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Abstract: We present detailed models of low and intermediate-mass asymptotic giantbranch AGB stars with and without the 18Fa,p21Ne reaction included in thenuclear network, where the rate for this reaction has been recentlyexperimentally evaluated for the first time. The lower and recommended measuredrates for this reaction produce negligible changes to the stellar yields,whereas the upper limit of the rate affects the production of 19F and 21Ne. Thestellar yields increase by ~50% to up to a factor of 4.5 for 19F, and byfactors of ~2 to 9.6 for 21Ne. While the 18}Fa,p21Ne reaction competes with18O production, the extra protons released are captured by 18O to facilitatethe 18Op,a15Na,g19F chain. The higher abundances of 19F obtained using theupper limit of the rate helps to match the F-O ratios observed in AGB stars,but only for large C-O ratios. Extra-mixing processes are proposed to help tosolve this problem. Some evidence that the 18Fa,p21Ne rate might be closer toits upper limit is provided by the fact that the higher calculated 21Ne-22Neratios in the He intershell provide an explanation for the Ne isotopiccomposition of silicon-carbide grains from AGB stars. This needs to beconfirmed by future experiments of the 18Fa,p21Ne reaction rate. Theavailability of accurate fluorine yields from AGB stars will be fundamental forinterpreting observations of this element in carbon-enhanced metal-poor stars.



Autor: Amanda Karakas ANU, Hye Young Lee Argonne, Maria Lugaro Utrecht, Joachim Goerres JINA, Notre Dame, Michael Wiescher JINA, Notre D

Fuente: https://arxiv.org/







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