Sachiko Amari

Interstellar grains in meteorites: I. Isolation of SiC, graphite and diamond; size distributions of SiC and graphite

A procedure has been developed for isolating three types of interstellar grains from primitive meteorites, in >90% purity and yields of generally ≥70%, and is here applied to the Murchison C2M meteorite. Silicates are dissolved in HF-HCl, kerogen (macromolecular organic matter) is destroyed by Cr2O7=, KOH, and H2O2, and microdiamonds (~400 ppm) are recovered as a colloid. Graphite (<1 ppm) is isolated by density and size separations. Spinel in the residue is dissolved in H2SO4, leaving SiC (~6 ppm), hibonite, and corundum. The size distribution of SiC has been measured in the range 0.2 to 6 μm. Over part of this range, it can be fit either to a power-law or a log-normal distribution, but the deficiency of small grains strongly favors the latter. Statistics are more limited for graphite spherules, but they, too, follow a log-normal distribution, and so does interstellar diamond (Lewis et al., 1989). Apparently the primary condensation process in stellar atmospheres consistently yields a log-normal distribution. The power-law distribution commonly inferred for interstellar grains may have been produced by secondary processes such as fragmentation.

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.

CONSTRAINTS ON STELLAR GRAIN FORMATION FROM PRESOLAR GRAPHITE IN THE MURCHISON METEORITE

We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3E9 km) extracted from the Murchison meteorite. The spherules have 12C/13C ratios ranging over 3 orders of magnitude (from 0.02 to 80 times solar), clearly establishing their presolar origin as stellar condensates. These and other isotopic constraints point to a variety of stellar types as sources of the carbon, including low-mass asymptotic giant branch (AGB) stars and supernovae. Transmission elec- tron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite struc- ture consisting of a core of nanocrystalline carbon surrounded by a mantle of well-graphitized carbon. The nanocrystalline cores are compact masses consisting of randomly oriented graphene sheets, from PAH-sized units up to sheets 3E4 nm in diameter, with little graphitic layering order. These sheets prob- ably condensed as isolated particles that subsequently coalesced to form the cores, after which the sur- rounding graphitic mantles were added by vapor deposition. We also detected internal crystals of metal carbides in one-third of the spherules. These crystals (5E200 nm) have compositions ranging from nearly pure TiC to nearly pure Zr-Mo carbide. Some of these car- bides occur at the centers of the spherules and are surrounded by well-graphitized carbon, having evi- dently served as heterogeneous nucleation centers for condensation of carbon. Others were entrained by carbon as the spherules grew. The chemical and textural evidence indicates that these carbides formed prior to carbon condensation, which indicates that the C/O ratios in the stellar sources were very close to unity. Only one of the 67 spherules studied in the TEM contained SiC, from which we infer that carbon condensation nearly always preceded SiC formation. This observation places stringent limits on the possible delay of graphite formation and is consistent with the predictions of equilibrium thermody- namics in the inferred range of pressure and C/O ratios. We model the formation of the observed refractory carbides under equilibrium conditions, both with and without s-process enrichment of Zr and Mo, and show that the chemical variation among internal crystals is consistent with the predicted equilibrium condensation sequence. The compositions of most of the Zr-Mo-Ti carbides require an s-process enrichment of both Zr and Mo to at least 30 times their solar abundances relative to Ti. However, to account for crystals in which Mo is also enriched relative to Zr, it is necessary to suppose that Zr is removed by separation of the earliest formed ZrC crystals from their parent gas. We also explore the formation constraints imposed by kinetics, equilibrium thermodynamics, and the observation of clusters of carbide crystals in some spherules, and conclude that relatively high formation pressures dynes cm~2), and/or condensable carbon number densities cm~3) are required. (Z0.1 (Z108 The graphite spherules with 12C/13C ratios less than the solar value may have originated in AGB stellar winds. However, in the spherically symmetric AGB atmospheres customarily assumed in models of stellar grain formation, pressures are much too low (by factors of to produce carbide crystals or Z102) graphite spherules of the sizes observed within plausible timescales. If some of the graphite spherules formed in the winds from such stars, it thus appears necessary to assume that the regions of grain forma- tion are density concentrations with length scales less than a stellar radius. Some of the spherules with both 12C/13C ratios greater than the solar value and 28Si excesses probably grew in the ejecta of super- novae. The isotopic compositions and growth constraints imply that they must have formed at high den- sities (e.g., with g cm~3) from mixtures of inner-shell material with material from the C-rich

Interstellar grains in meteorites: II. SiC and its noble gases

Abstract We have analyzed He, Ne, Ar, Kr, and Xe in fourteen size fractions of interstellar SiC, isolated from the Murchison C2 chondrite. All are mixtures of a highly anomalous component bearing the isotopic signature of the astrophysical s-process and a more normal component, generally solar-like but with anomalies of up to 30% in the heavy isotopes. As these two components strikingly resemble predictions for the He-burning shells and envelopes of red giant carbon stars, it appears that the SiC grains are pristine circumstellar condensates from such stars. A number of elemental and isotopic ratios (such as K 80 K 82 and K 86 Kr 82 ) vary with grain size, suggesting that the SiC comes from carbon stars representing a range of masses, metallicities, temperatures, and neutron densities. The Ne 21 -content of the SiC suggests a presolar cosmic-ray irradiation of up to 130 Ma, representing the interval between formation of the grains in a circumstellar shell and arrival in the solar system 4.6 Ga ago. Actually there is evidence that most of the Ne 21 (and Ne 22 ) is in ≤ 10% of the grains, suggesting that much of the SiC was degassed during or shortly before formation of the solar system. Thus the true cosmic-ray ages may be 7 to 18 × longer. Apparently the gas-rich SiC grains predate the solar system by at least 130 Ma and possibly up to 2000 Ma.

