Presolar SiC Research Articles

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.

Structural and isotopic microanalysis of presolar SiC from supernovae

Abstract– We report on the microstructure, crystallography, chemistry, and isotopic compositions of seven SiC X grains and two mainstream grains from the Murchison meteorite. TEM crystallographic analysis revealed that the X grains (approximately 3 μm) are composed of many small crystals (24–457 nm), while the similarly sized mainstream grains are composed of only a few crystals (0.5–1.7 μm). The difference in crystal size likely results from differences in their formation environments: the X grain crystals evidently formed under conditions of greater supersaturation and rapid growth compared to their mainstream counterparts. However, the same polytypes are observed in both mainstream and X grains. Six X grains and both mainstream grains are entirely the 3C-SiC polytype and one X grain is an intergrowth of the 3C-SiC and 2H-SiC polytypes. EDXS measurements indicate relatively high Mg content in the X grains (≲5 atomic%), while Mg was undetectable in the mainstream grains. The high Mg content is probably from the decay of 26Al into 26Mg. Estimates of the 26Al/27Al ratios, which range from 0.44–0.67, were made from elemental Mg/Al ratios. This range is consistent with the 26Al/27Al ratios inferred from previous isotopic measurements of X grains. We also report the first direct observations of subgrains in X grains, including the first silicides [(Fe,Ni)nSim]. Diffraction data do not match any previously observed presolar phases, but are a good fit to silicides, which are predicted stable SN condensates. Eight subgrains with highly variable Ni/Fe ratios (0.12–1.60) were observed in two X grains.

Osmium isotope anomalies in chondrites: Results for acid residues and related leachates

We have investigated Os isotope anomalies in acid residues enriched in insoluble organic matter (IOM) extracted from ten primitive chondrites, acid leachates and residues of these fractions, as well as acid leachates of bulk chondrites. Osmium isotopic compositions of bulk carbonaceous, ordinary and enstatite chondrites are also reported. Consistent with prior results, bulk chondrites have homogeneous Os isotope compositions for s-, r-, and p-process nuclides that are indistinguishable from terrestrial, at the current level of resolution. In contrast, nearly all the IOM-rich residues are enriched in s-process Os, evidently due to the preferential incorporation of s-process enriched presolar grains (most likely presolar SiC). Presolar silicate grains that formed in red giant branch (RGB) or asymptotic giant branch (AGB) stars are also likely hosts of additional s-process Os in chondrites. Consistent with one prior study, Os released by weak acid leaching of bulk chondrites is slightly to strongly enriched in r-process nuclides, of which the carrier may be fine-grained presolar silicates formed in supernovae or unidentified solar phases. Collectively, the different, chemically concentrated components in these meteorites are variably enriched in s-, r-, and possibly p-process Os, of which the individual carriers must have been produced in multiple stellar environments. The lack of evidence for Os isotopic heterogeneity among bulk chondrites contrasts with evidence for isotopic heterogeneities for various other elements at approximately the same levels of resolution (e.g., Cr, Mo, Ru, Ba, Sm, and Nd). One possible explanation for this is that the heterogeneities for some elements in bulk materials reflect selective removal of some types of presolar grains as a result of nebular processes, and that because of the strong chemical differences between Os and the other elements, the Os was not significantly affected. Another possible explanation is that late-stage injection of isotopically distinct materials into the nebula led to heterogeneous distribution of some elements. Osmium may not have been affected because of a lack of isotopic contrast with the pre-injection, well mixed nebula.

Matrix and whole-rock fractionations in the Acfer 094 type 3.0 ungrouped carbonaceous chondrite

Abstract– We used the electron microprobe to study matrix in the ungrouped type 3.0 carbonaceous chondrite Acfer 094 using 7 × 7-point, focused-beam arrays; data points attributable to mineral clasts were discarded. The grid areas show resolvable differences in composition, but differences are less pronounced than we observed in studies of CR2 LaPaz Icefield (LAP) 02342 (Wasson and Rubin [2009]) and CO3.0 Allan Hills A77307 (Brearley [1993]). A key question is why Acfer shows an anomalously uniform composition of matrix compared with these other carbonaceous chondrites. Both whole-rock and matrix samples of Acfer 094 show enhancements of Ca and K; it appears that these reflect contamination during hot desert weathering. By contrast, the whole-rock abundance of Na is low. Although weathering effects are responsible for some fractionations, it appears that nebular effects are also resolvable in matrix compositions in Acfer 094. As with LAP 02342, we infer that the observed differences among different areas were inherited from the solar nebula and may have been carried by porous chondrules that experienced low (about 20%) degrees of melting. Acfer 094 has been comminuted by one or more impact events that may also have caused volatile loss. Thus, despite preserving evidence (e.g., an exceptionally high content of presolar SiC) implying a high degree of pristinity, Acfer 094 is far from pristine in other respects. This evidence of comminution and an O-isotopic composition similar to values measured in metamorphosed CM chondrites suggest that Acfer was hydrated before being outgassed by the inferred impact event. Convection within the plume associated with the impact event probably also contributed to the homogenization of the Acfer 094 matrix.

