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Presolar graphite from AGB stars: Microstructure
and s-process enrichment.
Croat T. K., Stadermann F. J., and Bernatowicz T. J.
(2005) Astrophys. J. 631, 976-987.
ABSTRACT
Correlated transmission electron microscopy and secondary
ion mass spectrometry with submicron spatial resolution
(NanoSIMS) investigations of the same presolar graphites
spherules from the Murchison meteorite were conducted, to
link the isotopic anomalies with the mineralogy and chemical
composition of the graphite and its internal grains.
Refractory carbide grains (especially titanium carbide) are
commonly found within the graphite spherules, and most have
significant concentrations of Zr, Mo, and Ru in solid
solution, elements primarily produced by s-process
nucleosynthesis. The effect of chemical fractionation on the
Mo/Ti ratio in these carbides is limited, and therefore from
this ratio one can infer the degree of s-process
enrichment in the gas from which the graphite condensed. The
resulting s-process enrichments within carbides are large (~
200 times solar on average), showing that most of the
carbide-containing graphites formed in the mass outflows of
asymptotic giant branch (AGB) stars. NanoSIMS measurements
of these graphites also show isotopically light carbon
(mostly in the 100<12C/13C < 400
range). The enrichment of these presolar graphites in both
s-process elements and 12C considerably
exceeds that astronomically observed around carbon stars.
However, a natural correlation exists between 12C
and s-process elements, as both form in the He intershell
region of thermally pulsing AGB stars and are dredged up
together to the surface. Their observation together suggests
that these graphites may have formed in chemically and
isotopically inhomogeneous regions around AGB stars, such as
high-density knots or jets. As shown in the companion paper,
a gas density exceeding that expected for smooth mass
outflows is required for graphite of the observed size to
condense at all in circumstellar environments, and the
spatially inhomogeneous, high-density regions from which
they condense may also be incompletely mixed with the
surrounding gas. We have greatly expanded the available data
set of presolar graphites (N = 847) and characterized
them by their morphology (onion type and cauliflower type).
This effort has also revealed two new, rare presolar phases
(iron carbide and metallic osmium). Due to the peculiar gas
composition needed to form these rare presolar grain types,
the graphites containing them are more likely to originate
in supernova outflows.
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