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

Abstract

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 M ☉ 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 12 C 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 13 C and 22 Ne during interpulse phases and the thermal pulses, respectively. A 13 C 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 13 C 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 13 C abundances in the pocket. In low-mass AGB stars (1.5 M ☉ ≤ M ≤ 4 M ☉ ), a second small burst of neutrons is released by marginal 22 Ne 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 (4 M ☉ < M ≤ 8 M ☉ ), the 22 Ne neutron source is more efficiently activated. The neutron capture process has been followed with a postprocessing code that considers all relevant nuclei from 4 He to 210 Po. The predicted isotopic compositions of strontium, zirconium, molybdenum, and barium in the envelopes of low-mass AGB stars of solar metallicity are in agreement with the isotopic ratios measured in individual presolar SiC grains, whereas predictions for intermediate-mass stars exclude them as the sources of these grains. A multiplicity of low-mass AGB stars with metallicity around solar, having different masses and experiencing different neutron exposures, are required to account for the measured spread in heavy-element isotopic compositions among single presolar SiC grains. The range of neutron exposures corresponds, on average, to a lower mean neutron exposure than that required to reproduce the s -process main component in the solar system.