Abundances Of Presolar Grains Research Articles

Presolar grain isotopic ratios as constraints to nuclear physics inputs for s-process calculations

The isotopic abundances in presolar SiC grains of AGB origin provide important and precise constraints to those star nucleosynthesis models. By comparing the values of the s-element abundances resulting from calculations with the ones measured in these dust grains, it turns out that new measurements of weak-interaction rates in ionized plasmas, as well as of neutron-capture cross sections, are needed, especially in the region near the neutron magic numbers 50 and 82.

Diverse assemblage of presolar and solar system materials in anhydrous interplanetary dust particles: Coordinated NanoSIMS and TEM analyses

A coordinated TEM and NanoSIMS isotopic imaging study of microtome sections of three anhydrous interplanetary dust particles (IDPs) revealed a diverse collection of primitive materials having disparate origins and histories. Presolar silicate grains that likely originated in asymptotic giant branch (AGB) stars were present in each IDP at abundances ranging from 140 (+320/-120) ppm to 2000 (+4600/-1700) ppm. A unique compound presolar grain was identified that consisted of a crystalline spinel core and amorphous silicate mantle having heterogeneous Fe content. This compound grain traces the changing conditions in the circumstellar region during condensation and is the first identified presolar spinel in an IDP. A presolar SiC grain, also rare in IDPs, was found to be enriched in 13C, 14N, and 28Si, consistent with mainstream SiC that originated in ∼ solar metallicity AGB stars. We determine presolar spinel and presolar SiC abundances of 760 (+1700/-630) ppm and 190 (+440/-160) ppm, respectively, in the individual IDPs.Two elongate whisker-like enstatite grains and one platy enstatite were found to have near-terrestrial O isotopic compositions (δ18O = -17 – 18 ‰) and show chemical evidence of equilibrium condensation from a high temperature gas. Two highly 16O-rich silicates with near-solar O isotopic compositions (δ18O = -79 ‰ and -83 ‰) were also identified and may represent the primordial dust reservoir. These silicates were crystalline equilibrated aggregates. The wide range of isotopic compositions observed in these silicate grains suggests they condensed from isotopically diverse reservoirs in the protoplanetary disk in different locations and/or times. The 16O-rich grains likely condensed in the inner solar system and were subsequently transported to the outer solar system, while grains having terrestrial O isotopic compositions likely condensed from the gas phase in the terrestrial planet forming region or beyond.The IDPs showed bulk 15N enrichments (δ15N = 15 – 129 ‰) and contained 15N-rich hotspots up to 1150 ‰, consistent with the presence of molecular cloud material. IDPs U2015D21 and W7013E17 had bulk O isotopic compositions that were offset from the carbonaceous chondrite anhydrous minerals line by ∼ 10 ‰ to more 17O-rich compositions. This 17O enrichment cannot be explained by the observed abundance of 17O-rich presolar grains in these particles and the source remains unknown. IDP W7027E6 had an unusual isotopically heavy bulk O isotopic composition (δ17,18O = 40 ‰, Δ17O = 19 ‰). W7027E6 lacked hydrous phases and was therefore not likely altered by isotopically heavy primordial water. We propose that the high temperature mineral assemblage in W7027E6 condensed in the inner solar system from an 16O-poor reservoir that existed prior to O isotope homogenization in the early nebula and was subsequently transported to the outer solar system.

Slow Neutron-Capture Process: Low-Mass Asymptotic Giant Branch Stars and Presolar Silicon Carbide Grains

Presolar grains are microscopic dust grains that formed in the stellar winds or explosions of ancient stars that died before the formation of the solar system. The majority (~90% in number) of presolar silicon carbide (SiC) grains, including types mainstream (MS), Y, and Z, came from low-mass C-rich asymptotic giant branch (AGB) stars, which is supported by the ubiquitous presence of SiC dust observed in the circumstellar envelope of AGB stars and the signatures of slow neutron-capture process preserved in these grains. Here, we review the status of isotope studies of presolar AGB SiC grains with an emphasis on heavy element isotopes and highlight the importance of presolar grain studies for nuclear astrophysics. We discuss the sensitives of different types of nuclei to varying AGB stellar parameters and how their abundances in presolar AGB SiC grains can be used to provide independent, detailed constraints on stellar parameters, including 13C formation, stellar temperature, and nuclear reaction rates.

