[论文解读] Thermohaline instability and rotation-induced mixing. I - Low- and intermediate-mass solar metallicity stars up to the end of the AGB

C. Charbonnel, N. Lagarde|arXiv (Cornell University)|Jun 28, 2010

Stellar, planetary, and galactic studies参考文献 90被引用 139

一句话总结

本研究探讨了低质量至中等质量（1–4 M⊙）太阳金属丰度恒星中由³He燃烧和自转引起的混合过程，结果表明热盐混合可解释演化巨星在红巨星分支凸起之后的Li、C/N和¹²C/¹³C趋势；而自转则增强了热盐混合的启动，并解释了恒星间丰度差异，特别是中等质量恒星中CN处理物质的丰度变化。

ABSTRACT

(abridged) Numerous spectroscopic observations provide compelling evidence for non-canonical processes that modify the surface abundances of low- and intermediate-mass stars beyond the predictions of standard stellar theory. We study the effects of thermohaline instability and rotation-induced mixing in the 1-4 Msun range at solar metallicity. We present evolutionary models by considering both thermohaline and rotation-induced mixing in stellar interior. We discuss the effects of these processes on the chemical properties of stars from the zero age main sequence up to the end of the second dredge-up on the early-AGB for intermediate-mass stars and up to the AGB tip for low-mass stars. Model predictions are compared to observational data for lithium,12C/13C,[N/C],[Na/Fe],16O/17O, and 16O/18O in Galactic open clusters and in field stars with well-defined evolutionary status,as well as in planetary nebulae. Thermohaline mixing simultaneously accounts for the observed behaviour of 12C/13C,[N/C], and lithium in low-mass stars that are more luminous than the RGB bump, and its efficiency is increasing with decreasing initial stellar mass. On the TP-AGB,thermohaline mixing leads to lithium production, although the 7Li yields remain negative. Although the 3He stellar yields are much reduced thanks to this process, we find that solar-metallicity, low-mass stars remain net 3He producers. Rotation-induced mixing is found to change the stellar structure so that in the mass range between \sim 1.5 and 2.2 Msun the thermohaline instability occurs earlier on the red giant branch than in non-rotating models. Finally rotation accounts for the observed star-to-star abundance variations at a given evolutionary status, and is necessary to explain the features of CN-processed material in intermediate-mass stars.

研究动机与目标

• 理解标准恒星理论之外影响低质量与中等质量恒星表面丰度的非经典混合过程。
• 研究由³He(³He,2p)⁴He反应触发的热盐不稳定性如何改变首次混合上翻后的表面成分。
• 评估自转诱导混合如何改变恒星结构并影响热盐混合的启动与效率。
• 将模型预测与开放星团、场星族巨星及行星状星云中Li、¹²C/¹³C、[N/C]、[Na/Fe]及氧同位素的观测数据相协调。
• 确定在联合混合过程下³He与⁷Li的净产量，并评估其对银河系核合成的影响。

提出的方法

• 针对太阳金属丰度下1–4 M⊙的恒星构建了包含热盐混合与自转诱导混合的恒星演化模型。
• 热盐扩散系数采用Ulrich（1972）的方案，该方案经实验室实验验证，并在低金属丰度恒星中已有成功应用。
• 自转诱导混合通过湍流扩散系数形式实现，可改变内部成分并延迟对流驱动混合的启动。
• 模型演化从零龄主序开始，直至第二次混合上翻（早期渐近巨支）阶段，部分案例进一步演化至AGB顶端。
• 将模型预测与红巨星、红巨星团星及行星状星云中表面丰度（Li、¹²C/¹³C、[N/C]、[Na/Fe]、¹⁶O/¹⁷O、¹⁶O/¹⁸O）的观测数据进行对比。
• 核反应速率取自NACRE，关键反应如¹⁴C(p,γ)¹⁵N、¹⁴C(p,n)¹⁴N、¹⁴C(p,α)¹¹B、¹⁴N(p,γ)¹⁵O等则来自专门文献。

实验结果

研究问题

• RQ1 热盐混合在多大程度上能解释质量较小的巨星在红巨星分支凸起之后¹²C/¹³C降低与[N/C]升高的观测现象？
• RQ2 自转诱导混合如何影响中等质量恒星（1.5–2.2 M⊙）中热盐混合的启动时间与效率？
• RQ3 在质量与演化阶段相近的恒星中，观测到的丰度离散性是否可由自转诱导混合解释？
• RQ4 当同时考虑热盐混合与自转诱导混合时，低质量恒星中³He与⁷Li的净产量是多少？
• RQ5 热盐混合与自转诱导混合如何影响演化恒星中氧同位素比值（¹⁶O/¹⁷O与¹⁶O/¹⁸O）？

主要发现

• 热盐混合可同时再现质量较小的巨星在红巨星分支凸起之后¹²C/¹³C降低、[N/C]升高及锂元素耗竭的观测特征。
• 热盐混合的效率随初始恒星质量减小而提高，在质量低于2.2 M⊙的恒星中效应最强。
• 在热脉动AGB阶段，热盐混合可导致锂的产生，但净⁷Li产量仍为负值，因此这些恒星不会使银河系中的锂丰度增加。
• 尽管热盐混合显著减少了³He含量，但太阳金属丰度下的低质量恒星在整个寿命中仍为净³He生产者。
• 自转诱导混合使质量在约1.5至2.2 M⊙之间的恒星在红巨星分支阶段提前启动热盐混合，相较于无自转模型。
• 自转是解释相同演化状态下表面丰度离散性的必要条件，也是再现中等质量恒星中CN处理物质的关键因素。