Deep Mixing in Evolved Stars: I. The Effect of Reaction Rate Revisions from C to Al - Astrophysics > Solar and Stellar AstrophysicsReport as inadecuate

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Abstract: We present computations of nucleosynthesis in low-mass red-giant-branch andasymptotic-giant-branch stars of Population I experiencing extended mixing. Weadopt the updated version of the FRANEC evolutionary model, a new post-processcode for non-convective mixing and the most recent revisions for solarabundances. In this framework, we discuss the effects of recent improvements inrelevant reaction rates for proton captures on intermediate-mass nuclei fromcarbon to aluminum. For each nucleus we briefly discuss the new choices andtheir motivations. The calculations are then performed on the basis of aparameterized circulation, where the effects of the new nuclear inputs are bestcompared to previous works. We find that the new rates and notably the one forthe 14Np,g15O reaction imply considerable modifications in the compositionof post-main sequence stars. In particular, the slight temperature changes dueto the reduced efficiency of proton captures on 14N induce abundance variationsat the first dredge up especially for 17O, whose equilibrium ratio to 16O isvery sensitive to the temperature. In this new scenario presolar oxide grainsof AGB origin turn out to be produced almost exclusively by very-low mass starsM<=1.5-1.7Msun, never becoming C-rich. The whole population of grains with18O-16O below 0.0015 the limit permitted by first dredge up is now explained.Also, there is now no forbidden area for very low values of 17O-16O below0.0005, contrary to previous findings. A rather shallow type of transportseems to be sufficient for the CNO changes in RGB stages. Both thermohalinediffusion and magnetic-buoyancy-induced mixing might provide a suitablephysical mechanism for this. Thermohaline mixing is in any case certainlyinadequate to account for the production of 26Al on the AGB. Other transportmechanisms must therefore be at play.

Author: S. Palmerini, M. La Cognata, S. Cristallo, M. Busso


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