Isotopic Abundances in Presolar SiC Grains accounted by s-Processing from MHD-induced Mixing in low mass AGB stars

In the past, the observational evidence that s-process elements from Sr to Pb are produced by stars ascending the so-called Asymptotic Giant Branch (or AGB) could not be explained by self-consistent models, forcing researchers to extensive parameterisations. The crucial point is to understand how protons can be injected from the envelope into the He-rich layers, yielding to the formation of 13C and then the activation of the 13C(α,n)16O reaction. In the last decade, some physically-based mixing mechanisms have been considered to solve this problem. Nowadays, a big step forward in s-process studies would be to understand what is among the suggested ones the physical model better accounts for the observational constrains. In this paper we analyse a model where the 13C forms as a feedback of MHD processes in the stellar plasma. We compare results of nucleosynthesis models for low mass AGB stars (M<3M©), developed from the MHD scenario, with the record of isotopic abundance ratios of s-elements in presolar SiC grains, which were shown to offer precise constraints on the 13C reservoir. We find that n-captures driven by magnetically-induced mixing can well account for the SiC data and that this is due to the fact that our 13C distribution fullfills the above constraints rather accurately. We show comparisons between model predictions and measurements for isotopes of Sr, Zr, Ba, Mo and Ru as representative examples of light and heavy s-elements.

The effects of secondary processing in the unique carbonaceous chondrite Miller Range 07687

AbstractOur detailed mineralogical, elemental, and isotopic study of the Miller Range (MIL) 07687 meteorite showed that, although this meteorite has affinities to CO chondrites, it also exhibits sufficient differences to warrant classification as an ungrouped carbonaceous chondrite. The most notable feature of MIL 07687 is the presence of two distinct matrix lithologies that result from highly localized aqueous alteration. One of these lithologies is Fe‐rich and exhibits evidence for interaction with water, including the presence of fibrous (dendritic) ferrihydrite. The other lithology, which is Fe‐poor, appears to represent relatively unaltered protolith material. MIL 07687 has presolar grain abundances consistent with those observed in other modestly altered carbonaceous chondrites: the overall abundance of O‐rich presolar grains is 137 ± 3 ppm and the overall abundance of SiC grains is 71 ± 11 ppm. However, there is a large difference in the observed O‐rich and SiC grain number densities between altered and unaltered areas, reflecting partial destruction of presolar grains (both O‐ and C‐rich grains) due to the aqueous alteration experienced by MIL 07687 under highly oxidizing conditions. Detailed coordinated NanoSIMS‐TEM analysis of a large hotspot composed of an isotopically normal core surrounded by a rim composed of 17O‐rich grains is consistent with either original condensation of the core and surrounding grains in the same parent AGB star, or with grain accretion in the ISM or solar nebula.

Quantitative models for the elemental and isotopic fractionations in chondrites: The carbonaceous chondrites

