The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge-up
The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge-up
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CNO同位素：红巨星的深层循环以及第一次和第二次挖掘
DOI：
10.1086/306546
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发表时间：
1995
期刊：
影响因子：
--
通讯作者：
I. Sackmann
中科院分区：
文献类型：
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作者：
A. I. Boothroyd;I. Sackmann
结果表明，标准对流包层底部下方的深层循环混合以及随后的 CNO 同位素“冷底处理”（CBP）可以重现低质量红巨星中 12C/13C 观测结果的恒星质量趋势。 （这一趋势与标准首次疏浚的预期相反。）我们的模型假设额外的混合总是达到距 H 燃烧壳相同的温度距离，并且当 H 燃烧壳消除由第一次疏浚建立的分子量不连续性（“μ 屏障”）时，CBP 开始。对于 I 族恒星，除了 15N 之外，其他 CNO 同位素预计不会被 CBP 改变。 （如果 18O 耗尽发生在渐近巨星分支 [AGB] 上，正如一些观测表明的那样，则需要额外的混合比红巨星分支 [RGB] 更接近 AGB 上的 H 燃烧壳层，并且还应该导致比在相关 AGB 恒星中观察到的低得多的 12C/13C 比率。）随着恒星质量或金属丰度的减少，CBP 急剧增加 - 大致与 RGB 上的 M-2 一样，因为 RGB 更长低质量恒星，大致为 Z-1，因为低金属丰度恒星的 H 壳层燃烧温度较高。在低质量星族 II 恒星中，所有 CNO 同位素预计都会被 CBP 显着改变。现场星族 II 恒星表现出的 RGB 丰度与我们的 CBP 模型的预测一致，该模型已标准化以重现星族 I RGB 丰度。另一方面，观察到球状星团恒星会经历更广泛的处理；此外，观察到 CBP 开始于球状星团 RGB 的底部附近（克服任何“μ 势垒”）。对于 CNO 同位素 12C、13C、14N、16O、17O 和 18O，我们还对第一次和第二次疏浚后果（即 RGB 和 AGB 阶段的标准对流）在各种恒星质量 (0.8-9 M☉) 和金属丰度 (Z=0.02-0.0001) 上进行了自洽计算。我们证明常见的低质量和中等质量恒星是宇宙中 13C、14N 和 17O 的主要来源。轻元素（3He、4He、7Li、9Be、10B 和 11B）在配套论文中进行了讨论。
It is demonstrated that deep circulation mixing below the base of the standard convective envelope, and the consequent "cool bottom processing" (CBP) of the CNO isotopes, can reproduce the trend with stellar mass of the 12C/13C observations in low-mass red giants. (This trend is opposite to what is expected from standard first dredge-up.) Our models assume that extra mixing always reaches to the same distance in temperature from the H-burning shell and that CBP begins when the H-burning shell erases the molecular weight discontinuity ("μ-barrier") established by first dredge-up. For Population I stars, none of the other CNO isotopes except 15N are expected to be altered by CBP. (If 18O depletion occurs on the asymptotic giant branch [AGB], as some observations suggest, it would require that extra mixing reach closer to the H-burning shell on the AGB than on the red giant branch [RGB]—and should also result in a much lower 12C/13C ratio than is observed in the relevant AGB stars.) CBP increases dramatically as one reduces the stellar mass or metallicity—roughly as M-2 on the RGB, because of the longer RGB of low-mass stars, and roughly as Z-1, because of the higher H-shell burning temperatures of low-metallicity stars. In low-mass Population II stars, all the CNO isotopes are expected to be significantly altered by CBP. Field Population II stars exhibit RGB abundances consistent with the predictions of our CBP models that have been normalized to reproduce the Population I RGB abundances. On the other hand, globular cluster stars are observed to encounter much more extensive processing; additionally, CBP is observed to start near the base of the globular cluster RGB (overcoming any "μ-barrier"). For the CNO isotopes 12C, 13C, 14N, 16O, 17O, and 18O, we also present self-consistent calculations of the consequences of both first and second dredge-up, i.e., of standard convection during the RGB and AGB stages, over a wide range of stellar masses (0.8-9 M☉) and metallicities (Z=0.02-0.0001). We demonstrate that the common low- and intermediate-mass stars are a prime source of 13C, 14N, and 17O in the universe. The light elements (3He, 4He, 7Li, 9Be, 10B, and 11B) are discussed in a companion paper.