Graphite Grains Research Articles

Dust Formation and Heiiλ4686 Emission in the Dense Shell of the Peculiar Type Ib Supernova 2006jc

We present evidence for the formation of dust grains in an unusual Type Ib SN based on late-time spectra of SN 2006jc. The progenitor suffered an LBV-like outburst just 2 yr earlier, and we propose that the dust formation is a consequence of the SN blast wave overtaking that LBV-like shell. The key evidence for dust formation is (a) the appearance of a red/near-IR continuum source fit by 1600 K graphite grains, and (b) fading of the redshifted sides of He I emission lines, yielding progressively more asymmetric blueshifted lines as dust obscures receding material. This provides the strongest case yet for dust formation in any SN Ib/c. Both developments occurred between 51 and 75 d after peak, while other SNe observed to form dust did so after a few hundred days. Geometric considerations indicate that dust formed in the dense swept-up shell between the forward and reverse shocks, and not in the freely expanding SN ejecta. Rapid cooling leading to dust formation may have been aided by extremely high shell densities, as indicated by He I line ratios. The brief epoch of dust formation is accompanied by He II 4686 emission and enhanced X-ray emission. These clues suggest that the unusual dust formation in this object was not due to properties of the SN itself, but instead -- like most peculiarities of SN 2006jc -- was a consequence of the dense environment created by an LBV-like eruption 2 yr before the SN.

Dust Sputtering by Reverse Shocks in Supernova Remnants

We consider sputtering of dust grains, believed to be formed in cooling supernovae ejecta, under the influence of reverse shocks. In the regime of self-similar evolution of reverse shocks, we can follow the evolution of ejecta density and temperature analytically as a function of time in different parts of the ejecta, and calculate the sputtering rate of graphite and silicate grains embedded in the ejecta as they encounter the reverse shock. Through analytic (1D) calculations, we find that a fraction of dust mass ($ 1\hbox{--}20$% for silicates and %$\le 5$% for graphites) can be sputtered by reverse shocks, the fraction varying with the grain size distribution and the steepness of the density profile of the ejecta mass. It is expected that many more grains will get sputtered in the region between the forward and reverse shocks, so that our analytical results provide a lower limit to the destroyed fraction of dust mass.

Experimental Demonstration of Plasma-Drag Acceleration of a Dust Cloud to Hypervelocities

Simultaneous acceleration of hundreds of dust particles to hypervelocities by collimated plasma flows ejected from a coaxial gun is demonstrated. Graphite and diamond grains with radii between 5 and 30 microm, and flying at speeds up to 3.7 km/s, have been recorded with a high-speed camera. The observations agree well with a model for plasma-drag acceleration of microparticles much larger than the plasma screening length.

Effects of graphite additives in electrolytes on the microstructure and corrosion resistance of Alumina PEO coatings

Abstract In the present work, graphite grains of different sizes were added into the electrolyte to prepare ceramic coatings on aluminum by plasma electrolytic oxidation (PEO). Scanning electron microscopy (SEM) coupled with an energy dispersive X-ray analysis system (EDX), Raman spectroscopy and X-ray diffractometer (XRD) were used to characterize the coatings. A three-electrode system was used to evaluate the corrosion performances of the coatings in a 3.5 wt.% NaCl solution. It was found that the morphology and corrosion performance of the coatings were significantly influenced by the size of the graphite grains. Compared with bigger graphite grains, finer ones were involved in the oxidation process and embedded within the ceramic coatings, which made the coatings less porous and more compact. Thus, the corrosion resistance of the coatings with embedded graphite grains was greatly improved.

The mineralogy of cosmic dust: astromineralogy

Stardust is newly-formed in the ejected shells of gas that surround stars towards the end of their lives. Observations of the thermal emission from this dust, which is at relatively low temperatures ( T = 50–200 K), in the circumstellar shells around these stars indicate that the dust consists of both amorphous and crystalline materials. The observed solid phases include: almost pure crystalline Mg-rich silicates (forsterite and clinoenstatite), amorphous silicates, diopside, spinel, oxides (corundum and Fe 0.9 Mg 0.1 O), and also carbon-rich solids such as: (hydrogenated) amorphous carbons, aromatic hydrocarbons and silicon carbide. Crystalline grains with isotopic signatures that indicate that they formed around evolved stars, and that therefore pre-date the formation of the solar system ( e.g. , the pre-solar silicate, nanodiamond, silicon carbide, graphite, corundum, spinel, hibonite, titanium carbide and silicon nitride grains), have now been extracted from primitive meteorites. Pre-solar forsterite and amorphous silicate grains have also been extracted from interplanetary dust particles. The dust formed around evolved stars is ejected into the surrounding interstellar medium by the relatively benign effects of stellar winds, where it is subject to stochastic and violent processing in fast supernova-generated shock waves. In this medium between the stars all the silicate dust appears to be completely amorphous and it is generally thought that dust processing, via ion irradiation/implantation in shocks and/or by cosmic rays, leads to the amorphisation of the crystalline silicate grains that were formed around the evolved stars. The formation of the dust, in circumstellar environments and subsequently ejected into the interstellar medium, is thus balanced by destructive processes that erode and eventually destroy it. This paper introduces the subject of interstellar, circumstellar and pre-solar dust composition (Astromineralogy), discusses where the dust comes from, how it evolves and what its eventual fate might be.

