渐近巨星分支星真的需要额外混合吗？
Is Extra Mixing Really Needed in Asymptotic Giant Branch Stars?
摘要：
我们证明了在第一巨星分支（FGB）恒星中所观察到的低C/N和12 C/13 C比值所需的额外混合量也足以解释银河系AGB恒星的C和N丰度。我们模拟的FGB上的额外的混合的效果，通过设置的信封的组成，在低质量的FGB恒星中观察到的，然后演变的模型的AGB的尖端。包含FGB额外的混合成分的变化有很强的影响，在我们的AGB模型的C和N丰度，导致在银河系富C星的测量一致的组成。模型的组成也与主流碳化硅晶粒中测得的C丰度一致。虽然我们的模型涵盖了NGC 1846中C星测量的C丰度范围，但我们无法同时匹配O和C富星的组成。我们的模型只匹配了K和一些M、MS巨星的O同位素组成，而不能匹配富C AGB星的O同位素组成。通过增加壳层间的16 O丰度（基于观测证据），有可能再现观察到的16 O/18 O和16 O/17 O比值随演化阶段而增加的趋势。我们的结论是：1）如果在AGB期间发生额外的混合，它可能只发生在低金属量的物体中，或者当恒星被严重遮蔽而难以进行光谱观测时，以及2）AGB恒星的壳层间成分需要进一步研究。
英文摘要：
(Abridged) 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 C and N abundances of Galactic 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 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 C-rich stars. The composition of the models is also consistent with C abundances measured in mainstream silicon carbide grains. While our models cover the range of C abundances measured in C stars in NGC 1846, we cannot simultaneously match the composition of the O and C-rich stars. Our models only match the O isotopic composition of K and some M, MS giants, and are not able to match the O composition of C-rich AGB stars. By increasing the 16O intershell abundance (based on observational evidence) it is possible to reproduce the observed trend of increasing 16O/18O and 16O/17O ratios with evolutionary phase. We conclude 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) that the intershell compositions of AGB stars needs further investigation.
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