Aluminum 26 production in asymptotic giant branch stars
Received 16 March 2000 / Accepted 27 June 2000
The production of in asymptotic giant branch (AGB) stars is studied based on evolutionary stellar models of different masses () and metallicities (). It is confirmed that 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 available in the intershell region follows, at a given metallicity, a very well defined pattern as a function of the H-burning shell temperature . Two zones must be distinguished. The first one comprises those He-rich layers containing H-burning ashes which escape pulse injection. The amount of in that zone ( at the first pulse in Z=0.02 stars) steadily decreases with pulse number. Its contribution to the surface 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 available in that zone is higher than that in the first zone ( at the first pulse in Z=0.02 stars), and keeps constant during about the first dozen pulses before decreasing when K. This zone is thus an important potential reservoir for surface enrichment.
Using third dredge-up (3DUP) efficiencies from model calculations, the surface abundance is predicted to reach 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 than in stars. In massive AGB stars, however, hot bottom burning enables to easily reach surface mass fractions above .
The / 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 present day mass of 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.
Key words: stars: abundances stars: interiors stars: evolution stars: white dwarfs stars: carbon stars: AGB and post-AGB
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000