Abstract A quantitative understanding of the elemental and isotopic fractionations recorded in the compositions of the chondritic meteorites would provide fundamental constraints for astrophysical models of early Solar System evolution. Here it is shown through least squares fitting that almost all features of the bulk elemental and isotopic compositions of the main carbonaceous chondrite (CC) groups, as well as the ungrouped Tagish Lake (C2) meteorite, can be reproduced using mixtures of the same four components. The fractionations amongst the non-CCs (ordinary, Rumuruti and enstatite chondrites) are distinctly different to those in the CCs and are the subject of a separate study (Alexander, 2019). The four CC components are: (1) a ‘chondrule’ (or chondrule precursor) component that partially lost Fe,Ni metal and volatiles, but is otherwise CI-like, (2) the cc -RI component that has a refractory inclusion-like bulk composition and is largely responsible for the refractory element enrichments and nucleosynthetic isotope anomalies in the bulk CCs, (3) anhydrous and reduced but otherwise CI-like matrix that accounts for almost all of the most volatile element (e.g., Zn, Se and C) contents of the CCs, and (4) water with relatively high Δ17O and δ18O values. Comparison of the inferred component compositions to additional meteoritic constraints produces some notable results. The e48Ca ≈ 8 and e50Ti ≈ 8 values for the cc -RI component are consistent with the average values for refractory inclusions. On the other hand, the e54Cr ≈ −10 is not, but is required by the negative correlation between e50Ti and e54Cr amongst the bulk CCs. The cc -RI component may be comprised of a more CAI-like sub-component that carries the e48Ca and e50Ti anomalies, and a more ferromagnesian sub-component that carries the negative e54Cr anomalies. The compositions of the volatile and metal subcomponents lost from the ‘chondrule’ component are consistent with condensation models, suggesting that the fractionations predated chondrule formation. The isotopic compositions of chondrules from the more cc -RI-rich CC groups (e.g., CV, CO and CM) seem to require the addition of some of the cc -RI component to their precursors. The assumption that matrix is CI-like is inconsistent with chondrule-matrix complementarity, but is justified by the success of the fits and the relatively uniform and CI-like abundances of organics and presolar grains in the matrices of the most primitive CCs. The inferred Δ17O = 3.5‰ for the water component is consistent with most constraints from secondary phases in the CCs. The large O isotopic mass fractionation (δ18O ≈ 18–21‰) of the water is consistent with ∼89–95% condensation of ice from a vapor under Rayleigh conditions at 150–170 K. The water was entirely accreted with the matrix with fairly constant (0.32 ± 0.06 by wt.) and CI-like (∼0.38 by wt.) water/matrix ratios. These water/matrix ratios are much less than the water/rock ratio of one that is often cited for a nebula of solar composition, but can be explained if much of the C in the CC formation regions was present as CO and CO2, and the abundance of CH4 was low.

The Impact of AGB Stars on Galaxies

AbstractAt the end of their evolution, asymptotic giant branch (AGB) stars undergo strong pulsation, mass loss, and dust production. Their mass loss results in substantial chemical and dust enrichment of the interstellar medium. Dust evolution models and isotope abundances in presolar grains suggest that AGB stars play a key role in both dust evolution and the star formation process. They are also the brightest stars in galaxies, potentially dominating in the near-infrared. As a result, AGB stars have a significant influence on the evolution and appearance of their host galaxies and thus must be accounted for when interpreting a galaxy’s integrated light. I will highlight new results that describe the impact AGB stars have on galaxies, including how AGB stars are used to probe galaxy evolution.

High abundances of presolar grains and 15N-rich organic matter in CO3.0 chondrite Dominion Range 08006

NanoSIMS C-, N-, and O-isotopic mapping of matrix in CO3.0 chondrite Dominion Range (DOM) 08006 revealed it to have in its matrix the highest abundance of presolar O-rich grains (257 +76/−96 ppm, 2σ) of any meteorite. It also has a matrix abundance of presolar SiC of 35 (+25/−17, 2σ) ppm, similar to that seen across primitive chondrite classes. This provides additional support to bulk isotopic and petrologic evidence that DOM 08006 is the most primitive known CO meteorite. Transmission electron microscopy of five presolar silicate grains revealed one to have a composite mineralogy similar to larger amoeboid olivine aggregates and consistent with equilibrium condensation, two non-stoichiometric amorphous grains, and two olivine grains, though one is identified as such solely based on its composition. We also found insoluble organic matter (IOM) to be present primarily as sub-micron inclusions with ranges of C- and N-isotopic anomalies similar to those seen in primitive CR chondrites and interplanetary dust particles. In contrast to other primitive extraterrestrial materials, H isotopic imaging showed normal and homogeneous D/H. Most likely, DOM 08006 and other CO chondrites accreted a similar complement of primitive and isotopically anomalous organic matter to that found in other chondrite classes and IDPs, but the very limited amount of thermal metamorphism experienced by DOM 08006 has caused loss of D-rich organic moieties, while not substantially affecting either the molecular carriers of C and N anomalies or most inorganic phases in the meteorite. One C-rich grain that was highly depleted in 13C and 15N was identified; we propose it originated in the Sun’s parental molecular cloud.

