26Al and 60Fe yields from AGB stars

We present yields for Al-26 and Fe-60 from asymptotic giant branch (AGB) stars. For AGB stars of masses lower than approximate to 4 M-circle dot yields are of the order of only 10(-7) M-circle dot, while for AGB stars of higher masses yields are up to 10(-5) M-circle dot In these massive AGB stars Al-26 is produced via Mg-25(p, gamma)Al-26 reactions when proton captures occur at the base of the convective envelope (hot bottom burning), while Fe-60 is produced via the operation of the Fe-59(n, gamma)Fe-60 reaction when high neutron densities result from the activation of the Ne-22(alpha, n)Mg-25 neutron source during thermal pulses. Large nuclear and stellar uncertainties are associated with these predictions, ranging from the rate of the Al-26 + p reaction to the amount of material carried from the He-rich shell to the convective envelope via the third dredge-up. When compared to the contribution from core-collapse supernovae, the overall contribution of AGB stars to the Galactic inventory of Al-26 and Fe-60 is insignificant. On the other hand, a massive AGB star may have polluted the early solar system with short lived radioactive nuclei since we obtain a self-consistent match for the abundances of Ca-41, Al-26, Fe-60, and Pd-107 using our 6.5 M. model. Finally. the interpretation of the Al-26/Al-27 ratios in the majority of meteoritic stellar grains from low-mass AGB stars is hindered by the three orders of magnitude error bar of the Al-26(p, gamma)Si-27 reaction. Grains with very high Al-26/Al-27 ratios may represent evidence for extra-mixing phenomena in AGB stars or for a post-AGB origin. (C) 2008 Elsevier B.V. All rights reserved.