Nanosims Studies of Presolar Graphite Grains: Are C-isotopic Ratios Grain-size- Dependent?

Introduction: After silicon carbide (SiC), graphite is the best studied presolar mineral. A wealth of information exists on the isotopic compositions of C and of many minor/trace elements in micrometer-sized grains [e.g., 1-5]. Presolar graphite grains exhibit large C-isotopic anomalies with C/C ratios between 2 and 7000. Although this range is compatible with what is observed for SiC [6, 7], the distribution of C/C ratios is completely different. Most presolar graphite grains have isotopically light C (i.e., C/C > 89) while most SiC grains have heavy C with C/C ratios between 40 and 80, indicative of different relative proportions of the stellar sources that contributed carbonaceous dust to the solar nebula. Many low-density (ρ < 2.05 g/cm) graphite grains show enrichments in N, O, and Si as well as evidence for the former presence of radioactive Ti and V. This points to an origin in Type II supernovae (SNII) of a large fraction (several 10 %) of presolar graphite grains [3]. An important source of high-density graphite grains with isotopically light C are probably low-metallicity AGB stars [4]. This contrasts with what was inferred for the stellar sources of presolar SiC, namely, solar metallicity AGB stars for the vast majority of the grains [8] and SNII for about 1% of the grains [7,9].