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Amari, Sachiko

Publications and source records attributed to Amari, Sachiko.

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

SIMS chemical and isotopic analysis of impact features from LDEF experiments AO187-1 and AO187-2

Previous secondary ion mass spectrometry (SIMS) studies of extended impact features from LDEF capture cell experiment AO187-2 showed that it is possible to distinguish natural and man-made particle impacts based on the chemical composition of projectile residues. The same measurement technique has now been applied to specially prepared gold target impacts from experiment AO187-1 in order to identify the origins of projectiles that left deposits too thin to be analyzed by conventional energy-dispersive x-ray (EDX) spectroscopy. The results indicate that SIMS may be the method of choice for the analysis of impact deposits on a variety of sample surfaces. SIMS was also used to determine the isotopic compositions of impact residues from several natural projectiles. Within the precision of the measurements all analyzed residues show isotopically normal compositions.

Stadermann, Frank J.↗
NASA NTRS1994

Carbon, nitrogen, magnesium, silicon, and titanium isotopic compositions of single interstellar silicon carbide grains from the Murchison carbonaceous chondrite

Seven hundred and twenty SiC grains from the Murchison CM2 chondrite, ranging in size from 1 to 10 micrometers, were analyzed by ion microprobe mass spectrometry for their C-isotopic compositions. Subsets of the grains were also analyzed for N (450 grains), Si (183 grains), Mg (179 grains), and Ti (28 grains) isotopes. These results are compared with previous measurements on 41 larger SiC grains (up to 15 x 26 micrometers) from a different sample of Murchison analyzed by Virag et al. (1992) and Ireland, Zinner, & Amari (1991a). All grains of the present study are isotopically anomalous with C-12/C-13 ratios ranging from 0.022 to 28.4 x solar, N-14/N-15 ratios from 0.046 to 30 x solar, Si-29/Si-28 from 0.54 to 1.20 x solar, Si-30/Si-28 from 0.42 to 1.14 x solar, Ti-49/Ti-48 from 0.96 to 1.95 x solar, and Ti-50/Ti-48 from 0.94 to 1.39 x solar. Many grains have large Mg-26 excesses from the decay of Al-26 with inferred Al-26/Al-27 ratios ranging up to 0.61, or 12,200 x the ratio of 5 x 10(exp -5) inferred for the early solar system. Several groups can be distinguished among the SiC grains. Most of the grains have C-13 and N-14 excesses, and their Si isotopic compositions (mostly excesses in Si-29 and Si-30) plot close to a slope 1.34 line on a Delta Si-29/Si-28 versus Delta Si-30/Si-28 three-isotope plot. Grains with small C-12/C-13 ratios (less than 10) tend to have smaller or no N-14 excesses and high Al-26/Al-27 ratios (up to 0.01). Grains with C-12/C-13 greater than 150 fall into two groups: grains X have N-15 excesses and Si-29 and Si-30 deficits and the highest (0.1 to 0.6) Al-26/Al-27 ratios; grains Y have N-14 excesses and plot on a slope 0.35 line on a Si three-isotope plot. In addition, large SiC grains of the Virag et al. (1992) study fall into three-distinct clusters according to their C-, Si-, and Ti-isotopic compositions. The isotopic diversity of the grains and the clustering of their isotopic compositions imply distinct and multiple stellar sources. The C- and N-isotopic compositions of most grains are consistent with H-burning in the CNO cycle. These and s-process Kr, Xe, Ba, and Nd suggest asymptotic giant branch (AGB) or Wolf-Rayet stars as likely sources for the grains, but existing models of nucleosynthesis in these stellar sites fail to account in detail for all the observed isotopic compositions. Special problems are posed by grains with C-12/C-13 less than 10 and almost normal and heavy N-isotopic compositions. Also the Si- and Ti-isotopic compositions, with excesses in Si-29 and Si-30 relative to Si-28 and excesses in all Ti isotopes relative to Ti-48, do not precisely conform with the compositions predicted for slow neutron capture. Additional theoretical efforts are needed to achieve an understanding of the isotopic composition of the SiC grains and their stellar sources.

Hoppe, Peter↗
NASA NTRS1993

Fingerprints of carbon, nitrogen, and silicon isotopes in small interstellar SiC grains from the Murchison meteorite

We report ion microprobe determinations of the carbon, nitrogen, and silicon isotopic compositions of small SiC grains from the Murchison CM2 chondrite. Analyses were made on samples containing variable numbers of grains and on 14 individual grains. In some cases the multiple-grain sample compositions were probably dominated by only one or two grains. Total ranges observed are given. Only a few grains show values near the range limits. Both the total ranges of carbon and nitrogen isotopic compositions, and even the narrower ranges typical for the majority of the grains, are similar to those observed for larger SiC grains. Two rare components appear to be present in the smaller-size fraction, one characterized by C-12/C-13 about 12-16 and the other by very heavy nitrogen. The carbon and nitrogen isotopic compositions qualitatively may reflect hydrostatic H-burning via the CNO cycle and He-burning in red giants, as well as explosive H-burning in novae. The silicon isotopic compositions of most grains qualitatively show what is the signature of He-burning. The silicon isotopic composition of one grain, however, suggests a different process.