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.

The innermost region of AGN tori: implications from the HST/NICMOS type 1 point sources and near-IR reverberation

Spatially resolving the innermost region of the putative torus-like structure in an active galactic nucleus (AGN) is one of the main goals of high-spatial-resolution studies. This could be done in the near-IR observations of type 1 AGNs where we directly see the hottest dust grains in the torus. We discuss two critical issues in such studies. One is the possible contribution from the central putative accretion disk (the near-IR part of the big blue bump emission), which should be taken into account for the torus measurements. The other is the expected size of the inner boundary of the torus, essential for the feasibility of spatially resolving the region. We examine the nuclear near-IR point sources in the HST/NICMOS images of nearby type 1 AGNs to evaluate the accretion disk contribution. After the subtraction of the host bulge flux through two-dimensional decompositions, we show that near-IR colors of the point sources appear quite interpretable simply as a composite of a black-body-like spectrum and a relatively blue distinct component, as expected for a torus and an accretion disk in the near-IR. The near-IR colors of our radiative transfer models for clumpy tori also support this simple two-component interpretation. The observed near-IR colors for the available sample suggest a fractional accretion disk contribution of ~25% or less at 2.2 μ m. We also show that the innermost torus radii as indicated by recent near-IR reverberation measurements are systematically smaller by a factor of ~3 than the predicted dust sublimation radius with a reasonable assumption for graphite grains of a sublimation temperature 1500 K and size 0.05 μ m in radius. The discrepancy might indicate a much higher sublimation temperature or a typical grain size much larger in the innermost tori, though the former case appears to be disfavored by the observed colors of the HST point sources studied above. Alternatively, the central engine radiation might be significantly anisotropic. The near-IR interferometry with a baseline of ~100 m should be able to provide important, independent size measurements for the innermost torus region, based on the low fractional contribution from the accretion disk obtained above.

Supernova Mixtures Reproducing Isotopic Ratios of Presolar Grains

Most of supernova-originating presolar grains, such as silicon carbide type X (SiC X) and low-density graphite, show excesses of 28Si. Some of them also indicate evidence for the original presence of short-lived nuclei 44Ti. In order to reproduce isotopic and elemental signatures of these grains, large-scale heterogeneous mixing in supernova ejecta is required. I investigate supernova mixtures that reproduce as many isotopic ratios as possible of 18 individual SiC X and 26 individual low-density graphite grains. The supernova ejecta are divided into seven layers, i.e., the Ni, Si/S, O/Si, O/Ne, C/O or O/C, He/C, and He/N layers. Then, mixtures indicating isotopic ratios with small differences from those of individual single grains are sought. Mixtures reproducing five isotopic ratios in 12C/13C, 14N/15N, 26Al/ 27Al, 29Si/28Si, 30Si/ 28Si, and 44Ti/48Ti are obtained for three SiC grains. For 10 SiC X grains for which five, four, or three isotopic ratios have been measured, the mixtures reproducing all measured isotopic ratios are also found. For 20 low-density graphite grains, the mixtures reproducing six isotopic ratios in 12C/13C, 14N/ 15N, 16O/17O, 16O/18O, 26Al/27Al, 29Si/28Si, 30Si/28Si, and 44Ti/48Ti are obtained. The mixing ratios of each mixture strongly depend on reproduced isotopic ratios. The main component of most mixtures is one of the Ni, He/C, and He/N layers. The carbon-to-oxygen ratio of the mixtures is larger than unity for most cases. The ratios of Fe/C and Fe/Si in the mixtures have large varieties. Most of the mixtures for low-density graphite indicate Fe/Si larger than unity. The elemental signatures of the mixtures will constrain grain formation in supernovae.

Electrical properties of a graphite-rich quartzite from a former lower continental crust exposed in the Serre San Bruno, Calabria (southern Italy)

In this study electrical properties of a graphite-rich quartzite from the former lower crust exposed in the Serre San Bruno, Calabria (southern Italy) have been analysed by means of complex electrical conductivity measurements in the frequency range 10 −3 to 10 6 Hz on plug samples of 25 mm in diameter and 22 mm high. The samples were identified as a former oil sandstones containing up to 12–15% flake-like graphite, which present highly metamorphosed relics of its former hydrocarbon filling. Though high concentrations of graphite were detected, the graphite quartzite is highly resistive. The spatial ordering of isolated graphite grains like tiles prevent the generation of interconnected electrical pathways. These isolated good conductors are interconnected by electrolyte bridges, thus causing the high polarizability, indicating redox reactions at the electrolyte/graphite interfaces that could be modelled using CPE elements. The type of graphite ordering was caused by the early formation of isolated oil droplets in the reservoir sandstone. After high-grade metamorphism and shearing, they appear as flat graphite grains in the foliation plane and were smeared together because of the rigidity of sillimanite and quartz, the main rock constituents. Thus this graphite quartzite is by no means a candidate to enhance the electrical conductivity of the deep continental crust. Syngenetic graphite in former lower crustal rocks is largely resistant against varying geochemical conditions during prograde and retrograde metamorphism. This had also been shown for graphite that is contained in normal metapelites as accessory mineral, or enriched in former black shales where it may cause high conductivity [Jödicke, H., Kruhl, J.H., Ballhaus, C., Giese, P., Untiedt, J., 2004. Syngenetic, thin graphite-rich horizons in lower crustal rocks from the Serre San Bruno, Calabria (Italy), and implications for the nature of high-conducting deep crustal layers. PEPI 141, 37–58; Nover, G., 2005. Electrical properties of crustal and mantle rocks—a review of laboratory measurements and their explanations. Surv. Geophys. 26 (5), 593–651].

