The uncertainties in the [superscript 22]Ne+[alpha]-capture reaction rates and the production of the heavy magnesium isotopes in asymptotic giant branch stars of intermediate mass

Abstract

We present new rates for the [superscript 22]Ne([alpha], n)[superscript 25]Mg and [superscript 22]Ne([alpha], [gamma])[superscript 26]Mg reactions, with uncertainties that have been considerably reduced compared to previous estimates, and we study how these new rates affect the production of the heavy magnesium isotopes in models of intermediate-mass asymptotic giant branch (AGB) stars of different initial compositions. All the models have deep third dredge-up, hot bottom burning, and mass loss. Calculations have been performed using the two most commonly used estimates of the [superscript 22]Ne+[alpha] rates as well as the new recommended rates, and with combinations of their upper and lower limits. The main result of the present study is that, with the new rates, uncertainties on the production of isotopes from Mg to P coming from the [superscript 22]Ne+[alpha]-capture rates have been considerably reduced.We have therefore removed one of the important sources of uncertainty to effect models of AGB stars. We have studied the effects of varying the mass-loss rate on nucleosynthesis and discuss other uncertainties related to the physics employed in the computation of stellar structure, such as the modeling of convection, the inclusion of a partial mixing zone, and the definition of convective borders. These uncertainties are found to be much larger than those coming from [superscript 22]Ne+[alpha]-capture rates, when using our new estimates. Much effort is needed to improve the situation for AGB models.