Hoppe, Peter↗
NASA NTRS1993

Interstellar graphite in Murchison: Continued search for isotopically distinct components

We have measured C-, N-, and O-isotopic ratios of individual graphite grains from the Murchison density fractions KFB1 (2.1-2.15 g/cc) and KFC1 (2.15 -2.20 g/cc). According to the C-isotopic composition, KFC1 consists of two populations: one has light C while the other has heavy C. In addition to these two populations, KFB1 also contains grains whose C-isotopic ratios are approximately normal. In view of these isotopic distributions, it is difficult to understand why the Kr-isotopic compositions of these two separates are so different. In contrast to the lighter density fractions KE1 (1.6-2.05 g/cc) and KFA1 (2.05-2.10 g/cc), the separates KFB1 and KFC1 do not seem to contain many grains with significant O-18 excess.

Amari, Sachiko↗
NASA NTRS1992

Interstellar SiC with unusual isotopic compositions - Grains from a supernova?

Results are presented from an ion microprobe mass spectrometric analyses of five SiC grains from the Murchison carbonaceous meteorite. Unlike most interstellar SiC grains from primitive meteorites, the five grains from the Murchison meteorite show large excesses of C-12 (up to 28 times solar) and N-15 (up to 22 times solar), depletion in Si-29 and Si-30 (up to 59 percent), Al-26/Al-27 ratios between 0.1 and 0.6, and Ti-49 excesses up to 95 percent; in addition, one grain has a large Ca-44 excess (300 percent). The Ca and Ti anomalies point toward explosive nucleosynthesis in supernovae and the in situ decay of the radioactive precursors Ti-44 and V-49 in SiC grains formed in supernova ejecta. However, there is no simple formation scenario that can give a consistent explanation for the isotopic compositions of these grains.

Amari, Sachiko↗
NASA NTRS1992

Isotopic, optical, and trace element properties of large single SiC grains from the Murchison meteorite

Ion probe mass spectrometry is used to analyze 41 large SiC grains from the Murchison CM2 chondrite, ranging up to 15 x 26 microns, for the isotopic compositions of C, N, Mg, and Si, and the concentrations of Al, Ti, V, Fe, Zr, and Ba. The majority were found to have large isotopic anomalies. Only two grains, characterized by extremely heavy carbon, give evidence for fossil Mg-26. On the basis of C and Si isotopic composition, 29 of the grains fall into three compact clusters, presumably from three discrete sources. The clustering of coarse-grained SiC stands in sharp contrast to the quasi-continuous distribution of finer-grained SiC and suggests that the top 0.1 percent of the mass distribution is a distinct population. The C and N isotopic compositions of the anomalous grains are not very diagnostic, being consistent with H-burning in the CNO cycle. The Si-isotopic compositions qualitatively show the signature of neutron capture in He-burning shells of highly evolved stars.

Virag, Alois↗
NASA NTRS1992

SIMS chemical analysis of extended impact features from the trailing edge portion of experiment AO187-2

One hundred capture cells from the trailing edge, which had lost their cover foils during flight, were optically scanned for extended impact features caused by high velocity projectiles impinging on the cells while the foils were still intact. Of the 53 candidates, 24 impacts were analyzed by secondary ion mass spectroscopy for the chemical composition of the deposits. Projectile material was found in all impacts, and at least 75 percent of them appear to be caused by interplanetary dust particles. Elemental ratios are fractionated, with refractory elements enriched in the impacts relative to interplanetary dust particles collected in the stratosphere. Although this could be due to systematic differences in the compositions, a more likely explanation is volatility fractionation during the impact process.

Amari, Sachiko↗
NASA NTRS1992

TEM studies of a circumstellar rock

Reported here is the discovery of crystals of titanium carbide in a grain of silicon carbide which formed as a circumstellar dust particle in the atmosphere of a carbon-rich star. Just as in the case of terrestrial rocks, whose assemblage of minerals gives us clues to the composition and conditions of the environment in which they formed, the titanium carbide crystals and their textural relationship to the silicon carbide give us important clues to the nature of the stellar atmosphere in which they formed. From microscopic studies of the relationships between the atomic planes of the silicon carbide and the titanium carbide, we can show that the titanium carbide cannot have existed as already-formed crystals in a gas around which silicon carbide subsequently condensed. An alternative possibility is that both minerals grew quickly and simultaneously from condensing gas in the rapidly cooling and expanding stellar atmosphere. Other microscopic features of the silicon carbide, such as abundant atomic layer disorder and crystal twinning, similarly suggest rapid grain growth. However, another possibility is that the titanium carbide grew inside of the silicon carbide by diffusion of titanium atoms. Our calculations suggest that this scenario is less likely, given the relatively short times (a year or less) for which stellar condensates can be expected to be exposed to temperatures high enough to make diffusion sufficiently rapid.

