DEEP MIXING IN EVOLVED STARS. I. THE EFFECT OF REACTION RATE REVISIONS FROM C TO Al

Abstract

We present computations of nucleosynthesis in low-mass red-giant-branch and\nasymptotic-giant-branch stars of Population I experiencing extended mixing. We\nadopt the updated version of the FRANEC evolutionary model, a new post-process\ncode for non-convective mixing and the most recent revisions for solar\nabundances. In this framework, we discuss the effects of recent improvements in\nrelevant reaction rates for proton captures on intermediate-mass nuclei (from\ncarbon to aluminum). For each nucleus we briefly discuss the new choices and\ntheir motivations. The calculations are then performed on the basis of a\nparameterized circulation, where the effects of the new nuclear inputs are best\ncompared to previous works. We find that the new rates (and notably the one for\nthe 14N(p,g)15O reaction) imply considerable modifications in the composition\nof post-main sequence stars. In particular, the slight temperature changes due\nto the reduced efficiency of proton captures on 14N induce abundance variations\nat the first dredge up (especially for 17O, whose equilibrium ratio to 16O is\nvery sensitive to the temperature). In this new scenario presolar oxide grains\nof AGB origin turn out to be produced almost exclusively by very-low mass stars\n(M<=1.5-1.7Msun), never becoming C-rich. The whole population of grains with\n18O/16O below 0.0015 (the limit permitted by first dredge up) is now explained.\nAlso, there is now no forbidden area for very low values of 17O/16O (below\n0.0005), contrary to previous findings. A rather shallow type of transport\nseems to be sufficient for the CNO changes in RGB stages. Both thermohaline\ndiffusion and magnetic-buoyancy-induced mixing might provide a suitable\nphysical mechanism for this. Thermohaline mixing is in any case certainly\ninadequate to account for the production of 26Al on the AGB. Other transport\nmechanisms must therefore be at play.\n