Study of astrophysically important resonant states in 30S using the 32S(p,t)30S reaction

A small fraction (< 1%) of presolar SiC grains is suggested to have been formed in the ejecta of classical novae. The 29P(p,γ)30S reaction plays an important role in understanding the Si isotopic abundances in such grains, which in turn provide us with information on the nature of the probable white dwarf progenitor's core, as well as the peak temperatures achieved during nova outbursts, and thus the nova nucleosynthetic path. This rate at nova temperatures is determined by two low-lying 3+ and 2+ resonances above the proton threshold at 4399 keV in 30S. Despite several experimental studies in the past, however, these two states have only been observed very recently. We have studied the 30S nuclear structure via the 32S(p,t)30S reaction at 5 laboratory angles between 9° to 62°. We have observed 14 states, eleven of which are above the proton threshold, including two levels at 4692.7 ± 4.5 keV and 4813.8 ± 3.4 keV that are candidates for the 3+ and the previously "missing" 2+ state, respectively.

Resonance ionization mass spectrometry for precise measurements of isotope ratios

Resonance ionization mass spectrometry offers extremely high sensitivity and elemental selectivity in microanalysis, but the isotopic precision attainable by this technique has been limited. Measured isotope ratios are sensitive to small fluctuations in the pointing, pulse timing, and wavelength of the resonance lasers. We show that, by minimizing these fluctuations using feedback controls and by power-broadening the optical transitions, we are able to measure chromium isotope ratios with statistics-limited precision better than 1%. Small additional improvements in reproducibility come from careful shaping of the electric field in the region where atoms are photoionized and from minimizing pulse-to-pulse variations in the time-of-flight mass spectrometer through which the photoions travel. The increased reproducibility of isotopic measurements on standard materials has enabled us to detect anomalous chromium isotopic abundances in presolar SiC grains extracted from primitive meteorites.

STARDUST FROM ASYMPTOTIC GIANT BRANCH STARS

The formation of dust in the outflows of low- and intermediate-mass stars on the first giant branch and asymptotic giant branch (AGB) is studied and the relative contributions of stars of different initial masses and metallicities to the interstellar medium (ISM) at the instant of solar system formation are derived. These predictions are compared with the characteristics of the parent stars of presolar dust grains found in primitive meteorites and interplanetary dust particles (IDPs) inferred from their isotopic compositions. For this purpose, model calculations for dust condensation in stellar outflows are combined with synthetic models of stellar evolution on the first giant branch and AGB and an evolution model of the Milky Way for the solar neighborhood. The dust components considered are olivine, pyroxene, carbon, SiC, and iron. The corresponding dust production rates are derived for the solar vicinity. From these rates and taking into account dust destruction by supernova shocks in the ISM, the contributions to the inventory of presolar dust grains in the solar system are derived for stars of different initial masses and metallicities. It is shown that stars on the first giant branch and the early AGB are not expected to form dust, in accord with astronomical observations. Dust formation is concentrated in the last phase of evolution, the thermally pulsing AGB. Due to the limited lifetime of dust grains in the ISM only parent stars from a narrow range of metallicities are expected to contribute to the population of presolar dust grains. Silicate and silicon carbide dust grains are predicted to come from parent stars with metallicities not less than about Z ≈ 0.008 (0.6 × solar). This metallicity limit is higher than that inferred from presolar SiC grain isotope data. The population of presolar carbon dust grains is predicted to originate from a wider range of metallicities, down to Z ≈ 0.004. Masses of AGB stars that produce C-rich dust are in the range ≈1.5-4 M ☉, in good agreement with what was inferred from the isotope data of presolar grains. The mass distribution of AGB stars that produce O-rich dust is essentially bimodal, with roughly equal contributions from stars in the ranges 1.3-2.5 M ☉ and ≈4-8 M ☉. These model predictions are in conflict with the O-isotope data of presolar grains that indicate contributions essentially only from 1 to 2.5 M ☉ AGB stars.

