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Carbon, Nitrogen, Magnesium, Silicon, and Titanium Isotopic Compositions of Single Interstellar Silicon Carbide Grains from the Murchison Carbonaceous Chondrite

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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.
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... The first SiC grain to be discovered with a markedly different isotopic composition, presumably formed in a Type II supernova, was called "Type X" Nittler et al. 1996;Hoppe et al. 2000). Subsequent distinctive finds of presolar SiC were called types Y and Z (Alexander 1993;Hoppe et al. 1994Hoppe et al. , 1997Amari et al. 2001a;Nittler and Alexander 2003), followed by A, B, and C. Types A and B were later merged into "Type AB," though Liu et al. (2017c) subsequently proposed a split into AB1 and AB2 based on nitrogen isotopes. In addition, Liu et al. (2016) suggested splitting C into C1 and C2 based on carbon isotopes, while Type X is sometimes subdivided into X0, X1, and X2 (Lin et al. 2010). ...
... Several moissanite grains (a few percent of presolar SiC), dubbed "Y" type, are presumed to come from AGB stars with ~50% solar metallicity (Hoppe et al. 1994). These grains have the unusual combination of high 12 C/ 13 C and high 14 N/ 15 N (Amari et al. 2001a;Nguyen et al. 2018), possibly with excesses of s-process Ti and Mo isotopes (Larry Nittler, personal communications). ...
... The first SiC grain to be discovered with a markedly different isotopic composition, presumably formed in a Type II supernova, was called "Type X" Nittler et al. 1996;Hoppe et al. 2000). Subsequent distinctive finds of presolar SiC were called types Y and Z (Alexander 1993;Hoppe et al. 1994Hoppe et al. , 1997Amari et al. 2001a;Nittler and Alexander 2003), followed by A, B, and C. Types A and B were later merged into "Type AB," though Liu et al. (2017c) subsequently proposed a split into AB1 and AB2 based on nitrogen isotopes. In addition, Liu et al. (2016) suggested splitting C into C1 and C2 based on carbon isotopes, while Type X is sometimes subdivided into X0, X1, and X2 (Lin et al. 2010). ...
... Several moissanite grains (a few percent of presolar SiC), dubbed "Y" type, are presumed to come from AGB stars with ~50% solar metallicity (Hoppe et al. 1994). These grains have the unusual combination of high 12 C/ 13 C and high 14 N/ 15 N (Amari et al. 2001a;Nguyen et al. 2018), possibly with excesses of s-process Ti and Mo isotopes (Larry Nittler, personal communications). ...
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Minerals preserve records of the physical, chemical, and biological histories of their origins and subsequent alteration, and thus provide a vivid narrative of the evolution of Earth and other worlds through billions of years of cosmic history. Mineral properties, including trace and minor elements, ratios of isotopes, solid and fluid inclusions, external morphologies, and other idiosyncratic attributes, represent information that points to specific modes of formation and subsequent environmental histories—information essential to understanding the co-evolving geosphere and biosphere. This perspective suggests an opportunity to amplify the existing system of mineral classification, by which minerals are defined solely on idealized end-member chemical compositions and crystal structures. Here we present the first in a series of contributions to explore a complementary evolutionary system of mineralogy—a classification scheme that links mineral species to their paragenetic modes. The earliest stage of mineral evolution commenced with the appearance of the first crystals in the universe at >13 Ga and continues today in the expanding, cooling atmospheres of countless evolved stars, which host the high-temperature (T > 1000 K), low-pressure (P < 10-2 atm) condensation of refractory minerals and amorphous phases. Most stardust is thought to originate in three distinct processes in carbon- and/or oxygen-rich mineral-forming stars: (1) condensation in the cooling, expanding atmospheres of asymptotic giant branch stars; (2) during the catastrophic explosions of supernovae, most commonly core collapse (Type II) supernovae; and (3) classical novae explosions, the consequence of runaway fusion reactions at the surface of a binary white dwarf star. Each stellar environment imparts distinctive isotopic and trace element signatures to the micro- and nanoscale stardust grains that are recovered from meteorites and micrometeorites collected on Earth’s surface, by atmospheric sampling, and from asteroids and comets. Although our understanding of the diverse mineral-forming environments of stars is as yet incomplete, we present a preliminary catalog of 41 distinct natural kinds of stellar minerals, representing 22 official International Mineralogical Association (IMA) mineral species, as well as 2 as yet unapproved crystalline phases and 3 kinds of non-crystalline condensed phases not codified by the IMA.
