出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Primitive meteorites contain graphite spherules whose anomalous isotopic compositions indicate
a stellar origin. The isolation of presolar graphite grains is difficult and they have been less well
studied than presolar silicon carbide grains. While previous isotopic measurements have been
made on graphite from the Murchison meteorite and mostly on low-density grains, a new
separation of graphite from the Orgueil meteorite and the application of a new ion microprobe,
the NanoSIMS, have provided a wealth of C, N, O, Al, and Si isotopic data on grains with a
range of densities. These data confirm that low-density grains come from supernovae. These
grains are characterized by 15N, 18O, 28Si and 49Ti excesses, high inferred 26Al/27Al ratios, as well
as evidence for the initial presence of the short-lived radionuclides 22Na, 41Ca, and 44Ti. All
these signatures point to an origin in Type II supernovae. The Ne-E(L) component, almost pure
22Ne, is characteristic of presolar graphite and led to its discovery. In SN grains it is due to the
decay of short-lived (T1/2 = 2.6 yr) 22Na, which apparently was implanted into the grains. Many
high-density grains have high 12C/13C ratios (>300) and large excesses in 30Si and smaller
excesses in 29Si. These signatures are best explained by an origin in low-metallicity AGB stars.
In these stars the enrichments of the envelope in the heavy Si isotopes and in 12C, products of
nucleosynthesis in the He shell, are much larger than those expected for solar-metallicity parent
stars. The high 12C/13C ratios imply also high C/O ratios, which probably caused the preferential
condensation of graphite grains over SiC grains. An AGB origin of high-density grains is
compatible with the presence of internal sub-grains with high concentrations of the s-process
elements Zr, Mo, and Ru, reflecting the high abundances of these elements in the envelope of
such stars. The stellar source of high-density grains with 12C/13C ratios around 10 remains
elusive.
