87 research outputs found
High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum
Corundum (-AlO) and amorphous or metastable
AlO are common components of circumstellar dust observed around
O-rich asymptotic giant branch (AGB) stars and found in primitive meteorites.
We report a detailed isotopic and microstructural investigation of a unique
presolar corundum grain, QUE060, identified in an acid residue of the Queen
Alexandra Range 97008 (LL3.05) meteorite. Based on its O and Mg isotopic
compositions, this 1.4 m diameter grain formed in a low- or
intermediate-mass AGB star. It has four developed rhombohedral 011
faces of corundum and a rough, rounded face with cavities. High Mg contents
(Mg/Al 0.004) are due to the decay of radioactive Al. No spinel
(MgAlO) inclusions that might have exsolved from the corundum are
observed, but there are several high-Mg domains with modulated structures. The
subhedral shape of grain QUE060 is the first clear evidence that corundum
condenses and grows to micrometer sizes in the extended atmospheres around AGB
stars. The flat faces indicate that grain QUE060 experienced little
modification by gas-grain and grain-grain collisions in the interstellar medium
(ISM) and solar nebula. The Mg distribution in its structure indicates that
grain QUE060 has not experienced any severe heating events since the exhaustion
of Al. However, it underwent at least one very transient heating event
to form the high-Mg domains. A possible mechanism for producing this transient
event, as well as the one rough surface and cavity, is a single grain-grain
collision in the ISM. These results indicate that grain QUE060 is the most
pristine circumstellar corundum studied to date
Determination of Interface Atomic Structure and Its Impact on Spin Transport Using Z-Contrast Microscopy and Density-Functional Theory
We combine Z-contrast scanning transmission electron microscopy with
density-functional-theory calculations to determine the atomic structure of the
Fe/AlGaAs interface in spin-polarized light-emitting diodes. A 44% increase in
spin-injection efficiency occurs after a low-temperature anneal, which produces
an ordered, coherent interface consisting of a single atomic plane of
alternating Fe and As atoms. First-principles transport calculations indicate
that the increase in spin-injection efficiency is due to the abruptness and
coherency of the annealed interface.Comment: 16 pages (including cover), 4 figure
Coordinated Analyses of Presolar Grains in the Allan Hills 77307 and Queen Elizabeth Range 99177 Meteorites
We report the identification of presolar silicates (~177 ppm), presolar
oxides (~11 ppm), and one presolar SiO2 grain in the Allan Hills (ALHA) 77307
chondrite. Three grains having Si isotopic compositions similar to SiC X and Z
grains were also identified, though the mineral phases are unconfirmed. Similar
abundances of presolar silicates (~152 ppm) and oxides (~8 ppm) were also
uncovered in the primitive CR chondrite Queen Elizabeth Range (QUE) 99177,
along with 13 presolar SiC grains and one presolar silicon nitride. The O
isotopic compositions of the presolar silicates and oxides indicate that most
of the grains condensed in low-mass red giant and asymptotic giant branch
stars. Interestingly, unlike presolar oxides, few presolar silicate grains have
isotopic compositions pointing to low-metallicity, low-mass stars (Group 3).
The 18O-rich (Group 4) silicates, along with the few Group 3 silicates that
were identified, likely have origins in supernova outflows. This is supported
by their O and Si isotopic compositions. Elemental compositions for 74 presolar
silicate grains were determined by scanning Auger spectroscopy. Most of the
grains have non-stoichiometric elemental compositions inconsistent with
pyroxene or olivine, the phases commonly used to fit astronomical spectra, and
have comparable Mg and Fe contents. Non-equilibrium condensation and/or
secondary alteration could produce the high Fe contents. Transmission electron
microscopic analysis of three silicate grains also reveals non-stoichiometric
compositions, attributable to non-equilibrium or multistep condensation, and
very fine scale elemental heterogeneity, possibly due to subsequent annealing.
The mineralogies of presolar silicates identified in meteorites thus far seem
to differ from those in interplanetary dust particles.Comment: 23 pages, 16 figure
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