[论文解读] Lithium and sodium in the globular cluster M4. A Main Sequence star with Li compatible with the cosmological value: nature or nurture?

L. Monaco, S. Villanova+6 more|arXiv (Cornell University)|Jul 31, 2011

Stellar, planetary, and galactic studies被引用 2

一句话总结

本研究利用VLT/Giraffe光谱仪对球状星团M4中的91颗主序星和亚巨星的锂和钠丰度进行了研究。研究发现锂-钠反相关关系较浅，并识别出一颗恒星（编号37934）的锂丰度（A(Li) = 2.87）与宇宙学原初值一致，表明其可能为原初锂的保存，或由前代恒星产生。

ABSTRACT

Context. The abundance inhomogeneities of light elements observed in Globular Clusters (GCs), and notably the ubiquitous Na-O anti-correlation, are generally interpreted as evidence that GCs comprise several generations of stars. There is an on-going debate as to the nature of the stars which produce the inhomogeneous elements, and investigating the behavior of several elements is a way to shed new light on this problem. Aims. We aim at investigating the Li-Na anti-correlation in the GC M 4, which is known to have a well defined Na-O anti-correlation. Methods. We obtained moderate resolution (R=17 000-18 700) spectra for 91 main sequence (MS)/sub-giant branch stars of M 4 with the Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres analysis we measured lithium and sodium abundances. Results. We detect a well defined Li-Na anti-correlation among un-evolved MS stars, albeit with a shallow slope d(A(Li))/d(A(Na))' −0.2. One star in the sample, # 37934, shows the remarkably high lithium abundance A(Li)=2.87, in good agreement with current estimates of the primordial lithium abundance, and much higher than the Spite plateau. Conclusions. The shallow slope found for the Li-Na anti-correlation suggests that lithium is produced in parallel to sodium. The high lithium abundance of star # 37934 can be interpreted as preservation of the primordial lithium abundance, or as lithium produced by a previous generation of stars. The fact that the star is also sodium-rich favors the latter hypothesis, yet the former cannot be easily

研究动机与目标

• 研究已知具有强烈Na-O反相关的球状星团M4中的Li-Na丰度相关性。
• 确定M4中锂的不均匀性是否与钠和氧不均匀性产生的过程相关。
• 评估样本中任意恒星的锂丰度是否与宇宙学原初锂值一致。
• 区分原初锂保存与原位锂生成作为极端锂丰度解释的可能性。

提出的方法

• 利用VLT/FLAMES设施的Giraffe光谱仪获取M4中91颗主序星和亚巨星的中等分辨率（R ≈ 17,000–18,700）光谱。
• 通过模型大气分析从观测光谱中推导出锂和钠的丰度。
• 比较测量得到的锂和钠丰度，以评估Li-Na反相关性的存在及其斜率。
• 将单颗恒星的锂丰度与宇宙学原初锂值（A(Li) ≈ 2.73）及Spite平台（A(Li) ≈ 2.2）进行对比。
• 利用丰度比和恒星运动学推断其演化状态和污染历史。

实验结果

研究问题

• RQ1 M4主序星中锂与钠丰度之间是否存在相关性？
• RQ2 Li-Na反相关关系的斜率是否表明锂与钠在星团自污染过程中共同生成？
• RQ3 样本中是否存在某颗恒星的锂丰度与宇宙学原初值一致？
• RQ4 恒星#37934的高锂丰度最合理的解释是原初锂的保存，还是前代恒星原位生成？
• RQ5 恒星#37934的富钠特性如何影响其锂丰度的解释？

主要发现

• 在M4主序星中检测到较浅的Li-Na反相关关系，斜率为d(A(Li))/d(A(Na)) ≈ −0.2。
• 恒星#37934的锂丰度为A(Li) = 2.87，与当前宇宙学原初锂丰度估计值高度一致。
• 该锂丰度显著高于Spite平台的A(Li) ≈ 2.2，表明其起源非标准。
• 恒星#37934的高锂丰度与原初锂保存或前代恒星生成均一致。
• 恒星#37934同时为富钠星，支持原位锂生成的假设，但原初保存仍是合理替代解释。
• Li-Na反相关关系的浅斜率表明，锂的生成可能与钠的生成并行发生，而非通过独立机制。