[论文解读] Lithium-7 surface abundance in pre-MS stars. Testing theory against clusters and binary systems

E. Tognelli, S. Degl’Innocenti+1 more|arXiv (Cornell University)|Oct 15, 2012

Stellar, planetary, and galactic studies参考文献 73被引用 35

一句话总结

本研究利用更新的恒星演化模型，调查了预主序（pre-MS）恒星中锂-7表面丰度，以检验理论不确定性。通过在预主序和主序阶段改变混合长度参数（α），发现预主序阶段的对流效率较低（α_PMS = 1.0）能显著改善模型与年轻星团及双星系统观测结果的一致性，表明预主序阶段的对流效率低于主序阶段。

ABSTRACT

The disagreement between theoretical predictions and observations for surface lithium abundance in stars is a long-standing problem, which indicates that the adopted physical treatment is still lacking in some points. However, thanks to the recent improvements in both models and observations, it is interesting to analyse the situation to evaluate present uncertainties. We present a consistent and quantitative analysis of the theoretical uncertainties affecting surface lithium abundance in the current generation of models. By means of an up-to-date and well tested evolutionary code, FRANEC, theoretical errors on surface 7Li abundance predictions, during the pre-main sequence (pre-MS) and main sequence (MS) phases, are discussed in detail. Then, the predicted surface 7Li abundance was tested against observational data for five open clusters, namely Ic 2602, αPer, Blanco1, Pleiades, and Ngc 2516, and for four detached double-lined eclipsing binary systems. Stellar models for the aforementioned clusters were computed by adopting suitable chemical composition, age, and mixing length parameter for MS stars determined from the analysis of the colour-magnitude diagram of each cluster. We restricted our analysis to young clusters, to avoid additional uncertainty sources such as diffusion and/or radiative levitation efficiency. We confirm the disagreement, within present uncertainties, between theoretical predictions and 7Li observations for standard models. However, we notice that a satisfactory agreement with observations for 7Li abundance in both young open clusters and binary systems can be achieved if a lower convection efficiency is adopted during the pre-MS phase with respect to the MS one.

研究动机与目标

• 评估影响预主序星与主序星表面7Li丰度预测的理论不确定性。
• 将标准恒星模型与年轻开放星团及非食双星系统的观测数据进行对比。
• 探究在演化阶段间采用可变的混合长度参数（α）是否能解决长期存在的锂损耗差异问题。
• 评估预主序与主序阶段对流效率差异对7Li表面丰度的影响。
• 通过分析年龄、组成和质量均较确定的系统，改进模型约束。

提出的方法

• 采用更新的物理输入（消光系数、反应速率、状态方程）的FRANEC演化代码计算恒星模型。
• 通过匹配五个年轻开放星团（Ic 2602、α Per、Blanco1、Pleiades、Ngc 2516）的观测颜色-星等图，标定主序阶段的混合长度参数（α_MS）。
• 探索预主序阶段其他α_PMS取值（如α_PMS = 1.0），以检验预主序演化期间7Li表面丰度预测对参数的敏感性。
• 将模型预测与四个非食双线双星系统（ASAS J052821+0338.5、EK Cep、RXJ 0529.4+0041 A、V1174 Ori）中观测到的7Li丰度进行比较。
• 通过评估初始组成、消光系数、反应速率和状态方程的不确定性，量化理论误差范围。
• 利用赫罗图与等龄线拟合，验证预主序恒星的模型轨迹与观测数据的一致性。

实验结果

研究问题

• RQ1 在预主序阶段采用较低的混合长度参数是否能改善理论7Li表面丰度与观测结果的一致性？
• RQ2 物理输入（消光系数、反应速率、状态方程）的不确定性在多大程度上影响7Li丰度的理论预测？
• RQ3 年轻开放星团与预主序双星系统中观测到的7Li丰度在多大程度上约束了α_PMS的取值？
• RQ4 理论与观测之间7Li丰度差异的主要原因是否源于预主序模型中对流效率假设不准确？
• RQ5 通过采用可变的对流效率（α_PMS ≠ α_MS）是否能解释锂的“ dip”现象与预主序阶段的损耗模式，而无需引入非标准物理机制？

主要发现

• 采用恒定混合长度参数（α_PMS = α_MS ≈ 1.68）的标准模型无法再现年轻预主序星的观测7Li丰度。
• 在预主序阶段采用较低的对流效率（α_PMS = 1.0）显著改善了所有五个研究的年轻开放星团中7Li观测结果的一致性。
• α_PMS = 1.0的模型能准确再现ASAS J052821+0338.5与RXJ 0529.4+0041 A双星系统中预主序恒星的7Li表面丰度，尤其是在Hayashi轨迹附近。
• 对于EK Cep的次星成分，低对流模型（α_PMS = 1.0）与观测完全兼容，尽管由于误差范围较大，仍存在不确定性。
• 由于锂丰度的观测不确定性可达0.5 dex，无法对RXJ 0529.4+0041 A得出明确结论，该系统仅能提供下限值。
• 结果表明，预主序阶段的对流效率可能低于主序阶段，这挑战了在演化各阶段采用恒定α的假设。