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.

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.

Low-Density Graphite Grains and Mixing in Type II Supernovae

Primitive meteorites contain presolar grains that originated in stellar outflows and supernova ejecta. Low-density graphite grains from the Murchison carbonaceous meteorite were analyzed for the isotopic compositions of C, N, O, Mg, Si, K, Ca, and Ti by ion microprobe mass spectrometry. The grains are characterized by a large range of 12C/13C ratios (from 3.6 to 7200 compared to the solar ratio of 89), excesses in 15N (15N/14N up to 10 times solar) and 18O (18O/16O up to 185 times solar), large inferred 26Al/27Al ratios (from 26Mg excesses) ranging up to 0.15, a large range in Si isotopic ratios (from 50% deficits in 29Si and 30Si relative to 28Si up to more than 120% excesses), large excesses in 41K and 44Ca from the prior presence of now-extinct 41Ca (T?=105 yr) and 44Ti (T?=59 yr), respectively, and excesses in 42Ca, 43Ca relative to 40Ca, and 49Ti, and 50Ti relative to 48Ti. Several of these isotopic signatures indicate a supernova origin. In particular, the initial presence of 44Ti and excesses of 28Si as well as the size of the inferred 41Ca/40Ca ratios are proof that the carrier grains formed in supernova ejecta. We explored the possibility that the low-density graphite grains originated from C-rich ejecta of Type II supernovae. In such stars 44Ti and 28Si are produced in the inner layers and the presence of these two isotopes in carbonaceous grains is evidence for extensive mixing of different supernova layers in the explosion. We performed mixing calculations of different layers of the SN models by Woosley & Weaver under the imposed boundary condition that C?O and compare the resulting isotopic ratios with the isotopic ratios measured in the meteoritic grains. The mixing model can explain the observed 12C/13C, 16O/18O, 30Si/28Si, and 44Ti and 41Ca fairly well as long as jets of material from the Si-rich zone, carrying 44Ti and pure 28Si, are assumed to penetrate the O-rich zone and are ejected into and mixed with the C-rich layers, where carbonaceous grains can form, without overwhelming these layers with the massive amounts of oxygen. Problems with the model are that it produces not enough 15N and consistently yields lower 29Si/30Si ratios than those in the grains. Furthermore, large excesses in 42Ca and 50Ti found in several grains, which can be attributed to neutron capture, in the model are obtained only in layers with O>C. It remains to be seen whether adjustment of cross sections and/or multidimensional SN models can overcome some of these problems. It also remains to be seen whether Type Ia supernovae, which have been proposed as a source of SN grains in meteorites, can provide a better explanation. The fact that essentially all supernova grains identified so far are diamond, graphite, SiC of Type X and Si3N4, and only one oxide grain with a supernova signature has been found remains a puzzle.

Silicon Nitride from Supernovae

Seven presolar silicon nitride (Si3N4) dust grains have been identified (five unambiguously and two probably) in separates of the Tieschitz (H3.6) and Murchison (CM2) meteorites, confirming previous tentative identifications of this mineral as a presolar component. These rare (2 ppb in Murchison) grains have isotopic compositions similar to those of the uncommon class of meteoritic SiC known as grains X (~60 ppb in Murchison), namely 28Si and 15N excesses relative to solar, both 13C excesses and deficits, and extremely high inferred 26Al/27Al ratios. These isotopic compositions coupled with Ca and Ti isotopic anomalies seen in some SiC grains X point to an origin in Type II supernova ejecta for SiC grains X, and by analogy for the Si3N4 grains as well. However, substantial discrepancies exist between the isotopic characteristics of the grains and the compositions predicted by supernova models.

Presolar Grains from Novae

We report the discovery of five SiC grains and one graphite grain isolated from the Murchison carbonaceous meteorite whose major-element isotopic compositions indicate an origin in nova explosions. The grains are characterized by low 12C/13C (4-9) and 14N/15N (5-20) ratios, large excesses in 30Si (30Si/28Si ratios range to 2.1 times solar), and high 26Al/27Al ratios. These isotopic signatures are theoretically predicted for the ejecta from ONe novae and cannot be matched by any other stellar sources. Previous studies of presolar grains from primitive meteorites have shown that the vast majority formed in red giant outflows and supernova ejecta. Although a classical nova origin was suggested for a few presolar graphite grains on the basis of 22Ne enrichments, this identification is somewhat ambiguous since it is based on only one trace element. Our present study presents the first evidence for nova grains on the basis of major element isotopic compositions of single grains. We also present the results of nucleosynthetic calculations of classical nova models and compare the predicted isotopic ratios with those of the grains. The comparison points toward massive ONe novae if the ejecta are mixed with material of close-to-solar composition.

Extinct 44Ti in Presolar Graphite and SiC: Proof of a Supernova Origin

Large excesses in 44Ca, from the radioactive decay of short-lived 44Ti, have been observed in four low-density graphite grains and five SiC grains of type X extracted from the Murchison meteorite. Titanium-46,49Ti, and 50Ti excesses were also observed in several of these grains. Because 44Ti is only produced in supernovae, these grains must have a supernova origin. Moreover, Si-, C-, N-, Al-, O-, and Ti-isotopic compositions of the grains require a Type II supernova source, and indicate extensive and heterogeneous mixing of different supernova regions, including the nickel core.