26Al and 60Fe yields from AGB stars

Abstract

We present yields for 26Al and 60Fe from asymptotic giant branch (AGB) stars. For AGB stars of masses lower than ≈4 M yields are of the order of only 10−7 M, while for AGB stars of higher masses yields are up to 10−5 M. In these massive AGB stars 26Al is produced via 25Mg(p, γ)26Al reactions when proton captures occur at the base of the convective envelope (hot bottom burning), while 60Fe is produced via the operation of the 59Fe(n, γ)60Fe reaction when high neutron densities result from the activation of the 22Ne(α, n)25Mg neutron source during thermal pulses. Large nuclear and stellar uncertainties are associated with these predictions, ranging from the rate of the 26Al + 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 26Al and 60Fe 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 41Ca, 26Al, 60Fe, and 107Pd using our 6.5 M model. Finally, the interpretation of the 26Al/27Al 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 26Al(p, γ) 27Si reaction. Grains with very high 26Al/27Al ratios may represent evidence for extra-mixing phenomena in AGB stars or for a post-AGB origin.