INTERSTELLAR RESIDENCE TIMES OF PRESOLAR SiC DUST GRAINS FROM THE MURCHISON CARBONACEOUS METEORITE

The time span between the formation of presolar grains in stellar outflows and their incorporation into early solar-system solids is poorly constrained. Knowledge of this time span is essential for a better understanding of the processing of grains in the interstellar medium (ISM) and formation processes of the solar system. Here, we report interstellar residence times of ∼3–1100 Myr for large (∼5–50 μm) presolar SiC grains, based on their content of He and Ne produced by Galactic cosmic rays. A majority of these grains have interstellar residence times on the order of a few tens up to less than 200 Myr, considerably shorter than theoretical estimates of interstellar dust lifetimes (∼500 Myr), but long enough to require formation of the majority of the grains before that of the presolar molecular cloud. The age distribution may be explained by starburst activity 1–2 billion years prior to the birth of the Sun. Our findings provide essential "ground truth" to constrain models of interstellar dust lifetimes and destructive processes in the ISM.

HIGH ABUNDANCES OF CIRCUMSTELLAR AND INTERSTELLAR C-ANOMALOUS PHASES IN THE PRIMITIVE CR3 CHONDRITES QUE 99177 AND MET 00426

QUE 99177 and MET 00426 are the first known CR3 chondrites and have largely escaped the aqueous alteration experienced by most of the CR chondrite group. In addition to high presolar silicate and oxide grain abundances, these meteorites also contain high abundances of phases with anomalous C isotopic compositions. Presolar SiC abundances (~65 ppm) are consistent with independent estimates for CR1 and CR2 chondrites, indicating that aqueous alteration does not destroy SiC, but are significantly higher than earlier abundance estimates based on noble gas contents. Particularly notable are the high abundances (~120 ppm) of carbonaceous matter with anomalous C isotopic compositions that probably formed in cold molecular clouds via ion-molecule reactions. The observation of both 13C-enriched and 13C-depleted compositions suggests that multiple chemical pathways contribute to the formation of this material. The abundance of such material is significantly higher in QUE 99177 and MET 00426 than in other primitive meteorites and attests to the primitive nature of the CR3 chondrites.

INTERSTELLAR EXPOSURE AGES OF LARGE PRESOLAR SiC GRAINS FROM THE MURCHISON METEORITE

ABSTRACT We report the Li isotopic compositions of nine large, greater than 5 μm, presolar SiC grains from the Murchison (CM) meteorite. Most of the SiC grains analyzed are isotopically similar in C and Si and morphologically distinct from other presolar SiC grains. Their 7Li/6Li ratios range from ∼9.3 to the solar value (∼12). The 6Li enrichments observed in the grains can only arise from galactic cosmic ray spallation off C atoms during travel through the interstellar medium (ISM) and before incorporation into the meteorite parent body. Using appropriate production rates, we calculate recoil-loss-corrected individual exposure ages of eight grains with 6Li excesses that range from 40 Myr to about 1 Gyr. The long exposure ages (&gt;500 Myr) are consistent with calculations of grain survival lifetimes in the ISM, while the shorter ages (&lt;100 Myr) of two grains are more consistent with previous cosmogenic noble gas studies. Although the sample size is only eight grains, there appears to be little evidence for clustering of exposure ages and no obvious correlation between exposure age and grain size.

He and Ne Ages of Large Presolar Silicon Carbide Grains: Solving the Recoil Problem

AbstractKnowledge about the age of presolar grains provides important insights into Galactic chemical evolution and the dynamics of grain formation and destruction processes in the Galaxy. Determination from the abundance of cosmic ray interaction products is straightforward, but in the past has suffered from uncertainties in correcting for recoil losses of spallation products. The problem is less serious in a class of large (tens of μm) grains. We describe the correction procedure and summarise results for He and Ne ages of presolar SiC ‘Jumbo’ grains that range from close to zero to ∼850 Myr, with the majority being less than 200 Myr. We also discuss the possibility of extending our approach to the majority of smaller SiC grains and explore possible contributions from trapping of cosmic rays.

