Tungsten isotopic compositions in stardust SiC grains from the Murchison meteorite: Constraints on the s-process in the Hf-Ta-W-Re-Os region

Abstract

We report the first tungsten isotopic measurements in stardust silicon carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The isotopes 182W, 183W, 184W, 186W and 179Hf, 180Hf were measured on both an aggregate (KJB fraction) and single stardust SiC grains (LS+LU fraction) believed to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The SiC aggregate shows small deviations from terrestrial (=solar) composition in the 182W/184W and 183W/184W ratios, with deficits in 182W and 183W with respect to 184W. The 186W/184W ratio, however, shows no apparent deviation from the solar value. Tungsten isotopic measurements in single mainstream stardust SiC grains revealed lower than solar 182W/184W, 183W/184W, and 186W/184W ratios. We have compared the SiC data with theoretical predictions of the evolution of W isotopic ratios in the envelopes of AGB stars. These ratios are affected by the slow neutron-capture process and match the SiC data regarding their 182W/184W, 183W/184W, and 179Hf/180Hf isotopic compositions, although a small adjustment in the s-process production of 183W is needed in order to have a better agreement between the SiC data and model predictions. The models cannot explain the 186W/184W ratios observed in the SiC grains, even when the current 185W neutron-capture cross section is increased by a factor of two. Further study is required to better assess how model uncertainties (e.g., the formation of the 13C neutron source, the mass-loss law, the modelling of the third dredge-up, and the efficiency of the 22Ne neutron source) may affect current s-process predictions.

Accepted for Publication on The Astrophysical Journal 43 pages, 2 tables, 7 figures

References in corpus (5)

• Updated stellar yields from Asymptotic Giant Branch models
• Ratio Estimation in SIMS Analysis
• Is Extra Mixing Really Needed in Asymptotic Giant Branch Stars?
• Nucleosynthetic osmium isotope anomalies in acid leachates of the Murchison meteorite
• s-process branching at 185W revised

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• Stellar origin of the 182Hf cosmochronometer and the presolar history of solar system matter
• Application of a Theory and Simulation based Convective Boundary Mixing model for AGB Star Evolution and Nucleosynthesis
• The s Process and Beyond
• The impact of updated Zr neutron-capture cross sections and new asymptotic giant branch models on our understanding of the s process and the origin of stardust
• Origin of large meteoritic SiC stardust grains in metal-rich AGB stars
• s-Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains
• Europium s-process signature at close-to-solar metallicity in stardust SiC grains from AGB stars
• The 95zr(n, gamma)96zr cross section from the surrogate ratio method and its effect on the s-process nucleosynthesis
• Representation of s-process abundances for comparison to data from bulk meteorites
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