Presolar graphite from the Murchison meteorite: An isotopic study

Abstract

We studied presolar graphite grains from four density fractions, KE3 (1.65–1.72g/cm3), KFA1 (2.05–2.10g/cm3), KFB1 (2.10–2.15g/cm3), and KFC1 (2.15–2.20g/cm3), 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 15N and 18O excesses, Si isotopic anomalies, high 26Al/27Al ratios (∼0.1), and Ca and Ti isotopic anomalies, including the initial presence of short-lived 41Ca and 44Ti. 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 (18O/16O) and Al isotopic ratios of the KE3 grains, using 15M⊙ model calculations by Rauscher et al. (2002). Isotopic ratios of grains with high 12C/13C ratios (>200) can be reproduced, whereas those of grains with ratios ⩽200 are hard to explain if we assume that graphite grains form in C-rich conditions.We compared the distributions of the 12C/13C ratios of KFB1 and KFC1 grains and their s-process 86Kr/82Kr 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 12C/13C⩾100 formed in the outflow of low-mass (1.5, 2 and 3M⊙) low-metallicity (Z=3×10−3 for 1.5, 2 and 3M⊙, Z=6×10−3 for 3M⊙ only) AGB stars and that KFC1 grains with 12C/13C⩾60 formed in those stars as well as in 5M⊙ stars of solar and/or half-solar metallicities. Grains with 12C/13C<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 0.24ppm and 0.44ppm, respectively, whereas those of SiC grains from supernovae and AGB stars are 0.063ppm and 5.6ppm, 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 (0.24ppm vs. 0.063ppm), indicating that supernovae are a prolific producer of graphite grains. Graphite grains from AGB stars are less abundant than SiC grains from AGB stars (0.44ppm vs. 5.6ppm). 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.