On the stellar sources of presolar graphite

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 15 N, 18 O, 28 Si and 49 Ti excesses, high inferred 26 Al/ 27 Al ratios, as well as evidence for the initial presence of the short-lived radionuclides 22 Na, 41 Ca, and 44 Ti. All these signatures point to an origin in Type II supernovae. The Ne-E(L) component, almost pure 22 Ne, 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) 22 Na, which apparently was implanted into the grains. Many high-density grains have high 12 C/ 13 C ratios (>300) and large excesses in 30 Si and smaller excesses in 29 Si. 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 12 C, products of nucleosynthesis in the He shell, are much larger than those expected for solar-metallicity parent stars. The high 12 C/ 13 C 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 12 C/ 13 C ratios around 10 remains elusive.