Presolar silicates in the matrix and fine-grained rims around chondrules in primitive CO3.0 chondrites: Evidence for pre-accretionary aqueous alteration of the rims in the solar nebula

To investigate the origin of fine-grained rims around chondrules (FGRs), we compared presolar grain abundances, elemental compositions and mineralogies in fine-grained interstitial matrix material and individual FGRs in the primitive CO3.0 chondrites Allan Hills A77307, LaPaz Icefield 031117 and Dominion Range 08006. The observation of similar overall O-anomalous (∼155ppm) and C-anomalous grain abundances (∼40ppm) in all three CO3.0 chondrites suggests that they all accreted from a nebular reservoir with similar presolar grain abundances. The presence of presolar silicate grains in FGRs combined with the observation of similar estimated porosity between interstitial matrix regions and FGRs in LAP 031117 and ALHA77307, as well as the identification of a composite FGR (a small rimmed chondrule within a larger chondrule rim) in ALHA77307, all provide evidence for a formation of FGRs by accretion of dust grains onto freely-floating chondrules in the solar nebula before their aggregation into their parent body asteroids. Our study also shows systematically lower abundances of presolar silicate grains in the FGRs than in the matrix regions of CO3 chondrites, while the abundances of SiC grains are the same in all areas, within errors. This trend differs from CR2 chondrites in which the presolar silicate abundances are higher in the FGRs than in the matrix, but similar to each other within 2σ errors. This observation combined with the identification of localized (micrometer-scaled) aqueous alteration in a FGR of LAP 031117 suggests that the lower abundance of presolar silicates in FGRs reflects pre-accretionary aqueous alteration of the fine-grained material in the FGRs. This pre-accretionary alteration could be due to either hydration and heating of freely floating rimmed chondrules in icy regions of the solar nebula or melted water ice associated with 26Al-related heating inside precursor planetesimals, followed by aggregation of FGRs into the CO chondrite parent-body.

S-Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains

In the past years the observational evidence that s-process elements from Sr to Pb are produced by stars ascending the so-called Asymptotic Giant Branch (or “AGB”) could not be explained by self-consistent models, forcing researchers to extensive parameterizations. The crucial point is to understand how protons can be injected from the envelope into the He-rich layers, yielding the formation of 13C and then the activation of the 13C(α,n)16O reaction. Only recently, attempts to solve this problem started to consider quantitatively physically-based mixing mechanisms. Among them, MHD processes in the plasma were suggested to yield mass transport through magnetic buoyancy. In this framework, we compare results of nucleosynthesis models for Low Mass AGB Stars (M≲3M⊙), developed from the MHD scenario, with the record of isotopic abundance ratios of s-elements in presolar SiC grains, which were shown to offer precise constraints on the 13C reservoir. We find that n-captures driven by magnetically-induced mixing can indeed account for the SiC data quite well and that this is due to the fact that our 13C distribution fulfils the above constraints rather accurately. We suggest that similar tests should be now performed using different physical models for mixing. Such comparisons would indeed improve decisively our understanding of the formation of the neutron source.

Diffusion of Oxygen Isotopes in Thermally Evolving Planetesimals and Size Ranges of Presolar Silicate Grains

Abstract Presolar grains are small particles found in meteorites through their isotopic compositions, which are considerably different from those of materials in the solar system. If some isotopes in presolar grains diffused out beyond their grain sizes when they were embedded in parent bodies of meteorites, their isotopic compositions could be washed out, and hence the grains could no longer be identified as presolar grains. We explore this possibility for the first time by self-consistently simulating the thermal evolution of planetesimals and the diffusion length of 18O in presolar silicate grains. Our results show that presolar silicate grains smaller than ∼0.03 μm cannot keep their original isotopic compositions even if the host planetesimals experienced a maximum temperature as low as 600 °C. Since this temperature corresponds to that experienced by petrologic type 3 chondrites, isotopic diffusion can constrain the size of presolar silicate grains discovered in such chondrites to be larger than ∼0.03 μm. We also find that the diffusion length of 18O reaches ∼0.3–2 μm in planetesimals that were heated up to 700–800°C. This indicates that, if the original size of presolar grains spans a range from ∼0.001 μm to ∼0.3 μm like that in the interstellar medium, then the isotopic records of the presolar grains may be almost completely lost in such highly thermalized parent bodies. We propose that isotopic diffusion could be a key process to control the size distribution and abundance of presolar grains in some types of chondrites.

