出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Silicon carbide grains of type X constitute ~1% of all presolar SiC grains found in primitive meteorites. These grains have 15N and 28Si excesses, mostly light C, high inferred 26Al/27Al ratios, and evidence (from 44Ca and 49Ti excesses) for the initial presence of the short-lived radioisotopes 44Ti and 49V. These isotopic signatures clearly indicate an origin in the ejecta of Type II supernovae (SNe) and material from different SN zones had to contribute to the mix from which the grains formed: He/N (26Al), He/C (12C and 15N), and the Si/S zone (28Si, 44Ti, 49V). However, in detail there are many discrepancies between grain data and theoretical predictions from SN models. One major discrepancy concerns the distribution of the Si isotopic ratios. In a Si 3-isotope plot, most grains plot along a line, which points to a primary isotopic composition with 29Si/28Si ≈ 1/3×solar and almost no 30Si. However, such a composition is not produced by any of the SN models. Another set of discrepancies involves the correlation between C, N and Al isotopic ratios: 12C/13C and 15N/14N ratios are high in the He/C and low in the He/N zone, whereas the 26Al/27Al ratios show the opposite behavior. As a consequence, mixing between these zones results in negative correlations between these ratios, in contrast to the grain data. Another example is the lack of large 54Fe excesses in X grains. While mixing with material from the Si/S zone, rich in 28Si, is necessary to explain the 28Si excesses in these grains, the 54Fe, also abundant in this zone, must have been separated from the 28Si before grain condensation by a process still not understood.
