IRON AND NICKEL ISOTOPIC RATIOS IN PRESOLAR SiC GRAINS

We report the first Fe isotopic anomalies and the first Ni isotopic ratio measurements in presolar SiC grains of separate KJG from the Murchison meteorite. With NanoSIMS, we analyzed Fe and Ni in 37 X grains from Type II supernovae and 53 SiC grains of other types. TheNi/Fe and Co/Fe ratios in grains of all types aremuch higher than in the gas from which the grains are believed to have condensed. Amajority of the X grains and a couple of mainstream grains contain Fe-rich subgrains. Most X grains have large excesses in 57Fe, Ni, and Ni. Ni excesses are small and the Fe/Fe ratios of almost all X grains are normal. These isotopic compositions are best explained by mixing of material from theHe/N zone of Type II supernovaewithmaterial from the He/C zone. The lack of any Fe excesses is puzzling in view of the fact that the Si /S zone, whose contribution resulted in the Si excesses in X grains, is very rich in Fe. It has yet to be seen whether elemental fractionation between Si and Fe is an explanation. The Fe deficits observed in a few X grains remain unexplained. In comparison to the X grains, fewer mainstream and AB grains have anomalies. Observed Ni excesses in some mainstream grains are larger than predicted for AGB stars of solar metallicity and are not accompanied by corresponding Ni excesses. AY grain and a Z grain have excesses in Fe and Ni, but close to normal Fe/Fe and 60,61Ni/58Ni ratios. These isotopic compositions are not expected for grains from low-metallicity AGB stars. Subject headinggs: dust, extinction — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — supernovae: general