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

We report the first tungsten isotopic measurements in stardust silicon carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The isotopes ^{182,183,184,186}W and ^179,180Hf 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 modeling of the third dredge-up, and the efficiency of the ²²Ne neutron source) may affect current s-process predictions.