An investigation into the origin of Fe-rich presolar
silicates in Acfer 094
Presolar
silicate and oxide grains from primitive meteorites are recognized as
'stardust' on the basis of their extremely anomalous O isotopic
compositions. We report data on forty-eight O-anomalous grains
that were identified in grain size-separates of the ungrouped
carbonaceous chondrite Acfer 094. A majority of these grains
exhibit high 17O/16O ratios along with solar to
sub-solar 18O/16O ratios. and may have
originated in low-mass stars with close-to-solar metallicity.
Four silicate grains that contain 18O-enrichments were also
measured for their Si isotopes. A comparison of their O and Si
isotopic compositions with model predictions indicate that the 18O-rich
grains may have formed in supernova ejecta.
Four of the O-anomalous grains are oxides while the
remaining forty-four are silicates, based on elemental compositions
determined by Auger spectroscopy. The presolar oxides include a
TiO2 grain and a grain with spinel stoichiometry. The
silicate grains exhibit large ferromagnesian compositions, although a
few grains also contain small amounts of Ca and/or Al.
Stoichiometric silicates were further classified as either olivine-like
or pyroxene-like and in this study pyroxene-like grains are more
abundant than olivine-like ones.
The majority of silicates contain more Fe than Mg, including a
few grains with Fe-rich end-member compositions. Spectroscopic
observations indicate the presence of Mg-rich silicates in the
atmospheres of stars and the interstellar medium. Minerals such
as forsterite and enstatite form by equilibrium condensation in stellar
environments. Non-equilibrium condensation can result in higher
Fe contents and the occurrence of such processes in the outflows of
stars may account or the Fe-rich grains. Alternatively, secondary
processes may play a role in producing the Fe enrichments observed in
the presolar silicate garins identified in the matrix of Acfer 094.
Bose et al. (2009) Astrophys. J., submitted.
Full publication:
Last revised: 10-2009