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One of the primary focuses of our research has been the isolation of individual grains of silicon carbide, graphite, aluminum oxide, and silicon nitride whose unusual isotopic properties show that they must have formed around other stars prior to incorporation into the pre-solar nebula (circumstellar grains). |
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Surface Properties of Pristine Circumstellar SiC Grains
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In this investigation we are studying the morphologies and surface properties of circumstellar SiC grains gently isolated from their parent meteorites. The object of the work is to understand the history of the grains from initial production to incorporation into the meteorite parent bodies. Circumstellar SiC grains were first found in acid resistant residues of carbonaceous meteorites. The crystalline grains typically appear corroded and their properties prior to chemical etching were unknown. This issue was partially resolved when we developed a x-ray mapping technique that located the SiC grains in situ in polished sections. To our surprise, we found that the grains are isolated entities distributed at random in the fine-grained meteorite matrix material. We have now used x-ray mapping to isolate, for the first time, circumstellar grains that have been neither etched nor polished. High-resolution scanning electron images show several distinct morphologies. While some grains are highly angular, most are rounded indicating various degrees of erosion. The nature of this rounding and where and when it took place - perhaps by oxygen chemical attack in the solar nebula or possibly by processing by supernovae shock waves in interstellar space, are questions that we are attempting to answer. If the rounding has resulted from oxidation, a thin layer of SiO2 may be present on some grains. It is also possible that the least modified grains were protected by an ice or organic mantle acquired in the interstellar medium. These issues are being pursued by applying a variety of analytic techniques to study the surface properties of the grains in more detail. |
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Al-26 in Presolar Grains and Deep Mixing in AGB Stars
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Many presolar grains show excesses in 26Mg due to the decay of the radioisotope 26Al (half life = 730,000 years). The figure shows inferred 26Al/27Al ratios in presolar oxide and SiC grains. The highest ratios are found in SiC grains of type X, which originated in supernova ejecta. Mainstream, Y, and Z SiC grains as well as almost all oxide grains are believed to come from Asymptotic Giant Branch (AGB) stars, whereas the stellar sources of A+B grains are still not well established. It is remarkable that oxide grains have, on average, much higher 26Al/27Al ratios than SiC grains from AGB stars. 18O/16O ratios less than ~10-3 are lower than those predicted by standard stellar evolution models and have been explained by invoking extra deep mixing (also called cool bottom processing) of material from the star's envelope to hot zones close to the H-burning shell. Deep mixing is also required to explain 26Al/27Al ratios larger than ~3x10-3. Thus, the parent stars of many oxide grains must have experienced deep mixing. In contrast, the 26Al/27Al ratios of most SiC grains having an AGB origin are within the limits expected from shell H burning in standard stellar evolution models. This is surprising because continued dredge-up of 12C produced in the He shell during thermal pulses is expected to turn O-rich AGB stars, the parents of oxide grains, into C-rich stars ("carbon stars"), from which SiC grains can condense. Thus it seems that most parent stars of SiC grains do not undergo deep mixing. An explanation for this observation is that only AGB stars that do not experience deep mixing become carbon stars or, conversely, that deep mixing prevents AGB stars from becoming carbon stars. The mechanism for deep mixing is still not well understood. The study of presolar grains not only can provide information about nucleosynthetic processes taking place in the interior of stars but also about mixing processes that bring the nucleosynthetic products to the star's surface where they can be observed astronomically and where they are incorporated into grains. |
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Coordinated TEM and NanoSIMS studies of presolar grains
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Transmission electron microscopy (TEM) can reveal the internal structure and chemical composition of presolar grains. To see through presolar grains with electrons in the TEM, they must be sliced into ultra-thin sections only several hundred atoms thick. However, thinned TEM samples do not have enough atoms for accurate isotopic analysis with most SIMS instruments. This limited earlier presolar grain studies, as we were unable to obtain isotopic and microstructural information from the same samples. Now, using the higher sensitivity of the nanoSIMS, we are pursuing coordinated TEM and isotopic studies on presolar already mounted on TEM grids. In this way, we can identify the likely stellar source of individual presolar grains from their isotopic anomalies, and correlate this information directly with the mineralogical results from TEM studies. We have discovered that presolar graphite and silicon carbide often contain internal crystals of carbide minerals along with metallic iron and other phases. From the microstructure we can determine the order in which the minerals condensed, yielding constraints on the ranges of temperature and pressure in the gaseous outflows from the stars around which the grains formed. Estimates show that the minimum pressure required to form large carbides is higher than astronomers had thought, and require the formation of "clumps" of high density near the stellar surfaces. This conclusion has been substantiated by high-resolution astronomical observation of mass-losing stars. We have also studied a number of individual graphites that are known to be of supernova origin based on their isotopic compositions. Results from the supernova graphites are presented in Presolar Grains: a complementary approach to studying supernovae.
