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)Wand Hf-179,Hf-180 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 W-182/(184)Wand W-183/(184)Wratios, with deficits in W-182 and W-183 with respect to W-184. The W-186/W-184 ratio, however, shows no apparent deviation from the solar value. Tungsten isotopic measurements in single mainstream stardust SiC grains revealed lower than solar W-182/W-184, W-183/W-184, and W-186/W-184 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 W-182/W-184, W-183/W-184, and Hf-179/Hf-180 isotopic compositions, although a small adjustment in the s-process production of W-183 is needed in order to have a better agreement between the SiC data and model predictions. The models cannot explain the W-186/W-184 ratios observed in the SiC grains, even when the current W-185 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 C-13 neutron source, the mass-loss law, the modeling of the third dredge-up, and the efficiency of the Ne-22 neutron source) may affect current s-process predictions.