Cosmic-Ray Exposure Ages of Large Presolar SiC Grains

AbstractAmong presolar SiC grains found in the Murchison carbonaceous meteorites (average size less than 0.5 μm) are very large grains, ranging in size up to 50 μm. We interpret 6Li excesses measured in eight of these grains as being the result of spallation reactions by Galactic cosmic rays during the time the grains spent in the interstellar medium before their incorporation into the meteorite. Derived interstellar exposure ages range from 40 My to 1 Gy, the highest values being consistent with theoretical expectations of interstellar grain lifetimes. Although six grains have almost identical C and Si isotopic compositions, their exposure ages are very different. This observation, combined with low trace element contents, and unusual grain sizes, raises fundamental questions about their stellar sources.

Co-Production of Light p-, s- and r-Process Isotopes in the High-Entropy Wind of Type II Supernovae

AbstractWe have performed large-scale nucleosynthesis calculations within the high-entropy-wind (HEW) scenario of Type II supernovae. The primary aim was to constrain the conditions for the production of the classical ‘p-only’ isotopes of the light trans-Fe elements. We find, however, that for electron fractions in the range 0.458 ≤ Ye ≤ 0.478, sizeable abundances of p-, s- and r-process nuclei between 64Zn and 98Ru are coproduced in the HEW at low entropies (S ≤ 100) by a primary charged-particle process after an α-rich freezeout. With the above Ye–S correlation, most of the predicted isotopic abundance ratios within a given element, e.g. 64Zn(p)/70Zn(r) or 92Mo(p)/94Mo(p), as well as of neighboring elements, e.g. 70Ge(s + p)/74Se(p) or 74Se(p)/78Kr(p) agree with the observed Solar-System ratios. Taking the Mo isotopic chain as a particularly challenging example, we show that our HEW model can account for the production of all 7 stable isotopes, from ‘p-only’ 92Mo, via ‘s-only’ 96Mo up to ‘r-only’ 100Mo. Furthermore, our model is able to reproduce the isotopic composition of Mo in presolar SiC X-grains.

AN UNUSUAL PRESOLAR SILICON CARBIDE GRAIN FROM A SUPERNOVA: IMPLICATIONS FOR THE PRODUCTION OF SILICON-29 IN TYPE II SUPERNOVAE

We report the discovery of a presolar SiC grain (KJB2-11-17-1) with unusual Si-isotopic composition. The grain has 29Si/28Si = 1.63 × solar, 30Si/28Si = 0.82 × solar, 12C/13C = 265 (= 3 × solar), and evidence for the presence of radiogenic 44Ca from the decay of 44Ti. A comparison of these isotopic signatures with stellar models suggests an origin in a 15 M☉ Type II supernova. It is possible to achieve a very good match between the 30Si/28Si, 12C/13C, and inferred 44Ti/48Ti ratios in KJB2-11-17-1 and the model predictions if matter from different supernova zones is mixed in appropriate proportions. The 29Si/28Si ratio, however, cannot be reproduced and is clearly higher than predicted. It was suggested previously by Travaglio et al. that supernova models underestimate the 29Si yield in the C- and Ne-burning regions by about a factor of 2. Because of its very high 29Si/30Si of two times the solar ratio, grain KJB2-11-17-1 provides the opportunity to make a stringent test of this hypothesis. With a twofold enhanced 29Si yield in the C- and Ne-burning zones, we find a perfect match for 29Si/28Si between the model predictions and the grain. Nuclear network calculations show that a twofold increase in the 29Si yield in the C- and Ne-burning regions requires roughly a threefold higher 26Mg(α, n)29Si reaction rate, the most important reaction for the production of 29Si, in the temperature range 1–3 × 109 K than currently used in supernova models. This increase is qualitatively within current uncertainties of this reaction rate.

