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Astron. Astrophys. 361, 959-976 (2000)


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Aluminum 26 production in asymptotic giant branch stars

N. Mowlavi 1,2 and G. Meynet 1

1 Geneva Observatory, 1290 Sauverny, Switzerland
2 INTEGRAL Science Data Center, Switzerland

Received 16 March 2000 / Accepted 27 June 2000

Abstract

The production of [FORMULA] in asymptotic giant branch (AGB) stars is studied based on evolutionary stellar models of different masses ([FORMULA]) and metallicities ([FORMULA]). It is confirmed that [FORMULA] 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 [FORMULA] available in the intershell region follows, at a given metallicity, a very well defined pattern as a function of the H-burning shell temperature [FORMULA]. Two zones must be distinguished. The first one comprises those He-rich layers containing H-burning ashes which escape pulse injection. The amount of [FORMULA] in that zone ([FORMULA] at the first pulse in [FORMULA] Z=0.02 stars) steadily decreases with pulse number. Its contribution to the surface [FORMULA] 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 [FORMULA] available in that zone is higher than that in the first zone ([FORMULA] at the first pulse in [FORMULA] Z=0.02 stars), and keeps constant during about the first dozen pulses before decreasing when [FORMULA] K. This zone is thus an important potential reservoir for surface [FORMULA] enrichment.

Using third dredge-up (3DUP) efficiencies from model calculations, the surface [FORMULA] abundance is predicted to reach [FORMULA] 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 [FORMULA] than in [FORMULA] stars. In massive AGB stars, however, hot bottom burning enables to easily reach surface [FORMULA] mass fractions above [FORMULA].

The [FORMULA]/[FORMULA] 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 [FORMULA] present day mass of [FORMULA] 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

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