Water and organics in interplanetary dust particles

AbstractInterplanetary dust particles (IDPs) and larger micrometeorites (MMs) impinge on the upper atmosphere where they decelerate at 90 km altitude and settle to the Earths surface. Comets and asteroids are the major sources and the flux, 30,000-40,000 tons/yr, is comparable to the mass of larger meteorites impacting the Earths surface. The sedimentary record suggests that the flux was much higher on the early Earth. The chondritic porous (CP) subset of IDPs together with their larger counterparts, ultracarbonaceous micrometeorites (UCMMs), appear to be unique among known meteoritic materials in that they are composed almost exclusively of anhydrous minerals, some of them contain &gt;&gt; 50% organic carbon by volume as well as the highest abundances of presolar silicate grains including GEMS. D/H and 15N abundances implicate the Oort Cloud or presolar molecular cloud as likely sources of the organic carbon. Prior to atmospheric entry, IDPs and MMs spend 104-105 year lifetimes in solar orbit where their surfaces develop amorphous space weathered rims from exposure to the solar wind (SW). Similar rims are observed on lunar soil grains and on asteroid Itokawa regolith grains. Using valence electron energy-loss spectroscopy (VEELS) we have detected radiolytic water in the rims on IDPs formed by the interaction of solar wind protons with oxygen in silicate minerals. Therefore, IDPs and MMs continuously deliver both water and organics to the earth and other terrestrial planets. The interaction of protons with oxygen-rich minerals to form water is a universal process.

Stellar abundances and presolar grains trace the nucleosynthetic origin of molybdenum and ruthenium

This work presents a large consistent study of molybdenum (Mo) and ruthenium (Ru) abundances in the Milky Way. These two elements are important nucleosynthetic diagnostics. In our sample of 71 Galactic metal-poor field stars, we detect Ru and/or Mo in 51 of these (59 including upper limits). The sample consists of high-resolution, high signal-to-noise spectra covering both dwarfs and giants from [Fe/H]=-0.63 down to -3.16. Thus we provide information on the behaviour of Mo I and Ru I at higher and lower metallicity than is currently known. We find a wide spread in the Mo and Ru abundances, which is typical of heavy elements. This indicates that several formation processes, in addition to high entropy winds, can be responsible for the formation of Mo and Ru. The formation processes are traced by comparing Mo and Ru to elements (Sr, Zr, Pd, Ag, Ba, and Eu) with known formation processes. We find contributions from different formation channels, namely p-, slow (s-), and rapid (r-) neutron-capture processes. Molybdenum is a highly convolved element that receives contributions from several processes, whereas Ru is mainly formed by the weak r-process as is silver. We also compare our absolute elemental stellar abundances to relative isotopic abundances of presolar grains extracted from meteorites. Their isotopic abundances can be directly linked to the formation process (e.g. r-only isotopes) providing a unique comparison between observationally derived abundances and the nuclear formation process. The comparison to abundances in presolar grains shows that the r-/s-process ratios from the presolar grains match the total elemental chemical composition derived from metal-poor halo stars with [Fe/H]~ -1.5 to -1.1 dex. This indicates that both grains and stars around and above [Fe/H]=-1.5 are equally (well) mixed and therefore do not support a heterogeneous presolar nebula... Abridged.

Abundances of presolar silicon carbide grains in primitive meteorites determined by NanoSIMS

