IS EXTRA MIXING REALLY NEEDED IN ASYMPTOTIC GIANT BRANCH STARS?
IS EXTRA MIXING REALLY NEEDED IN ASYMPTOTIC GIANT BRANCH STARS?
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渐近巨星分支星真的需要额外的混合吗？
DOI：
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发表时间：
2010
期刊：
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通讯作者：
R. Stancliffe
中科院分区：
文献类型：
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作者：
A. Karakas;S. Campbell;R. Stancliffe
我们证明了在第一巨支（FGB）恒星中观测到的低C/N和12C/13C比所需的额外混合量也足以解释银河系渐近/第二巨支（AGB）恒星的碳和氮丰度。我们模拟了额外的混合对FGB的影响，将包络层的组成设置为低质量（M≤2 M☉）FGB恒星的组成，然后将模型演化到AGB的尖端。在我们的AGB模型中，包含FGB的额外混合成分变化对C和N丰度有强烈的影响，导致成分与银河系富碳恒星的测量结果一致。模型的组成也与主流碳化硅晶粒中测得的C和N丰度一致。虽然我们的模型涵盖了LMC星团NGC 1846中碳星的碳丰度范围，但我们无法同时匹配富含O和C的恒星的组成。第二个重要的结果是，我们的模型只匹配K和一些M， MS巨星的氧同位素组成，而不能匹配富碳AGB恒星的氧组成。通过增加间壳中16O的丰度（基于观测证据），可以重现观测到的随进化阶段增加的16O/18O和16O/17O比值的趋势。我们还发现在AGB期间会产生一些锂，并且可以产生富含锂的碳星（log ε (Li) > 1）。这些模型显示了锂丰度的增加与碳含量的增加之间的相关性。这些模型不能解释大多数富含锂的碳星的组成，如果我们假设在FGB中由于3He破坏而发生了额外的混合，我们也不能产生富含锂的碳星。我们初步得出结论：(1)如果在AGB期间发生额外的混合，它可能只在低金属丰度的物体中有效发生，或者当恒星被严重遮挡使光谱观测变得困难时；(2)AGB恒星的壳间成分需要进一步研究。
We demonstrate that the amount of extra mixing required to fit the observed low C/N and 12C/13C ratios in first giant branch (FGB) stars is also sufficient to explain the carbon and nitrogen abundances of Galactic asymptotic/second giant branch (AGB) stars. We simulate the effect of extra mixing on the FGB by setting the composition of the envelope to that observed in low-mass (M ⩽ 2 M☉) FGB stars, and then evolve the models to the tip of the AGB. The inclusion of FGB extra mixing compositional changes has a strong effect on the C and N abundance in our AGB models, leading to compositions consistent with those measured in Galactic carbon-rich stars. The composition of the models is also consistent with C and N abundances measured in mainstream silicon carbide grains. While our models cover the range of C abundances measured in carbon stars in the LMC cluster NGC 1846, we cannot simultaneously match the composition of the O- and C-rich stars at the same time. A second important result is that our models only match the oxygen isotopic composition of K and some M, MS giants, and are not able to match the oxygen composition of carbon-rich AGB stars. By increasing the abundance of 16O in the intershell (based on observational evidence) it is possible to reproduce the observed trend of increasing 16O/18O and 16O/17O ratios with evolutionary phase. We also find that some Li production takes place during the AGB and that Li-rich carbon stars (log ϵ(Li) ≳ 1) can be produced. These models show a correlation between increasing Li abundances and C. The models cannot explain the composition of the most Li-enriched carbon stars, nor can we produce a Li-rich carbon star if we assume extra mixing occurs during the FGB owing to 3He destruction. We tentatively conclude that (1) if extra mixing occurs during the AGB it likely only occurs efficiently in low metallicity objects, or when the stars are heavily obscured making spectroscopic observations difficult and (2) the intershell compositions of AGB stars need further investigation.