Presolar graphite from the Murchison meteorite: An isotopic study

Abstract

We studied presolar graphite grains from four density fractions, KE3 (1.65–1.72 g/cm³), KFA1 (2.05–2.10 g/cm³), KFB1 (2.10–2.15 g/cm³), and KFC1 (2.15–2.20 g/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 15 M_⊙ 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 3 M_⊙) low-metallicity (Z = 3 × 10⁻³ for 1.5, 2 and 3 M_⊙, Z = 6 × 10⁻³ for 3 M_⊙ only) AGB stars and that KFC1 grains with ¹²C/¹³C ⩾ 60 formed in those stars as well as in 5 M_⊙ 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 0.24 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.

Introduction

Elemental and isotopic abundances of the Galaxy change with time. When stars reach the later stages of their lives, their nucleosynthetic products are expelled into space as gas and dust. Stars with masses less than 8 M_⊙ become asymptotic giant branch (AGB) stars and lose significant material as stellar wind during the thermally-pulsing AGB phase. Stars with masses ⩾ 8 M_⊙ become core-collapse supernovae and the nucleosynthetic products in these stars are distributed into space through explosion. Gas and dust expelled from these stars are eventually incorporated into molecular clouds. Our solar system, located at 8.5 kpc from the center of the Galaxy, formed from such a molecular cloud ∼4.6 billion years ago. The parent molecular cloud of the solar system was believed to have been completely homogenized during solar system formation (Cameron, 1962). This idea of an isotopically uniform solar system was reinforced by analyses of meteorites in the 50’s and 60’s, which showed uniform isotopic ratios (e.g., Podosek, 1978).

However, when Black and Pepin (1969) analyzed Ne in primitive meteorites by step-wise heating, they found a new ²²Ne-rich component (see Fig. 4 in Black and Pepin, 1969), which later was named Ne-E (Black, 1972). The lowest ²⁰Ne/²²Ne ratio (3.4, air: 9.8) observed in their experiment was such that it was difficult to explain the ²²Ne enrichment by processes that occurred in the solar system, thus a nucleosynthetic origin was proposed (Clayton, 1975). Subsequent studies indicated the presence of two kinds of Ne-E: Ne-E(H), released at high temperature (1200–1400 °C) and concentrated in high-density mineral separates (3–3.5 g/cm³), and Ne-E(L), released at low temperature (500–700 °C) and concentrated in low-density separates (<2.3 g/cm³) (Jungck, 1982).

Isotopically distinct components were identified also in heavy noble gases. Xe-HL is characterized by excesses in both light and p-process-only isotopes, 124 and 126, and heavy and r-process-only isotopes, 134 and 136 relative to solar (Lewis et al., 1975). Xe-S and Kr-S, with excesses in even-numbered isotopes, are components that show the signature of the slow neutron-capture process (the s-process) (Srinivasan and Anders, 1978). These Kr and Xe components were well hidden in bulk meteorites and their presence was detected only in chemically processed residues, after more than 99 percent of bulk meteorites had been dissolved. This indicates that minerals containing these noble gases have extremely low abundances in meteorites.

Edward Anders, Roy S. Lewis and their colleagues at The University of Chicago undertook the task to identify and isolate the minerals with the anomalous noble gas components (≡carriers). Without knowing the mineral types of these carriers, they used the anomalous noble gas components as a navigator. They separated small portions of meteorites from bulk meteorites by chemical and physical methods and analyzed noble gases in these fractions. Fractions enriched in anomalous noble gas components would be further processed to achieve a greater enrichment. This effort, on and off but spanning more than a decade, finally bore fruit when diamond, the carrier of Xe-HL, was isolated and identified from the Allende meteorite (Lewis et al., 1987). Soon the identification and isolation of silicon carbide (SiC), the carrier of Xe-S, Kr-S and Ne-E(H), followed (Bernatowicz et al., 1987, Tang and Anders, 1988), as did the isolation of graphite (Amari et al., 1990). Interestingly, all carriers of these anomalous noble gas components are carbonaceous and fairly resistant to chemicals. Otherwise, this method, “burning a haystack to find the needle” as Edward Anders once phrased it, would not have worked. A historical account of the discoveries of those noble gas components and their carriers has been given by Anders (1988).

Presolar grains identified to date include diamond (Lewis et al., 1987), SiC (Bernatowicz et al., 1987, Tang and Anders, 1988), graphite (Amari et al., 1990), oxides (Hutcheon et al., 1994, Huss et al., 1994, Nittler et al., 1994, Choi et al., 1999, Floss et al., 2008), silicon nitride (Si₃N₄) (Nittler et al., 1995), refractory carbides in SiC (Bernatowicz et al., 1992, Hynes et al., 2010) and graphite (Bernatowicz et al., 1991, Bernatowicz et al., 1996, Croat et al., 2003, Croat et al., 2010), and silicates (Messenger et al., 2003, Nguyen and Zinner, 2004, Nagashima et al., 2004, Mostefaoui and Hoppe, 2004, Haenecour et al., 2013). In addition, a variety of metal, carbide, oxide, sulfide and silicide subgrains were identified inside of SiC and graphite grains (Bernatowicz et al., 1996, Croat et al., 2003, Croat et al., 2008, Croat et al., 2013, Hynes et al., 2010, Hynes et al., 2011). Except for the three carbonaceous presolar grain types, they were identified by secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM). Abundances of presolar grains in meteorites range from ∼1000 ppm (diamond) to a few ppb (Si₃N₄). The isolation and identification of presolar grains have opened a new field of astronomy: we are able to study stardust in great detail with unprecedented precisions in the laboratory. Analyses of presolar grains have provided a wealth of information about nucleosynthesis in stars, mixing in stellar ejecta, and Galactic chemical evolution. Reviews of presolar grains and related topics can be found in various papers and book chapters (Bernatowicz and Zinner, 1997, Hoppe and Zinner, 2000, Clayton and Nittler, 2004, Zinner, 2004, Lodders and Amari, 2005, Davis, 2011, Zinner, 2013).

In this paper, we report the results of a comprehensive study of presolar graphite from the Murchison meteorite by SIMS. We will describe experimental procedures in Section 2, and results in Section 3. We will discuss the results and their implications in Section 4 and summarize our work in Section 5. Part of the data have already been reported by Amari et al., 1993, Amari et al., 1995c, Amari et al., 1996, Hoppe et al., 1995, Nittler et al., 1996, Travaglio et al., 1999, Croat et al., 2003, Heck et al., 2009a and Meier et al. (2012).