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标题：22Ne a primary source of neutron for the s-process and a major neutron poison in CEMP AGB stars
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作者：R. Gallino ; S. Bisterzo ; L. Husti 等
期刊名称：PoS - Proceedings of Science
印刷版ISSN：1824-8039
出版年度：2006
出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：In AGB stars of low mass and very low metallicity, [Fe/H] 2, a large abundance of primary 12C is mixed with the envelope by each third dredge up episode. The subsequent activation of the H shell converts almost all CNO nuclei into 14N. Thus the H burning ashes contain 14N from the original CNO nuclei, plus an increasing amount of primary 14N. During the subsequent convective thermal instability in the He shell, all the 14N nuclides are converted to 22Ne by 14N(a,g )18F(b n)18O, followed by 18O(a,g )22Ne. At the peak temperature reached at the base of the thermal pulse, the 22Ne(a,n)25Mg reaction is partly activated, giving rise to a small neutron exposure. At the same time, although the neutron capture cross section of 22Ne is very small (s (22Ne, 30keV)=0.059 0.0057 mbarn, Beer et al. 1991), the very large amount of primary 22Ne acts as a major poison against the s process. This poison effect is substantial also in case of addition of a 13C-pocket. Some fraction of primary 16O is also made in the thermal pulse by a-capture on 12C (with mass fraction X(16O) 0.003, while X(12C) 0.20). Besides 12C and 22Ne, a number of light isotopes are largely produced in a primary way, among which one finds 19F (from neutron capture on 18O), 23Na, 25Mg, 26Mg. An effort should be devoted to improve the accuracy of the cross sections of these light isotopes, in order to better constrain the s-process efficiency and their production in low metallicity AGB stars. At very low metallicity, iron is directly made starting from neutron captures on 22Ne and then used as a bridge for the build-up of the s elements.