科研之友
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摘要
We report C, N, O, Si, Al-Mg, K, Ca, and Ti isotopic analyses of presolar graphite grains from the Orgueil CI chondrite. NanoSIMS isotopic measurements were made on 345 grains from seven density fractions, with grain sizes %26gt;1 mu m: low-density grains from OR1b, OR1c, and OR1d; and high-density grains from OR1f, OR1g, OR1h, and OR1i. In all fractions, except OR1b and OR1h, we found presolar graphite as demonstrated by the large range of C-12/C-13 ratios (4-2480) measured in individual grains. Some isotopic properties are dependent on density: low-density grains contain O-18, N-15, and Si-28 excesses, while the majority of high-density grains contain normal N and O, and are generally enriched in Si-29 and Si-30. The N-15, O-18, and Si-28 excesses and very high derived isotopic ratios for the extinct radionuclides Al-26, Ca-41, and Ti-44 in low-density grains indicate an origin from supernovae. In order to explain the isotopic ratios measured in these grains, we present mixing scenarios between different layers of supernovae and discuss the limitations of various theoretical models. Silicon-30 and C-12 excesses in high-density grains and lower values for short-lived radionuclides (Al-26 and Ca-41) indicate an origin in asymptotic giant branch stars with low metallicities. Some supernova grains, with Ca-44 excesses, are also present amongst the high-density grains. Grains with low C-12/C-13 ratios (without evidence for Ti-44) and large excesses in Ca-42,Ca-43 and Ti-46,Ti-47,Ti-49,Ti-50 probably originate from post-asymptotic giant branch stars, that have suffered a very late thermal pulse, and can achieve low C-12/C-13 ratios and large neutron capture signatures in Ca and Ti isotopes. %26lt;br%26gt;We conclude that most low-density graphite grains originate from supernovae while high-density graphite grains have multiple stellar sources: low-metallicity and born-again asymptotic giant branch stars, Type II supernovae, and possibly, J-type stars.
- 出版日期2013-7-15
