Aluminum-, calcium- and titanium-rich oxide stardust in
ordinary chondrite meteorites.
Nittler L. R., Alexander C. M. O'D., Gallino R., Hoppe P.,
Nguyen A. N., Stadermann F. J., and Zinner E. K. (2008)
Astrophy. J., in press.
ABSTRACT
We report O-, Al-Mg, K, Ca, and Ti isotopic data for a total
of 96 presolar oxide grains found in residues of several unequilibrated
ordinary chondrite meteorites. Identified grain types include Al2O3,
MgAl2O4, hibonite (CaAl12O19)
and Ti oxide. This work greatly increases the presolar hibonite
database, and is the first report of presolar Ti oxide. O-isotopic
compositions of the grains span previously observed ranges and indicate
an origin in red giant and asymptotic giant branch (AGB) stars of low
mass (<2.5 Msolar) for most grains. Cool bottom
processing in the parent AGB stars is required to explain isotopic
compositions of many grains. Potassium-41 enrichments in hibonite
grains are attributable to in situ decay of now-extinct 41Ca.
Inferred initial 41Ca/40Ca ratios are in good
agreement with model predictions for low-mass AGB star envelopes,
provided that ionization suppresses 41Ca decay. Stable Mg
and Ca isotopic ratios of most of the hibonite grains reflect primarily
the initial compositions of the parent stars and are generally
consistent with expectations for Galactic chemical evolution, but
require some local interstellar chemical inhomogeneity. Very high 17O/16O
or 25Mg/24Mg ratios suggest an origin for some
grains in binary star systems where mass transfer from an evolved
companion has altered the parent star compositions. A supernova origin
for the hitherto enigmatic 18O-rich Group 4 grains is
strongly supported by multi-element isotopic data for two grains. The
Group 4 data are consistent with an origin in a single supernova in
which variable amounts of material from the deep 16O-rich
interior mixed with a unique end-member mixture of the outer layers.
The Ti oxide grains primarily formed in low-mass AGB stars. They are
smaller and rarer than presolar Al2O3, reflecting
the lower abundance of Ti than Al in AGB envelopes.
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