Aluminum 26 production in asymptotic giant branch stars

The production of ²⁶Al in asymptotic giant branch (AGB) stars is studied based on evolutionary stellar models of different masses (1.5 <= M/M_\odot <= 6) and metallicities (0.004 <= Z <= 0.02). It is confirmed that ²⁶Al is efficiently produced by hydrogen burning, but destruction of that nuclei by n-capture reactions during the interpulse and pulse phases becomes increasingly more efficient as the star evolves on the AGB. The amount of ²⁶Al available in the intershell region follows, at a given metallicity, a very well defined pattern as a function of the H-burning shell temperature T_H. Two zones must be distinguished. The first one comprises those He-rich layers containing H-burning ashes which escape pulse in jection. The amount of ²⁶Al in that zone (M_\odot1-2×10^-7 at the first pulse in M_\odot1.5-3 Z=0.02 stars) steadily decreases with pulse number. Its contribution to the surface ²⁶Al enhancement can only be important during the first pulses if dredge--up occurs at that stage. The second zone consists of the C-rich material emerging from the pulses. The amount of ²⁶Al available in that zone is higher than that in the first zone (M_\odot3-4×10^-7 at the first pulse in M_\odot1.5-3 Z=0.02 stars), and keeps constant during about the first dozen pulses before decreasing when T_H~> 55× 10⁶ K. This zone is thus an important potential reservoir for surface ²⁶Al enrichment. Using third dredge-up (3DUP) efficiencies from model calculations, the surface ²⁶Al abundance is predicted to reach 1-2× 10^-7 mass fractions in our low-mass solar metallicity stars, with an uncertainty factor of about three. It decreases with increasing stellar mass, being about three times lower in a M_\odot4 than in M_\odot2-3 stars. In massive AGB stars, however, hot bottom burning enables to easily reach surface ²⁶Al mass fractions above 10^-6. The ²⁶Al/²⁷Al ratios measured in meteoritic SiC and oxide grains are discussed, as well as that possibly measured in the nearby C-star IRC+10216. We also address the contribution of AGB stars to the 2-3 M_\odot present day mass of ²⁶Al detected in the Galaxy. Finally, we discuss the possibility of directly detecting an AGB star or a planetary nebula as a single source at 1.8 MeV with the future INTEGRAL satellite.