SIMBAD references

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 for Eu isotopes. The mainstream grains are believed to have condensed in the outflows of ∼1.5-3 M _☉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 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, in most cases being 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% conf.), but is approximately 12% 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, γ) stellar reaction rate.