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Europium s-process signature at close-to-solar metallicity in stardust SiC grains from AGB stars

Individual mainstream stardust silicon carbide (SiC) grains and a SiC-enriched bulk sample from the Murchison carbonaceous meteorite have been analyzed by the Sensitive High Resolution Ion Microprobe - Reverse Geometry (SHRIMP-RG) for Eu isotopes. The mainstream grains are believed to have condensed in the outflows of 1.5 to 3 Msun carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The 151Eu fractions [fr(151Eu) = 151Eu/(151Eu+153Eu)] derived from our measurements are compared with previous astronomical observations of carbon-enhanced metal-poor (CEMP) stars enriched in elements made by slow neutron captures (the s-process). Despite the difference in metallicity between the parent stars of the grains and the metal-poor stars, the fr(151Eu) values derived from our measurements agree well with fr(151Eu) values derived from astronomical observations. We have also compared the SiC data with theoretical predictions of the evolution of Eu isotopic ratios in the envelope of AGB stars. Because of the low Eu abundances in the SiC grains, the fr(151Eu) values derived from our measurements show large uncertainties, being in most cases larger than the difference between solar and predicted fr(151Eu) values. The SiC aggregate yields a fr(151Eu) value within the range observed in the single grains and provides a more precise result (fr(151Eu) = 0.54 +/- 0.03, 95 percent conf.), but is approximately 12 percent higher than current s-process predictions. The AGB models can match the SiC data if we use an improved formalism to evaluate the contribution of excited nuclear states in the calculation of the 151Sm(n,g) stellar reaction rate.