Iron and Nickel Isotopic Ratios in Presolar SiC Grains

We report the first Fe isotopic anomalies and the first Ni isotopic ratio measurements in presolar SiC grains of separate KJG from the Murchison meteorite. With NanoSIMS, we analyzed Fe and Ni in 37 X grains from Type II supernovae and 53 SiC grains of other types. The Ni/Fe and Co/Fe ratios in grains of all types are much higher than in the gas from which the grains are believed to have condensed. A majority of the X grains and a couple of mainstream grains contain Fe-rich subgrains. Most X grains have large excesses in 57Fe,61Ni, and 62Ni.60Ni excesses are small and the 54Fe/56Fe ratios of almost all X grains are normal. These isotopic compositions are best explained by mixing of material from the He/N zone of Type II supernovae with material from the He/C zone. The lack of any 54Fe excesses is puzzling in view of the fact that the Si/S zone, whose contribution resulted in the 28Si excesses in X grains, is very rich in 54Fe. It has yet to be seen whether elemental fractionation between Si and Fe is an explanation. The 57Fe deficits observed in a few X grains remain unexplained. In comparison to the X grains, fewer mainstream and AB grains have anomalies. Observed 62Ni excesses in some mainstream grains are larger than predicted for AGB stars of solar metallicity and are not accompanied by corresponding 61Ni excesses. A Y grain and a Z grain have excesses in 54Fe and 62Ni, but close to normal 57Fe/56Fe and 60,61Ni/58Ni ratios. These isotopic compositions are not expected for grains from low-metallicity AGB stars.

Stardust in Antarctic micrometeorites

Abstract— We report the discovery of presolar silicate, oxide (hibonite), and (possibly) SiC grains in four Antarctic micrometeorites (AMMs). The oxygen isotopic compositions of the eighteen presolar silicate (and one oxide) grains found are similar those observed previously in primitive meteorites and interplanetary dust particles, and indicate origins in oxygen‐rich red giant or asymptotic giant branch stars, or in supernovae. Four grains with anomalous C isotopic compositions were also detected. 12C/13C as well as Si ratios are similar to those of mainstream SiC grains; the N isotopic composition of one grain is also consistent with a mainstream SiC classification.Presolar silicate grains were found in three of the seven AMMs studied, and are heterogeneously distributed within these micrometeorites. Fourteen of the 18 presolar silicate grains and 3 of the 4 C‐anomalous grains were found within one AMM, T98G8. Presolar silicate‐bearing micrometeorites contain crystalline silicates that give sharp X‐ray diffractions and do not contain magnesiowüstite, which forms mainly through the decomposition of phyllosilicates and carbonates. The occurrence of this mineral in AMMs without presolar silicates suggests that secondary parent body processes probably determine the presence or absence of presolar silicates in Antarctic micrometeorites.

Characteristics and applications of RELAX, an ultrasensitive resonance ionization mass spectrometer for xenon

RELAX is an ultra sensitive, time of flight, resonance ionization mass spectrometer specifically for measuring xenon isotope ratios. RELAX combines a resonance ionization source, a cryogenic sample concentrator and a time of flight mass spectrometer. It is the most sensitive instrument currently available for determining xenon isotope ratios. We report recent modifications to the spectrometer and discuss the properties and performance of the instrument in its current configuration. We also present the first data from attempted analyses of individual presolar SiC grains from the Murchison meteorite.

NewP30(p,γ)S31resonances and oxygen-neon nova nucleosynthesis

Enriched isotopic abundance ratios of $^{30}\mathrm{Si}$/$^{28}\mathrm{Si}$ in several presolar SiC and graphite grains qualitatively indicate massive oxygen-neon (ONe) nova origins but fall short of hydrodynamic ONe nova model ejecta predictions by as much as an order of magnitude. The astrophysical $^{30}\mathrm{P}$($p,\ensuremath{\gamma}$)$^{31}\mathrm{S}$ reaction rate uncertainty at ONe nova temperatures ($0.10lTl0.35$ GK) spans several orders of magnitude through which the predicted amount of ejected $^{30}\mathrm{Si}$ can vary by a factor of 100. By measuring triton momenta from the $^{31}\mathrm{P}$($^{3}\mathrm{He}$,$t$)$^{31}\mathrm{S}$ reaction at 20 MeV, the energies of $^{30}\mathrm{P}$ $+p$ resonances in the Gamow window for ONe novae have been determined to better than \ifmmode\pm\else\textpm\fi{}3 keV, and two new resonances at ${E}_{\mathrm{c}.\mathrm{m}.}=194.0(25)$ and 266.4(27) keV that likely dominate the $^{30}\mathrm{P}$($p,\ensuremath{\gamma}$)$^{31}\mathrm{S}$ rate for $0.08lTl0.25$ GK have been resolved. A resulting increase in the experimentally determined $^{30}\mathrm{P}$($p,\ensuremath{\gamma}$)$^{31}\mathrm{S}$ reaction rate puts it in accord with Hauser-Feshbach statistical model estimates for $0.08lTl0.40$ GK, supporting conclusions drawn from ONe nova model studies that employed a Hauser-Feshbach rate.

