Study on presolar grains including diamond, silicon carbide, graphite, silicon nitrite (Si\-3N\-4), corundum and spinel isolated from meteorites is summarized in this paper. Except for nanometer\|sized diamond, the ot...Study on presolar grains including diamond, silicon carbide, graphite, silicon nitrite (Si\-3N\-4), corundum and spinel isolated from meteorites is summarized in this paper. Except for nanometer\|sized diamond, the other grains are micrometers to submicrometers in size. The presolar grains survived mainly in the fine \|grained matrix of primitive chondrites and were isolated by chemical treatments. Diamond contains Xe isotopes (Xe\|HL), typically produced in p\| and r\|processes, probably formed in supernovae. Mainstream silicon carbides are enriched in \{\{\}\+\{29,30\}Si\} and \{\{\}\+\{13\}C\}, but depleted in \{\{\}\+\{15\}N\}. They also contain various s\|process products, consistent with calculations of AGB stars. Other silicon carbides exhibit much larger isotopic anomalies and are classified as groups X, Y, Z and AB. Among them, group X of SiC is characterized by enrichment of \{\{\}\+\{28\}Si\} and daughter isotopes of various short\|lived nuclides, suggesting an origin from supernovae. Graphite can be divided into four density fractions with distinct isotopic compositions. They may form in AGB stars, novae and supernovae, respectively. Si\-3N\-4 is similar to X\|SiC in isotopic composition. Corundum is classified as four groups based on their oxygen isotopic compositions. AGB and red giant stars are possible sources for the oxide. More comprehensive study of presolar grains, especially discovery of the other types of oxides and silicates, isotopic analyses of individual submicrometer\|sized grains and distribution of presolar grains among various chemical groups and petrographic types of chondrites will provide new information on nucleosynthesis, stellar evolution and formation of the solar nebula.展开更多
文摘Study on presolar grains including diamond, silicon carbide, graphite, silicon nitrite (Si\-3N\-4), corundum and spinel isolated from meteorites is summarized in this paper. Except for nanometer\|sized diamond, the other grains are micrometers to submicrometers in size. The presolar grains survived mainly in the fine \|grained matrix of primitive chondrites and were isolated by chemical treatments. Diamond contains Xe isotopes (Xe\|HL), typically produced in p\| and r\|processes, probably formed in supernovae. Mainstream silicon carbides are enriched in \{\{\}\+\{29,30\}Si\} and \{\{\}\+\{13\}C\}, but depleted in \{\{\}\+\{15\}N\}. They also contain various s\|process products, consistent with calculations of AGB stars. Other silicon carbides exhibit much larger isotopic anomalies and are classified as groups X, Y, Z and AB. Among them, group X of SiC is characterized by enrichment of \{\{\}\+\{28\}Si\} and daughter isotopes of various short\|lived nuclides, suggesting an origin from supernovae. Graphite can be divided into four density fractions with distinct isotopic compositions. They may form in AGB stars, novae and supernovae, respectively. Si\-3N\-4 is similar to X\|SiC in isotopic composition. Corundum is classified as four groups based on their oxygen isotopic compositions. AGB and red giant stars are possible sources for the oxide. More comprehensive study of presolar grains, especially discovery of the other types of oxides and silicates, isotopic analyses of individual submicrometer\|sized grains and distribution of presolar grains among various chemical groups and petrographic types of chondrites will provide new information on nucleosynthesis, stellar evolution and formation of the solar nebula.
