The impact of the¹⁸F(α, p)²¹Ne reaction on asymptotic giant branch nucleosynthesis

We present detailed models of low- and intermediate-mass asymptotic giant branch (AGB) stars with and without the ¹⁸F(α, p) ²¹Ne reaction included in the nuclear network, where the rate for this reaction has been recently experimentally evaluated for the first time. The lower and recommended measured rates for this reaction produce negligible changes to the stellar yields, whereas the upper limit of the rate affects the production of ¹⁹F and ²¹Ne. The stellar yields increase by ∼50% to up to a factor of 4.5 for ¹⁹F, and by factors of ∼2 to 9.6 for ²¹Ne. While the ¹⁸F(α, p)²¹Ne reaction competes with¹⁸O production, the extra protons released are captured by ¹⁸O to facilitate the ¹⁸O(p,α) ¹⁵N(α, γ)¹⁹F chain. The higher abundances of ¹⁹F obtained using the upper limit of the rate helps to match the [ F/O] ratios observed in AGB stars, but only for large C/O ratios. Extramixing processes are proposed to help to solve this problem. Some evidence that the ¹⁸F(α, p)²¹Ne rate might be closer to its upper limit is provided by the fact that the higher calculated ²¹Ne/ ²²Ne ratios in the He inter/shell provide an explanation for the Ne isotopic composition of silicon-carbide grains from AGB stars. This needs to be confirmed by future experiments of the ¹⁸F(α, p)²¹Ne reaction rate. The availability of accurate fluorine yields from AGB stars will be fundamental for interpreting observations of this element in carbon-enhanced metal-poor stars.