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Circumstellar Fe oxide from the Acfer 094
carbonaceous chondrite.
Floss C., Stadermann F. J., and Bose M. (2008)
Astrophys. J. 672, 1266-1271.
ABSTRACT
We report the discovery of a unique Fe- and O-bearing
circumstellar grain from the Acfer 094 ungrouped
carbonaceous chondrite. The grain has close to solar
17O/16O and an
18O/16O ratio that is 1.34 times the
solar value. Iron isotopic compositions show depletions of
100 - 200 permil in both 54Fe and
57Fe, relative to 56Fe. No evidence of
excess 60Ni from the decay of extinct
60Fe was observed. Auger elemental spectra show
that the grain is compositionally similar to wüstite
(FeO), but may contain a small amount of Mg in addition to
Fe and O. The solid solution series magnesiowüstite,
(Mg,Fe)O, is predicted to form under non-equilibrium
conditions in oxygen-rich AGB stars with low mass-loss
rates, and Fe-rich magnesiowüstite has been proposed as
the carrier of the 19.5 µm feature observed in the
spectra of certain low-mass-loss AGB stars that show little
silicate emission. Although the isotopic data are ambiguous,
these observations argue in favor of an AGB source for this
Fe oxide grain.
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