Abstract
Carbon stars are an important source of presolar TiC, SiC, and graphite
grains found in meteorites. The elemental abundances in the stellar
sources of the SiC grains are -inferred by using condensation calculations.
These elemental abundances, together with C isotopic compositions,
are used to identify possible groups of carbon stars that may have
contributed SiC grains to the presolar dust cloud. The most likely
parent stars of meteoritic SiC mainstream grains are N-type carbon
stars and evolved subgiant CH stars. Both have s-process element
abundances higher than solar and 10 < C-12/C-13 < 100 ratios. The
J stars and giant CH stars, with solar and greater than solar abundances
of s-process elements, respectively, are good candidate parents for
the 'A' and 'B' SiC grains with low C-12/C-13 ratios. A special subgroup
of CH giant stars with very large C-12/C-13 ratios could have parented
the 'Y' SiC grains with C-12/C-13 ratios > 100. The carbon star population
(e.g., N, R, J, CH groups) needed to provide the observed SiC grains
is compared to the current population of carbon stars. This comparison
suggests that low-metallicity CH stars may have been more abundant
in the past (>4.5 Ga ago) than at present. This suggestion is also
supported by condensation-chemistry modeling of the trace element
patterns in the SiC grains that shows that subsolar Fe abundances
may be required in the stellar sources for many SiC grains. The results
of this study suggest that presolar SiC grains in meteorites can
provide information about carbon stars during galactic evolution.
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