... Compared to the t-distribution results shown in the previous section, the normal distribution results here (1) point out that the division of X grains into two clusters is not robust and (2) shows the division of AB grains into two clusters enriched or depleted in 15 N and 13 C, with a strong o v erlap at 14 N/ 15 N ≈ 500-2000. Note that AB grains were initially divided into A and B grains by adopting a divider of 12 C/ 13 C = 3.5 (Hoppe et al. 1994 ). The stability test further shows that the o v erlapping parts of the AB grains are not stable and are ambiguous regarding their cluster assignment. ...
Article
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... 48 Ti (73.81%), 49 Ti (5.50%), and 50 Ti (5.35%). These isotopes are of interest because they exhibit striking mass-independent variations ('isotopic anomalies'; Niederer et al., 1980;Niederer et al., 1981;Niemeyer and Lugmair, 1981;Heydegger et al., 1982), which have been studied primarily as tracers of nucleosynthetic processes, early solar-system processes, and cosmogenic effects (Niederer et al., 1980;Niederer et al., 1981;Niemeyer and Lugmair, 1981;1984;Niemeyer, 1985;Niederer et al., 1985;Zinner et al., 1986;Hinton et al., 1987;Fahey et al., 1987;Niemeyer, 1988a, b;Papanastassiou and Brigham, 1989;Ireland, 1990;Hoppe et al., 1994;Stadermann et al., 2005;Leya et al., 2007;Leya et al., 2008;Leya et al., 2009;Trinquier et al., 2009;Zhang et al., 2011;Zhang et al., 2012;2014;Larsen et al., 2018;Kommescher et al., 2020). No Ti isotope anomalies are produced during geological processes, although Ti may be activated, migrated, and redistributed during processes involving melts and fluids, e.g., mantle metasomatism, and this may lead to mass-dependent fractionation (Millet et al., 2016;Greber et al., 2017a, b;Deng et al., 2018a, b;Johnson et al., 2019). ...
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... Isotope measurements exist for thousands of 0.1-20 m-sized meteoritic grains with non-solar isotope ratios of pre-solar origin (e.g. Hoppe et al. 1994;Speck, Thompson, & Hofmeister 2005) suggesting that they somehow travelled from C-stars to the Solar Nebula. However, SiC has not been detected in the interstellar medium (e.g Whittet, Duley, & Martin 1990). ...
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6-14 micron Spitzer spectra obtained at 6 epochs between April 2005 and October 2008 are used to determine temporal changes in dust features associated with Sakurai's Object (V4334 Sgr), a low mass post-AGB star that has been forming dust in an eruptive event since 1996. The obscured carbon-rich photosphere is surrounded by a 40-milliarcsec torus and 32 arcsec PN. An initially rapid mid-infrared flux decrease stalled after 21 April 2008. Optically-thin emission due to nanometre-sized SiC grains reached a minimum in October 2007, increased rapidly between 21-30 April 2008 and more slowly to October 2008. 6.3-micron absorption due to PAHs increased throughout. 20 micron-sized SiC grains might have contributed to the 6-7 micron absorption after May 2007. Mass estimates based on the optically-thick emission agree with those in the absorption features if the large SiC grains formed before May 1999 and PAHs formed in April-June 1999. Estimated masses of PAH and large-SiC grains in October 2008, were 3 x 10 -9 Msun and 10 -8 Msun, respectively. Some of the submicron-sized silicates responsible for a weak 10 micron absorption feature are probably located within the PN because the optical depth decreased between October 2007 and October 2008. 6.9 micron absorption assigned to ~10 micron-sized crystalline melilite silicates increased between April 2005 and October 2008. Abundance and spectroscopic constraints are satisfied if about 2.8 per cent cent of the submicron-sized silicates coagulated to form melilites. This figure is similar to the abundance of melilite-bearing calcium-aluminium-rich inclusions in chondritic meteorites.
... Decay of the key 92-keV resonance in the 25 Mg(p, γ ) reaction to the ground and isomeric states of the cosmic γ -ray emitter 26 Al abundance ratios found in carbonaceous chondrites [4] and presolar grains [5]. 26 Al has a long-lived ground-state (T 1/2 = 0.7 My) with a spinparity J π = 5 + , and a short-lived (T 1/2 = 6.35 s) 0 + isomeric state ...
