Reaction Rate Uncertainties and the Production of19F in Asymptotic Giant Branch Stars

We present nucleosynthesis calculations and the resulting 19F stellar yieldsfor a large set of models with different masses and metallicity. We find thatthe production of fluorine depends on the temperature of the convective pulses,the amount of primary 12C mixed into the envelope by third dredge up and theextent of the partial mixing zone. Then we perform a detailed analysis of thereaction rates involved in the production of 19F and the effects of theiruncertainties. We find that the major uncertainties are associated with the14C(alpha,gamma)18O and the 19F(alpha,p)22Ne reaction rates. For these tworeactions we present new estimates of the rates and their uncertainties. Theimportance of the partial mixing zone is reduced when using our estimate forthe 14C(alpha,gamma)18O rate. Taking into account both the uncertaintiesrelated to the partial mixing zone and those related to nuclear reactions, thehighest values of 19F enhancements observed in AGB stars are not matched by themodels. This is a problem that will have to be revised by providing a betterunderstanding of the formation and nucleosynthesis in the partial mixing zone,also in relation to reducing the uncertainties of the 14C(alpha,gamma)18Oreaction rate. At the same time the possible effect of Cool Bottom Processingat the base of the convective envelope should be included in the computation ofAGB nucleosynthesis. This process could in principle help matching the highest19F abundances observed by decreasing the C/O ratio at the surface of the star,while leaving the 19F abundance unchanged.Comment: 40 pages, 8 figures, accepted for publication on the Astrophysical Journa