP）红巨星中，由红外OH线与[O I] 630 nm线得出的氧丰度之间长期存在的差异。
• 评估3D对流效应对EMP星大气中红外OH线和[O I] 630 nm线形成的影响。
• 确定3D流体动力学模型大气是否能够调和EMP巨星中不同光谱示踪指标得出的氧丰度。
• 评估在考虑3D效应校正后，红外OH线是否可作为EMP恒星中氧丰度的可靠示踪指标。

提出的方法

• 利用VLT CRIRES光谱仪获取了四颗EMP红巨星的高分辨率（R = 50,000）和高信噪比（S/N ≈ 200–600）光谱。
• 对1514–1548 nm和1595–1632 nm波段内的4–14条红外OH振动-转动谱线应用1D局部热动平衡（LTE）丰度分析，以获得初始氧丰度。
• 利用CO5BOLD 3D流体动力学模型和LHD 1D静态模型大气，计算3D–1D LTE丰度校正值，以考虑对流效应的影响。
• 将校正值应用于基于红外OH线的1D LTE氧丰度，以评估其对与[O I] 630 nm线结果一致性的影响。
• 通过比较不同示踪指标的最终氧丰度，并利用统计离散度估算不确定性。

实验结果

研究问题

• RQ1 3D流体动力学效应如何影响极端金属贫金属红巨星大气中红外OH线的形成？
• RQ2 与1D LTE分析相比，3D效应在多大程度上改变了由红外OH线得出的氧丰度？
• RQ3 经3D校正后，基于红外OH线的氧丰度是否与EMP巨星中[O I] 630 nm线得出的结果一致？
• RQ4 当使用3D模型大气时，红外OH线能否作为EMP恒星中氧丰度的可靠示踪指标？

主要发现

• [O I] 630 nm线的3D–1D丰度校正可忽略不计，Δ3D–1D ≤ −0.01 dex，表明对流对该线的影响极小。
• 相比之下，红外OH线受对流影响显著，导致−0.2至−0.3 dex的显著3D–1D丰度校正。
• 应用3D校正后，红外OH线的平均1D LTE氧丰度（⟨A(O)⟩ = 6.61 ± 0.21）与[O I] 630 nm线的结果（⟨A(O)⟩ = 6.52 ± 0.33）高度一致。
• 四颗恒星的3D校正氧丰度得出[O/Fe] = 0.7，对应[Fe/H] = −3.0，与理论模型和先前研究一致。
• 红外OH线显著强于[O I] 630 nm线，且由于存在多条未重叠的谱线，具有更高的统计显著性。
• 本研究证实，3D流体动力学模型大气对于从EMP巨星的红外OH线中准确测定氧丰度至关重要。