797 research outputs found
Automated NanoSIMS Measurements of Spinel Stardust from the Murray Meteorite
We report new O isotopic data on 41 presolar oxide grains, 38 MgAl2O4
(spinel) and 3 Al2O3 from the CM2 meteorite Murray, identified with a recently
developed automated measurement system for the NanoSIMS. We have also obtained
Mg-Al isotopic results on 29 of the same grains (26 spinel and 3 Al2O3). The
majority of the grains have O isotopic compositions typical of most presolar
oxides, fall well into the four previously defined groups, and are most likely
condensates from either red giant branch or asymptotic giant branch stars. We
have also discovered several grains with more unusual O and Mg compositions
suggesting formation in extreme astrophysical environments, such as novae and
supernovae. One of these grains has massive enrichments in 17O, 25Mg, and 26Mg,
which are isotopic signatures indicative of condensation from nova ejecta. Two
grains of supernova origin were also discovered: one has a large 18O/16O ratio
typical of Group 4 presolar oxides; another grain is substantially enriched in
16O, and also contains radiogenic 44Ca from the decay of 44Ti, a likely
condensate from material originating in the O-rich inner zones of a Type II
supernova. In addition, several Group 2 presolar spinel grains also have large
25Mg and 26Mg isotopic anomalies that are difficult to explain by standard
nucleosynthesis in low-mass stars. Auger elemental spectral analyses were
performed on the grains and qualitatively suggest that presolar spinel may not
have higher-than- stoichiometric Al/Mg ratios, in contrast to SIMS results
obtained here and reported previously.Comment: 58 pages, 10 figures, 1 table, published in Ap
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Isotopic and Micro-Raman investigation of Interplanetary Dust Particles Collected during 2003 Earth passage through Comet Grigg-Skjellerup Dust Stream
We report microscale H and N isotopic and Raman spectral data for IDPs collected in April 2003. The samples show extreme D and 15N enrichments carried by very primitive organic matter. A high abundance of D anomalies might indicate a cometary origin
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Remarkably high abundance of presolar grains in interplanetary dust particles collected from the comet Grigg-Skjellerup dust stream
Isotopic studies of IDPs collected from the comet Grigg-Skjellerup dust stream reveal extremely high abundances of presolar grains in two of four IDPs. These abundances exceed those of any other extraterrestrial material analyzed and support a cometary origin for these IDPs
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
On Presolar Stardust Grains from CO Classical Novae
About 30% to 40% of classical novae produce dust 20-100 days after the
outburst, but no presolar stardust grains from classical novae have been
unambiguously identified yet. Although several studies claimed a nova paternity
for certain grains, the measured and simulated isotopic ratios could only be
reconciled assuming that the grains condensed after the nova ejecta mixed with
a much larger amount of close-to-solar matter. However, the source and
mechanism of this potential post-explosion dilution of the ejecta remains a
mystery. A major problem with previous studies is the small number of
simulations performed and the implied poor exploration of the large nova
parameter space. We report the results of a different strategy, based on a
Monte Carlo technique, that involves the random sampling over the most
important nova model parameters: the white dwarf composition; the mixing of the
outer white dwarf layers with the accreted material before the explosion; the
peak temperature and density; the explosion time scales; and the possible
dilution of the ejecta after the outburst. We discuss and take into account the
systematic uncertainties for both the presolar grain measurements and the
simulation results. Only those simulations that are consistent with all
measured isotopic ratios of a given grain are accepted for further analysis. We
also present the numerical results of the model parameters. We identify 18
presolar grains with measured isotopic signatures consistent with a CO nova
origin, without assuming any dilution of the ejecta. Among these, the grains
G270 2, M11-334-2, G278, M11-347-4, M11-151-4, and Ag2 6 have the highest
probability of a CO nova paternity.Comment: 8 figure
On the Mass and Metallicity Distributions of the Parent AGB Stars of O-rich Presolar Stardust Grains
Presolar grains in meteorites formed in a sample of AGB stars that ended
their lives within ~1 Gyr of the origin of the Solar System 4.6 Gyr ago. The
O-isotopic compositions of presolar O-rich stardust reflect the masses and
metallicities of their parent stars. We present simple Monte Carlo simulations
of the parent AGB stars of presolar grains. Comparison of model predictions
with the grain data allow some broad conclusions to be drawn: 1) Presolar
O-rich grains formed in AGB stars of mass ~1.15 - 2.2 MSun. The upper-mass
cutoff reflects dredge-up of C in more massive AGB stars, leading to C-rich
dust rather than O-rich, but the lack of grains from intermediate-mass AGB
stars (>4MSun) is a major puzzle. 2) The grain O-isotopic data are reproduced
well if the Galaxy in presolar times was assumed to have a moderate
age-metallicity relationship, but with significant metallicity scatter for
stars born at the same time. 3) The Sun appears to have a moderately low
metallicity for its age and/or unusual 17O/16O and 18O/16O ratios for its
metallicity. 4) The Solar 17O/18O ratio, while unusual relative to present-day
molecular clouds and protostars, was not atypical for the presolar disk and
does not require self-pollution of the protosolar molecular cloud by supernova
ejecta.Comment: 8 pages, 5 figures; accepted for publication in the Publications of
the Astronomical Society of Australi
DECODING THE MESSAGE FROM METEORITIC STARDUST SILICON CARBIDE GRAINS
Micron-sized stardust grains that originated in ancient stars are recovered from meteorites and analyzed using high-resolution mass spectrometry. The most widely studied type of stardust is silicon carbide (SiC). Thousands of these grains have been analyzed with high precision for their Si isotopic composition. Here we show that the distribution of the Si isotopic composition of the vast majority of stardust SiC grains carries the imprints of a spread in the age-metallicity distribution of their parent stars and of a power-law increase of the relative formation efficiency of SiC dust with the metallicity. This result offers a solution for the long-standing problem of silicon in stardust SiC grains, confirms the necessity of coupling chemistry and dynamics in simulations of the chemical evolution of our Galaxy, and constrains the modeling of dust condensation in stellar winds as a function of the metallicity
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