Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars

Details Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars Isotopic Compositions of Strontium, Zirconium, Molybdenum, and Barium in Single Presolar SiC Grains and Asymptotic Giant Branch Stars

Aug 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...593..486l&link_type=abstract

The Astrophysical Journal, Volume 593, Issue 1, pp. 486-508.

Astronomy and Astrophysics

Astronomy

68

Nuclear Reactions, Nucleosynthesis, Abundances, Stars: Agb And Post-Agb

Scientific paper

The strontium, zirconium, molybdenum, and barium isotopic compositions predicted in the mass-losing envelopes of asymptotic giant branch (AGB) stars of solar metallicity and mass 1.5, 3, and 5 Msolar are discussed and compared with recent measurements in single presolar silicon carbide (SiC) grains from the Murchison meteorite. Heavy-element nucleosynthesis via the s-process occurs in the helium intershell, the region between the helium-burning and hydrogen-burning shells, producing heavy elements beyond iron. After a limited number of thermal runaways of the helium shell (thermal pulses), at the quenching of each instability, the convective envelope penetrates into the top layers of the helium intershell (third dredge-up), mixing newly synthesized 12C and s-process material to the stellar surface. Eventually, the envelope becomes carbon-rich (C>=O), a necessary condition for SiC grains to condense. In the helium intershell, neutrons are released by (α, n) reactions on 13C and 22Ne during interpulse phases and the thermal pulses, respectively. A 13C pocket is assumed to form in a tiny region in the top layers of the helium intershell by injection of a small amount of protons from the envelope during each third dredge-up episode. This 13C then burns radiately during the interpulse phase. The average neutron density produced is low, but of long duration, so the total neutron exposure is high. We have explored a large range of possible 13C abundances in the pocket. In low-mass AGB stars (1.5Msolar<=M<=4 Msolar), a second small burst of neutrons is released by marginal 22Ne burning in the thermal pulse. The neutron density reaches quite a high peak value but is of short duration, so the neutron exposure is low. In intermediate-mass AGB stars (4Msolar

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