NanoSIMS isotopic analysis of small presolar grains: Search for Si 3N 4 grains from AGB stars and Al and Ti isotopic compositions of rare presolar SiC grains

We report isotopic ratio measurements of small SiC and Si 3N 4 grains, with special emphasis on presolar SiC grains of type Z, and new nucleosynthesis models for 26Al/ 27Al and the Ti isotopic ratios in asymptotic giant branch (AGB) stars. With the NanoSIMS we analyzed 310 SiC grains from Murchison (carbonaceous CM2 chondrite) separate KJB (diameters 0.25–0.45 μm) and 153 SiC grains from KJG (diameters 1.8–3.7 μm), 154 SiC and 23 Si 3N 4 grains from Indarch (enstatite EH4 chondrite) separate IH6 (diameters 0.25–0.65 μm) for their C and N isotopic compositions, 549 SiC and 142 Si 3N 4 grains from IH6 for their C and Si isotopic compositions, 13 SiC grains from Murchison and 66 from Indarch for their Al–Mg compositions, and eight SiC grains from Murchison and 10 from Indarch for their Ti isotopic compositions. One of the original objectives of this effort was to compare isotopic analyses with the NanoSIMS with analyses previously obtained with the Cameca IMS 3f ion microprobe. Many of the Si 3N 4 grains from Indarch have isotopic anomalies but most of these apparently originate from adjacent SiC grains. Only one Si 3N 4 grain, with 13C and 14N excesses, has a likely AGB origin. The C, N, and Si isotopic data show that the percentage of SiC grains of type Y and Z increase with decreasing grain size (from ∼1% for grains >2 μm to ∼5–7% for grains of 0.5 μm), providing an opportunity for isotopic analyses in these rare grains. Our measurements expand the number of Al–Mg analyses on SiC Z grains from 4 to 23 and the number of Ti analyses on Z grains from 2 to 11. Inferred 26Al/ 27Al ratios of Z grains are in the range found in mainstream and Y grains and do not exceed those predicted by models of AGB nucleosynthesis. Cool bottom processing (CBP) has been invoked to explain the low 12C/ 13C ratios of Z grains, but this process apparently does not lead to increased 26Al production in the parent stars of these grains. This finding is in contrast to presolar oxide grains where CBP is needed to explain their high 26Al/ 27Al ratios. The low 46,47,49Ti/ 48Ti ratios found in Z grains and their correlation with low 29Si/ 28Si ratios extend the trend seen in mainstream grains and confirm an origin in low-metallicity AGB stars. The relatively large excesses in 30Si and 50Ti in Z grains are predicted by our models to be the result of increased production of these isotopes by neutron-capture nucleosynthesis in low-metallicity AGB stars. However, the predicted excesses in 50Ti (and 49Ti) are much larger than those found. Even lowering the strength of the 13C pocket cannot solve this discrepancy in a consistent way.

Constraining the 13C neutron source in AGB stars through isotopic analysis of trace elements in presolar SiC

Abstract— Analyses of the isotopic compositions of multiple elements (Mo, Zr, and Ba) in individual mainstream presolar SiC grains were done by resonant ionization mass spectrometry (RIMS). While most heavy element compositions were consistent with model predictions for the slow neutron capture process (s‐process) in low‐mass (1.5–3 M⊙) asymptotic giant branch stars of solar metallicity when viewed on single‐element three‐isotope plots, grains with compositions deviating from model predictions were identified on multi‐element plots. These grains have compositions that cannot result from any neutron capture process but can be explained by contamination in some elements with solar system material. Previous work in which only one heavy element per grain was examined has been unable to identify contaminated grains. The multi‐element analyses of this study detected contaminated grains which were subsequently eliminated from consideration. The uncontaminated grains form a data set with a greatly reduced spread on the three‐isotope plots of each element measured, corresponding to a smaller range of 13C pocket efficiencies in parent AGB stars. Furthermore, due to this reduced spread, the nature of the stellar starting material, previously interpreted as having solar isotopic composition, is uncertain. The constraint on 13C pocket efficiencies in parent stars of these grains may help uncover the mechanism responsible for formation of 13C, the primary neutron source for s‐process nucleosynthesis in low‐mass stars.