出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Approximately 1% of presolar SiC grains found in primitive meteorites condensed in the ejecta
of Type II supernovae. To explain the isotopic signatures of these so-called X grains, material
from different SN zones had to contribute to the mix from which the grains formed. We
measured the Fe and Ni isotopic ratios of individual X grains with the NanoSIMS. Most grains
have enhanced 57Fe/56Fe ratios ranging up to 2solar. A few grains have depleted 57Fe/56Fe
ratios down to 0.55solar. In contrast, 54Fe/56Fe ratios are close to solar. Anomalies in the Ni
isotopes are even more extreme. 61Ni/58Ni ratios range up to 3.5solar, 62Ni/58Ni ratios up to
2solar, but 60Ni/58Ni ratios only up to 1.1solar. One of the grains with 57Fe deficits has a 61Ni
deficit, the others have normal 61Ni/58Ni ratios. The grain with the largest 57Fe deficit has
62Ni/58Ni of 1.8solar but solar 61Ni/58Ni. The excesses in 57Fe and 61,62Ni observed in most of
the grains can be explained by admixture of material from the He/C zone to material from the
He/N (or outer) zones. In the He/C zone, neutron capture results in 57Fe and 61,62Ni excesses, and
the grain data can be fairly well reproduced by variable admixture from different layers of the
He/C zone. The lack of large 54Fe excesses in the grains is puzzling in view of the fact that
material from the Si/S zone, rich in 54Fe, is required to explain the 28Si excesses of X grains. We
still do not have a good explanation for the 57Fe deficits of several of the grains; they could
reflect the initial isotopic compositions of the parent stars.