It has been suggested that the matrices of all chondrites are dominated by a common material with Ivuna-like (CI) abundances of volatiles, presolar grains and insoluble organic matter (IOM) (e.g., Alexander, 2005). However, matrix-normalized abundances of presolar silicon carbide (SiC) grains estimated from their noble gas components show significant variations in even the most primitive chondrites (Huss and Lewis, 1995; Huss et al., 2003), in contradiction to there being a common chondrite matrix material. Here we report presolar SiC abundances determined by NanoSIMS raster ion imaging of IOM extracted from primitive members of different meteorite groups. We show that presolar SiC abundance determinations are comparable between NanoSIMS instruments located at three different institutes, between residues prepared by different demineralization techniques, and between microtomed and non-microtomed samples. Our derived SiC abundances in CR chondrites are comparable to those found in the CI chondrites (∼30ppm) and are much higher than previously determined by noble gas analyses. The revised higher CR SiC abundances are consistent with the CRs being amongst the most primitive chondrites in terms of the isotopic compositions and disordered nature of their organic matter. Similar abundances between CR1, CR2, and CR3 chondrites indicate aqueous alteration on the CR chondrite parent body has not progressively destroyed SiC grains in them. A low SiC abundance for the reduced CV3 RBT 04133 can be explained by parent body thermal metamorphism at an estimated temperature of ∼440°C. Minor differences between primitive members of other meteorite classes, which did not experience such high temperatures, may be explained by prolonged oxidation at lower temperatures under which SiC grains formed outer layers of SiO2 that were not thermodynamically stable, leading to progressive degassing/destruction of SiC.

Insight into the silicate and organic reservoirs of the comet forming region

Cometary interplanetary dust particles (IDPs), collected in Earth’s stratosphere, currently represent the best way to sample outer Solar System primordial dust. The fine-grained (sub-μm) minerals of IDPs show some strong similarities to the textures expected for primary condensates from the solar nebula. In this study we have analysed a set of IDPs for combined bulk carbon, nitrogen and hydrogen isotopes, and high precision oxygen isotopes by NanoSIMS 50L. This study concentrates on combining isotopic analyses, including high precision oxygen, on the silicate and organic components within the same sample on fine-grained primordial materials. Oxygen isotope analyses reveal that some IDPs are more 16O-rich than any bulk meteorite compositions, extending to O isotope compositions in between chondritic- and solar-like values (δ17O=−20‰, δ18O=−20‰). The 16O-rich IDPs display more primitive organic signatures than the chondritic-like 16O-poor IDPs but, rather interestingly, they also have lower presolar grain abundances. The 16O-poor signatures probably indicate an abundant component of chondritic-like material, processed in the inner protoplanetary disk. In order to explain the association of relatively processed material (silicates and organics) with high presolar grain abundances we propose a model whereby the initial dust component is 16O-rich (solar-like) and originally present homogenously throughout the protoplanetary disk. Outward radial transport, potentially associated with aerodynamic sorting, led to an influx of processed silicates from the inner Solar System with chondritic-like isotopic signatures. Comet accretion that occurred late, or at smaller heliocentric distance, included higher abundances of this chondritic-like component and hence had more 16O-poor O isotope compositions. Presolar grains, whose extreme isotopic signatures were destroyed during early homogenisation of solar nebula dust, continue to accrete onto the protoplanetary disk with time such that the later-formed comets, those with a higher proportion of processed silicates within their source, also had time to accumulate a higher abundance of presolar grains. A similar pattern is observed in the organics, with decreasing δD and C/H, from increased exchange with nebula gas, in IDPs that contain more 16O-poor chondritic-like material.

Heavily fractionated noble gases in an acid residue from the Klein Glacier 98300 EH3 chondrite

Noble gases were measured both in bulk samples (stepped pyrolysis and total extraction) and in a HF/HCl residue (stepped pyrolysis and combustion) from the Klein Glacier (KLE) 98300 EH3 chondrite. Like the bulk meteorite and as seen in previous studies of bulk type 3 E chondrites (“sub-Q”), the acid residue contains elementally fractionated primordial noble gases. As we show here, isotopically these are like those in phase-Q of primitive meteorites, but elementally they are heavily fractionated relative to these. The observed noble gases are different from “normal” Q noble gases also with respect to release patterns, which are similar to those of Ar-rich noble gases in anhydrous carbonaceous chondrites and unequilibrated ordinary chondrites (with also similar isotopic compositions). While we cannot completely rule out a role for parent body processes such as thermal and shock metamorphism (including a later thermal event) in creating the fractionated elemental compositions, parent body processes in general seem not be able to account for the distinct release patterns from those of normal Q noble gases. The fractionated gases may have originated from ion implantation from a nebular plasma as has been suggested for other types of primordial noble gases, including Q, Ar-rich, and ureilite noble gases. With solar starting composition, the corresponding effective electron temperature is about 5000 K. This is lower than inferred for other primordial noble gases (10,000–6000 K). Thus, if ion implantation from a solar composition reservoir was a common process for the acquisition of primordial gas, electron temperatures in the early solar system must have varied spatially or temporally between 10,000 and 5000 K. Neon and xenon isotopic ratios of the residue suggest the presence of presolar silicon carbide and diamond in abundances lower than in the Qingzhen EH3 and Indarch EH4 chondrites. Parent body processes including thermal and shock metamorphism and a late thermal event also cannot be responsible for the low abundances of presolar grains. KLE 98300 may have started out with smaller amounts of presolar grains than Qingzhen and Indarch.