Bernatowicz, Thomas J.↗
NASA NTRS1991

s-process Ba, Nd, and Sm in presolar SiC from the Murchison meteorite

Ion microprobe isotopic measurements of Ba, Nd, and Sm in the K-series SiC size separates on which noble gas measurements were made by Lewis et al. (1990) are reported. All elements show isotopic abundance patterns characteristic for the s-process. The Ba-134/Ba-136 ratios are distinct from solar, indicating that s-Ba in SiC is different from that in the solar system. Ba-138/Ba-136 ratios decrease with grain size; if interpreted as being due to different neutron exposures, this trend is opposite of that shown by the Kr-86/Kr-82 ratios. Although other isotropic ratios for Ba and those for Nd and Sm differ in detail from theoretical predictions, the disagreements probably being due to uncertainties in the n-capture cross sections and simplifications in the models, the general agreement of the data with models of s-process nucleosynthesis support an AGB star origin for the relatively fine-grain SiC under consideration. Ba-136 in SiC is up to 85 percent pure s-process.

Zinner, Ernst↗
NASA NTRS1991

An ion microprobe study of corundum in the Murchison meteorite - Implications for Al-26 and O-16 in the early solar system

Results are presented from measurements of O, Mg, and Ti isotopes and the abundances of Mg, Ca, Sc, Ti, V, Sr, Y, Zr, La, and Ce in 26 individual corundum grains from the Murchison CM2 chondrite. The results provide further evidence for live Al-26 in the early solar system and set strict upper limits on the amounts of fossil Mg-26. The analyses suggest that corundum in this chondrite formed from two components with live Al-26 and a mass fraction of 43 percent dead Al.

Virag, Alois↗
NASA NTRS1991

Interstellar grains within interstellar grains

Five interstellar graphite spherules extracted from the Murchison carbonaceous meteorite are studied. The isotopic and elemental compositions of individual particles are investigated with the help of an ion microprobe, and this analysis is augmented with structural studies of ultrathin sections of the grain interiors by transmission electron microscopy. As a result, the following procedure for the formation of the interstellar graphite spherule bearing TiC crystals is inferred: (1) high-temperature nucleation and rapid growth of the graphitic carbon spherule in the atmosphere of a carbon-rich star, (2) nucleation and growth of TiC crystals during continued growth of the graphitic spherule and the accretion of TiC onto the spherule, (3) quenching of the graphite growth process by depletion of C or by isolation of the spherule before other grain types could condense.

Bernatowicz, Thomas J.↗
NASA NTRS1991

Large amounts of extinct Al-26 in interstellar grains from the Murchison meteorite

It is reported here that interstellar graphite and SiC grains recovered from the Murchison CM2 chondritic meteorite have large abundances of Mg-26 from the decay of extinct Al-26. The deduced initial Al-26/Al-27 ratios range up to 0.06 in graphite and 0.2 in SiC. This is 1200 to 4000 times the maximum values found in refractory inclusions in primitive meteorites. All proposed stellar sources of carbonaceous dust also produced Al-26, but the highest Al-26/Al-27 ratios found in these grains seem to rule out Wolf-Rayet stars and supernovae. The aluminum abundance correlates with that of nitrogen, suggesting that the aluminum condensed as aluminum nitride.

Zinner, Ernst↗
NASA NTRS1991

Instellar grains within interstellar grains

The discovery of crystals of titanium carbide in an interstellar graphite spherule is reported. The new species is particularly interesting in that it came in a protective wrapping (the graphite spherule) which eliminated the possibility of chemical alteration during its residence in the interstellar medium and in the meteorite in which it was discovered.

Bernatowicz, Thomas J.↗
NASA NTRS1990

Meteoritic silicon carbide - Pristine material from carbon stars

All five gases in interstellar silicon carbide grains have grossly nonsolar isotopic and elemental abundances that vary with grain size but are strikingly similar to calculated values for the helium-burning shell of low-mass carbon stars. Apparently these grains formed in carbon-star envelopes, and were impregnated with noble gas ions from a stellar wind. Meteoritic SiC provides a detailed record of nuclear and chemical processes in carbon stars.

Lewis, Roy S.↗
NASA NTRS1990

Interstellar graphite in meteorites

Graphite grains 1-4 microns in diameter have been identified in the Murchison C2 chondrite. The interstellar origin of these grains is demonstrated by their C-12/C-13 ratio, which ranges from 0.09 to 16 times the solar system value, and by the presence of nearly monoisotopic Ne-22 from the decay of Na-22. The grains apparently formed in the outflows of n novae and red giants, and demonstrate that graphite can form as a circumstellar condensate.

Amari, Sachiko↗