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The 92-keV resonance in the ²⁵Mg(p,γ)26Al reaction plays a key role in the production of ²⁶Al at astrophysical burning temperatures of ≈100 MK in the Mg-Al cycle. However, the state can decay to feed either the ground, 26gAl, or isomeric state, 26mAl. It is the ground state that is critical as the source of cosmic γ rays. It is therefore important to precisely determine the ground-state branching fraction f0 of this resonance. Here we report on the identification of four γ-ray transitions from the 92-keV resonance, and determine the spin of the state and its ground-state branching fraction f0=0.52(2)stat(6)syst. The f0 value is the most precise reported to date, and at the lower end of the range of previously adopted values, implying a lower production rate of 26gAl and its cosmic 1809-keV γ rays.
... Error bars are smaller than symbol sizes in most cases, except for presolar grains. Modified from Torrano et al. (2019) with data from the literature (Alexander and Nittler 1999;Amari et al. 2001;Davis et al. 2018;Gerber et al. 2017;Gyngard et al. 2018;Hoppe et al. 1994;Huss and Smith 2007;Ireland et al. 1991;Kööp et al. 2016;Niemeyer 1988;Render et al. 2019;Torrano et al. 2019;Trinquier et al. 2009;Zhang et al. 2012;Zinner et al. 2007) that enables rapid growth of gas-giant planets and provides the critical mass necessary to allow efficient gravitational capture of nebular gas (e.g., Wasson 1988). This provides an explanation for the H-and He-rich atmospheres of the outer planets and their much larger masses than the terrestrial planets. ...
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The ³³S(n,$alpha$â) and ³³S(n,$gamma$) cross sections ; have been measured from approximately 10 to approximately 700 keV. Resonance ; parameters are given for 39 resonances. The level spacing is determined to be ; 9.1 +- 0.9 keV. The sigma (n,$alpha$â) and sigma (n,$gamma$) cross ; sections are averaged over a Maxwellian distribution for values of kT from 25 to ; 275 keV. When these cross sections are used in a nucleosynthesis calculation of ; the rare isotope ³⁶S, overproduction of this isotope, relative to the other ; nuclei formed in the universe, is reduced from a factor of 10 to 2.5.;
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The original CN cycle of Bethe (1939) and von Weiszaecker (1938) provides the main contribution to energy generation and the synthesis of CN nuclei in main sequence stars more massive than the sun. An extension of the considered reactions to processes involving also oxygen isotopes led to the CNO bi-cycle considered by Burbidge et al. (1957). A review is present of the possible reactions of the CNO nuclei. Attention is given to general CNO cycles, CNO cycles including F-17 and F-18 proton captures, the ratio of final nitrogen to final carbon abundances as function of temperature and density, and the conditions in fast CNO cycles.
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Interstellar SiC recovered from primitive chondrites is characterized by large enrichments in ^13C and ^14N, and by Si isotope compositions distinct from solar that define a linear array on a Si 3-isotope plot [1-3]. While most SiC >2 micrometers fit this description, the total population of presolar SiC is not homogeneous. A comparison [4] of C, N, Mg, and Si isotopic characteristic of 2-6 micrometer SiC crystals from Orgueil with Murchison SiC [2,3] found two significant differences: 1) Orgueil SiC do not exhibit the clustering on either an Si 3-isotope plot or on a plot of delta^29Si vs. delta^13C shown by large Murchison SiC, and 2) there were no Orgueil SiC with delta^15N between 0 and -350 per mil. The interpretation of these differences was tempered by the relatively small number of Orgueil SiC that had been measured. We report here new Si and C isotope data for an additional 70 Orgueil SiC grains, making a total of ~110 grains measured to date. We have extended our study to smaller grains, measuring 15 grains less than 2 micrometers. The great majonty (~90%) of Orgueil SiC have roughly similar isotopic compositions and define the main population (Fig. 1). Among the remaining SiC grains, one is extremely enriched in ^28Si (Fig. 1 inset) and extremely depleted in ^13C, like the Murchison 'X' grains [3]. Four grains have delta^13C 0. Eight Orgueil SiC have extreme enrichments in ^13C (8000 per mil < delta^13C < 3000 per mil); six of the eight lie on the ^28Si-rich end of the Si isotope array (Fig. 1), five with delta^29Si