EXTINCT93Zr IN SINGLE PRESOLAR SiC GRAINS FROM LOW MASS ASYMPTOTIC GIANT BRANCH STARS AND CONDENSATION FROM Zr-DEPLETED GAS

Synchrotron X-ray fluorescence was used in this study for the first time to measure trace element abundances in single presolar grains. The abundances of Zr and Nb were determined in SiC grains of the KJF size-separate. These grains are most likely from C-rich asymptotic giant branch stars (mainstream grains). Comparison of the data with s-process calculations suggests that the relatively short-lived isotope 93Zr (t1/2 = 1.5 × 106 yr) condensed into the grains. The Nb/Zr ratios of the majority of the grains are higher than the s-process and CI chondrite ratios. This is probably due to grains condensing from stellar gas that was depleted in Zr, potentially because of earlier condensation of ZrC, but not depleted in Nb. However, grain contamination with solar system Nb is possible as well. Upper limits on the initial 93Zr/Zr ratios in the grains agree with the ratios observed in late-type S stars.

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

The 29P(p, γ) 30S reaction plays an important role in understanding the Si isotopic abundances in presolar grains of nova origin, 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. This rate at nova temperatures is determined by two low-lying 3 + and 2 + resonances above the proton threshold at 4399 keV in 30S. 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 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.

Ultra-primitive interplanetary dust particles from the comet 26P/Grigg–Skjellerup dust stream collection

Cometary material and pristine interplanetary dust particles (IDPs) best resemble the unaltered components from which our solar system was built because they have remained largely unaltered in a cold undisturbed environment since accretion in the outer protoplanetary disk. IDPs might supply more primitive assemblages for laboratory analysis than Stardust samples from comet 81P/Wild 2 but their individual provenances are typically unknown. We speculate that some IDPs collected by NASA in April 2003 may be associated with comet 26P/Grigg–Skjellerup because their particularly pristine character coincides with the collection period that was predicted to show an enhanced flux of particles from this Jupiter-family comet. Some IDPs from this collection contain the most primitive assembly of interstellar matter found to date including an unusually high abundance of presolar grains and very isotopically anomalous and disordered organic matter as well as fine-grained carbonates and an amphibole associated with a GEMS-like object (glass with embedded metals and sulfides) that potentially imply formation in a nebular rather than planetary environment. The two most primitive IDPs may contain assemblages of molecular cloud material at the percent level which is supported by the presence of four rare 17O-depleted presolar silicate grains possibly of supernova(e) origin within one ~ 70 μm 2-sized IDP and the close association of a Group 1 Mg-rich olivine from a low-mass red giant star with a carbonaceous nano-globule of potentially interstellar origin. Our study together with observations of comet 9P/Tempel 1 during the Deep Impact experiment and 81P/Wild 2 dust analyses reveal some compositional variations and many similarities among three Jupiter-family comets. Specifically carbonates and primitive organic matter or amorphous carbon were widespread in the comet-forming regions of the outer protoplanetary disk and not all comets contain as much inner solar system material as has been inferred for comet 81P/Wild 2. The bulk and hotspot hydrogen and nitrogen isotopic anomalies as well as the carbon Raman characteristics of the organic matter in IDPs and the most primitive meteorites are remarkably similar. This implies that the same mixture of molecular cloud material had been transported inward into the meteorite-forming regions of the solar system.

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.