Presolar graphite from the Murchison meteorite: An isotopic study

We studied presolar graphite grains from four density fractions, KE3 (1.65-1.72 g/cm(3)), KFA1 (2.05-2.10 g/cm(3)), KFB1 (2.10-2.15 g/cm(3)), and KFC1 (2.15-2.20 g/cm(3)), extracted from the Murchison (CM2) meteorite, with the ion microprobe. One of the most interesting features of presolar graphite is that isotopic features depend on density. There are grains with N-15 and O-18 excesses, Si isotopic anomalies, high Al-26/Al-27 ratios (similar to 0.1), and Ca and Ti isotopic anomalies, including the initial presence of short-lived Ca-41 and Ti-44. These isotopic features are qualitatively explained by nucleosynthesis in core collapse supernovae. We estimate that 76%, 50%, 7% and 1% of the KE3, KFA1, KFB1 and KFC1 grains, respectively, are supernova grains. We performed 3- and 4-zone supernova mixing calculations to reproduce the C, O (O-18/O-16) and Al isotopic ratios of the KE3 grains, using 15 M-circle dot model calculations by Rauscher et al. (2002). Isotopic ratios of grains with high C-12/C-13 ratios (>200) can be reproduced, whereas those of grains with ratios 6200 are hard to explain if we assume that graphite grains form in C-rich conditions. We compared the distributions of the C-12/C-13 ratios of KFB1 and KFC1 grains and their s-process Kr-86/Kr-82 ratios inferred from bulk noble gas analysis to model calculations of asymptotic giant branch (AGB) stars with a range of mass and metallicity. We conclude that KFB1 grains with C-12/C-13 >= 100 formed in the outflow of low-mass (1.5, 2 and 3 M-circle dot) low-metallicity (Z = 3 x 10(-3) for 1.5, 2 and 3M(circle dot), Z = 6 x 10(-3) for 3M(circle dot) only) AGB stars and that KFC1 grains with C-12/C-13 >= 60 formed in those stars as well as in 5M(circle dot) stars of solar and/or half-solar metallicities. Grains with C-12/C-13 < 20 in all the fractions seem to have multiple origins. Some of them formed in the ejecta of core-collapse supernovae. J stars and born-again AGB stars are also possible stellar sources. We calculated the abundances of graphite grains from supernovae and AGB stars in the Murchison meteorite to be 0.24 ppm and 0.44 ppm, respectively, whereas those of SiC grains from supernovae and AGB stars are 0.063 ppm and 5.6 ppm, respectively. In contrast to graphite, AGB stars are a dominant source of SiC grains. Since different mineral types have different residence times in the interstellar medium, their abundances in meteorites may not reflect original yields in stellar sources. Even if graphite grains are more easily destroyed than SiC grains, graphite grains from supernovae are more abundant than SiC grains from supernovae (0.24 ppm vs. 0.063 ppm), indicating that supernovae are a prolific producer of graphite grains. Graphite grains from AGB stars are less abundant than SiC grains from AGB stars (0.44 ppm vs. 5.6 ppm). This difference may reflect the difference in their parent stars: graphite grains formed in low-metallicity stars, while SiC grains formed in close-to-solar metallicity stars. (C) 2014 Elsevier Ltd. All rights reserved.