Li-rich and super Li-rich giants produced by element diffusion
Li-rich and super Li-rich giants produced by element diffusion
复制标题
元素扩散产生的富锂和超富锂巨星
DOI：
10.1051/0004-6361/202243871
复制
发表时间：
2022-10
期刊：
影响因子：
--
通讯作者：
Guoliang Lü
中科院分区：
文献类型：
--
作者：
Jun Gao;Chunhua Zhu;Jinlong Yu;Helei Liu;Xizhen Lu;Jianrong Shi;Guoliang Lü
背景。约0.2% - 2%的巨星富含锂，也就是说它们的锂丰度（A(Li)）高于1.5 dex。在这些富锂巨星中，近6%是超富锂的，其A(Li)超过3.2 dex。与此同时，这些富锂和超富锂巨星的形成机制仍存在争议。
目的。考虑到红巨星致密的氦核，关注元素扩散对A(Li)的影响。特别是当氦核闪发生时，元素扩散使热盐混合区向内延伸并与恒星内部对流区相连。这样，氦闪产生的大量7Be就能被转移到恒星表面，最终转变为7Li。因此，这项工作的目标是提出A(Li)富集的机制，并使理论数据与观测数据达成一致。
方法。使用恒星天体物理学实验模块（MESA）恒星演化代码，我们模拟了低质量恒星的演化，同时考虑元素扩散对锂丰度的影响。利用星族合成方法估算富锂巨星与正常巨星的时间尺度比。然后我们得到A(Li)的理论值，并与观测结果进行比较。
结果。模型中考虑元素扩散的影响导致锂丰度增加至约1.8 dex，这可以解释富锂巨星的情况。同时，在模型中引入数值在10¹¹ 至10¹⁵ cm² s⁻¹ 之间的高恒定扩散混合系数（Dmix），能使A(Li)从2.4 dex增加到4.5 dex，这可以解释大多数富锂和超富锂巨星的情况。星族合成方法表明，富锂巨星的数量约占所有巨星的0.2% - 2%，这与观测估计水平相符。
结论。在我们的模型中，主要由引力场引发的元素扩散改变了恒星包层与氦核交界区域的平均分子量，这使得热盐混合区域扩展到恒星内部对流区。由此形成了一个传输通道，能有效地将恒星氢燃烧区域的7Be传输到对流包层，在那里7Be衰变为7Li。结合高恒定扩散混合系数，这个传输通道可以解释富锂和超富锂巨星，甚至是极超富锂巨星的成因。
Context. About 0.2−2% of giant stars are Li rich, that is to say their lithium abundance (A(Li)) is higher than 1.5 dex. Nearly 6% of these Li-rich giant stars are super Li rich, with an A(Li) exceeding 3.2 dex. Meanwhile, the formation mechanism of these Li-rich and super Li-rich giants is still under debate..Aims. Considering the compact He core of red giants, attention is paid to the effect of element diffusion on A(Li). In particular, when the He-core flash occurs, element diffusion makes the thermohaline mixing zone extend inward and connect to the inner convection region of stars. Then, a large amount of 7Be produced by the He flash can be transferred to the stellar surface, finally turning into 7Li. Thus, the goal of this work is to propose the mechanism of A(Li) enrichment and achieve consistency between the theoretical and observation data..Methods. Using the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code, we simulated the evolution of low-mass stars, considering the effects of element diffusion on the Li abundances. The timescale ratio of Li-rich giants to normal giants was estimated using the population synthesis method. Then we obtained the theoretical value of A(Li) and made a comparison with observations..Results. Considering the influence of element diffusion in the model results in the increase of the lithium abundance up to about 1.8 dex, which can reveal Li-rich giants. Simultaneously, introducing high constant diffusive mixing coefficients (Dmix) with the values from 1011 to 1015 cm2 s−1 in the model allows the A(Li) to increase from 2.4 to 4.5 dex, which can explain most of the Li-rich and super Li-rich giant stars. The population synthesis method reveals that the amount of Li-rich giants is about 0.2−2% of all giants, which is consistent with observation estimated levels..Conclusions. In our models the element diffusion, mainly triggered by the gravity field, changes the mean molecular weight at the junction zone between the stellar envelope and the He core, which makes the thermohaline mixing region expand to the inner convection region of stars. A transport channel, efficiently transporting 7Be in the hydrogen-burning region of the star to the convective envelope where 7Be decays into 7Li, is formed. Combining high constant diffusive mixing coefficients, the transport channel can explain the origin of Li-rich and super Li-rich giants, even the most super Li-rich giants.
DOI：
--
发表时间：
2009-07
期刊：
--
影响因子：
--
作者：
H. Maehara;T. Soyano;B. Skiff
通讯作者：
H. Maehara;T. Soyano;B. Skiff