On the left are two TEM images of sliced presolar graphites containing internal refractory carbides (indicated by red arrows): a) the upper dense graphite (of likely AGB star origin) contains a central (Ti,Zr,Mo) carbide which likely served as the nucleation center for the graphite; b) the lower less dense supernova graphite contains multiple titanium carbides. |
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Isotopic Composition of Presolar Dust Grains
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Presolar grains of SiC, graphite, corundum, and silicon nitride have sizes of up to several µm. The ion microprobe makes it possible to measure isotopic ratios in individual dust grains down to less than 1 µm in size. Such measurements have revealed a tremendous range in isotopic compositions. An example is shown in the graph on the left, which displays the C and N isotopic ratios measured by SIMS in individual SiC grains. On the basis of the C, N, and Si isotopic ratios we can distinguish between six different classes of presolar SiC grains. The abundances of these classes vary greatly and are indicated in the graph. Grains of different classes originated from different types of stars. Mainstream grains and grains of type Y and Z are believed to have condensed in the atmospheres of Asymptotic Giant Branch (AGB) stars of different metallicity. AGB stars are low-to-intermediate mass stars in the late stages of their evolution when they lose large amounts of material in stellar winds from which grains can condense. A minor class of grains, grains of type X come from supernovae, massive stars that explode after the exhaustion of the nuclear fuel in their interior . The isotopic compositions of a handful of grains indicate that they originated from the ejecta of nova, nuclear explosions of a H-rich layer accreted from a companion onto the surface of a white dwarf star. The isotopic of presolar SiC (and other) grains not only allow us to identify their stellar sources but they set constraints on existing models of nucleosynthesis and stellar evolution and thus provide new information on the workings of the nuclear furnaces in the interior of stars. |
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Presolar Silicate Grains in Meteorites
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Although astronomical observations indicate that silicate grains are produced in abundance in the atmospheres of young and evolved stars, previous searches for presolar silicates in meteorites have been unsuccessful. This situation has changed with the advent of the NanoSIMS and its capability to analyze the O isotopic compositions of tens of thousands of sub-µm grains by raster imaging. By applying this technique to size-separated matrix grains and to polished sections we discovered presolar silicates in the primitive carbonaceous chondrites Acfer 094 and the CO3 chondrite ALHA 77307. The figure on the left shows O isotopic ratio images of Acfer 094 grains. The grain the arrow is pointing to has a large excess in 17O and deficit in 18O. The images of 28Si and 24MgO obtained together with the O isotope images indicate that this grain is a Mg-rich silicate. Because during isotopic raster imaging we also detect secondary electrons (SE) along with secondary ions we can use the SE image to relocate presolar grains in the SEM and analyze them for their elemental compositions by energy dispersive X-ray analysis and/or Auger Spectroscopy. The presolar silicate shown in the figure has been extracted by focused ion beam (FIB) techniques and was analyzed in the TEM. It was found to have an Fe-rich non-stoichiometric composition and an amorphous structure. Along with presolar silicates we identified also presolar oxide grains in the two analyzed meteorites. The abundances of these presolar grains were found to be much higher (as high as ~350 ppm) than those of any other presolar grain types with the possible exception of nanodiamonds. They are also higher than the abundances of presolar silicate and oxide grains in ordinary chondrites and CM2 meteorites. In addition to O, we can also measure Si and Mg isotopes. One grain was found to be enriched in 26Mg from the in situ decay of 26Al. The high inferred initial 27Al/26Al of 0.12 gives more detailed information on the nature of deep mixing processes that occurred in the parent star. Identification of new presolar grain types introduces new opportunities for studying various stellar environments as well as solar system environments. |
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Presolar Grains from Supernovae
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Low-density graphite grains, SiC grains of type X, and silicon nitride originated in the ejecta of supernovae. Proof for such an origin comes from the initial presence of 44Ti in the grains at the time of their formation. This nuclide, which is produced only in supernovae, is radioactive and decays with a half life of 60 years. Its prior presence in the grains is inferred from huge excesses of its daughter isotope 44Ca. Supernova grains also have large excesses of 28Si and 18O and large inferred 26Al/27Al ratios (from excesses in 26Mg, the decay product of the short-lived 26Al). Just before its explosion as a supernova, a massive stars has an onion-type structure shown schematically in the figure on the left. It consists of different layers (indicated in the figure by the most abundant elements) that contain the products of nuclear burning at increasing temperatures from the surface to the core. Both 44Ti and 28Si are produced in an interior zone that consists mostly of 28Si and 32S. In contrast, 18O and 26Al are found in two of the more exterior zones, where He burning produces 18O from 14N and H burning makes 26Al from Mg. The presence of isotopes that are produced in very different supernova layers in the same grain is evidence for turbulent mixing during supernova explosions.
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