Aluminum 26 production in asymptotic giant branch stars

The production of Al-26 in asymptotic giant branch (AGB) stars is studied b ased on evolutionary stellar models of different masses (1.5 less than or e qual to M/M-circle dot less than or equal to 6) and metallicities (0.004 le ss than or equal to Z less than or equal to 0.02). It is confirmed that Al- 26 is efficiently produced by hydrogen burning, but destruction of that nuc lei by n-capture reactions during the interpulse and pulse phases becomes i ncreasingly more efficient as the star evolves on the AGE. The amount of Al-26 available in the intershell region follows, at a given metallicity, a very well defined pattern as a function of the H-burning she ll temperature T-H. TWO zones must be distinguished. The first one comprise s those He-rich layers containing H-burning ashes which escape pulse inject ion. The amount of Al-26 in that zone (1-2 x 10(-7) M-circle dot at the fir st pulse in 1.5-3 M-circle dot Z=0.02 stars) steadily decreases with pulse number. Its contribution to the surface Al-26 enhancement can only be impor tant 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 o f Al-26 available in that zone is higher than that in the first zone (3-4 x 10(-7) M-circle dot at the first pulse in 1.5-3 M-circle dot Z=0.02 stars) , and keeps constant during about the first dozen pulses before decreasing when T-H greater than or similar to 55 x 10(6) K. This zone is thus an impo rtant potential reservoir for surface Al-26 enrichment. Using third dredge-up (3DUP) efficiencies from model calculations, the surf ace Al-26 abundance is predicted to reach 1-2 x 10(-7) mass fractions in ou r low-mass solar metallicity stars, with an uncertainty factor of about thr ee. It decreases with increasing stellar mass, being about three times lowe r in a 4 M-circle dot than in 2-3 M-circle dot stars. In massive AGE stars, however, hot bottom burning enables to easily reach surface Al-26 mass fra ctions above 10(-6) The Al-26/Al-27 ratios measured in meteoritic SiC and oxide grains are disc ussed, as well as that possibly measured in the nearby C-star IRC+10216. We also address the contribution of AGE stars to the 2-3 M-circle dot present day mass of Al-26 detected in the Galaxy. Finally, we discuss the possibility of directly detecting an AGE star or a planetary nebula as a single source at 1.8 MeV with the future INTEGRAL sat ellite.