Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...361..959m&link_type=abstract
Astronomy and Astrophysics, v.361, p.959-976 (2000)
Astronomy and Astrophysics
Astrophysics
33
Stars: Abundances, Stars: Interiors, Stars: Evolution, Stars: White Dwarfs, Stars: Carbon, Stars: Agb And Post-Agb
Scientific paper
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 TH. 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 TH~> 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.
Meynet George
Mowlavi Nami
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