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Internal grains within KFC graphites:
Implications for their stellar source.
Croat T. K., Stadermann F. J., and Bernatowicz T. J.
(2005)
Lunar & Planet. Sci. XXXVI, Abstract #1507.
ABSTRACT
Despite the assumed contribution of AGB carbon stars to
the presolar grains population, few Murchison KFC graphites
(about 4%) appear in the 40 <=
12C/13C <= 70 range typically
observed in the photospheres of AGB stars and in isotopic
measurements of presolar silicon carbides. Instead, most
have lighter carbon (75% with 12C/13C
> 90) and a significant group has very heavy carbon (15%
between 0 <= 12C/13C <= 20). On
the basis of bulk isotopic composition alone, a supernova
(SN) origin was thought possible for many of the
isotopically light carbon graphites, as they fall into the
same carbon isotopic range as most SiC-X grains. Recent
coordinated TEM and NanoSIMS studies of the same graphite
population have shown that many in this light carbon group
also contain refractory carbides with s-process enrichments
(high Zr, Mo and Ru), indicative of an AGB origin. There is
a much smaller subset which likely do have a SN origin,
namely those containing pure TiCs without s-process elements
or other rare phases (such as iron, iron carbide, and
metallic osmium) unlikely to form before graphite around AGB
stars.
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