Structure and Field Emission Study of Hot Filament Chemical Vapor Deposited Nanocrystalline Diamond Films

Grain sizes of the hot filament chemical vapor deposited diamond films were found to decrease from 3 µm to 15 nm with increase in substrate temperature during deposition from 1023 K to 1173 K respectively. These films were deposited on p‐type silicon (100) substrate using methane and hydrogen as reactant gases. Surface scanning electron micrographs of the deposited films confirmed that diamond grain size was strongly dependent on the substrate temperature. High resolution transmission electron microscope studies of the nanodiamond films showed lattice fringes corresponding to graphitic planes in the interface region between two diamond nanocrystallites. Field emission studies carried out in a diode geometry revealed that nanodiamond films were better emitters than microcrystalline films, which is attributed to increase in graphitic grain boundaries with decrease in grain size.

Decomposing Dusty Galaxies. I. Multicomponent Spectral Energy Distribution Fitting

We present a new multicomponent SED decomposition method and use it to analyze the UV to millimeter wavelength SEDs of a sample of dusty infrared-luminous galaxies. Each SED is decomposed into emission from populations of stars, an AGN accretion disk, PAHs, atomic and molecular lines, and distributions of graphite and silicate grains. Decompositions of the SEDs of template starburst galaxies and AGNs provide baseline properties to aid in quantifying the strength of star formation and accretion in the composite systems NGC 6240 and Mrk 1014. We find that obscured radiation from stars is capable of powering the total dust emission from NGC 6240. The presence of a small quantity of 1260 K dust in this source suggests a ~2% AGN contribution, although we cannot rule out a larger contribution from a deeply embedded AGN visible only in X-rays. The decomposition of Mrk 1014 is consistent with ~65% of its power emerging from an AGN and ~35% from star formation. We suggest that many of the variations in our template starburst SEDs may be explained in terms of the different mean optical depths through the clouds of dust surrounding the young stars within each galaxy. Prompted by the divergent far-IR properties of our template AGNs, we suggest that variations in the relative orientation of their AGN accretion disks with respect to the disks of the galaxies hosting them may result in different amounts of AGN-heated cold dust emission emerging from their host galaxies. We estimate that 30%-50% of the far-IR and PAH emission from Mrk 1014 may originate from such AGN-heated material in its host galaxy disk.

Process development for the electroenzymatic synthesis of (R)‐methylphenylsulfoxide by use of a 3‐dimensional electrode

The electrogeneration of hydrogen peroxide via reduction of dissolved oxygen was carried out in a three-dimensional electrochemical cell using graphite grains as cathode material and combined with the enantioselective oxidation of thioanisole to (R)-methylphenylsulfoxide catalyzed by Chloroperoxidase from Caldariomyces fumago. The relevant parameters for process development (e.g., conductivity of the biotransformation medium) were identified and optimized, leading to an efficient electroenzymatic process with a space-time yield of 104 g x L(-1) x d(-1). The isolated product was produced on gram scale (1.2 g, purity >98%, ee > 98.5%).

Far-infrared dust properties in the Galaxy and the Magellanic Clouds

A recent data analysis of the far-infrared (FIR) map of the Galaxy and the Magellanic Clouds has shown that there is a tight correlation between two FIR colours: the 60 um-100 um and 100 um-140 um colours. This FIR colour relation called ``main correlation'' can be interpreted as indicative of a sequence of various interstellar radiation fields with a common FIR optical property of grains. In this paper, we constrain the FIR optical properties of grains by comparing the calculated FIR colours with the observational main correlation. We show that neither of the ``standard'' grain species (i.e. astronomical silicate and graphite grains) reproduces the main correlation. However, if the emissivity index at ~ 100--200 um is changed to ~ 1--1.5 (not ~ 2 as the above two species), the main correlation can be successfully explained. Thus, we propose that the FIR emissivity index is ~ 1--1.5 for the dust in the Galaxy and the Magellanic Clouds at ~ 100--200 um. We also consider the origin of the minor correlation called ``sub-correlation'', which can be used to estimate the Galactic star formation rate.

Infrared Emission from Interstellar Dust. IV. The Silicate‐Graphite‐PAH Model in the Post‐SpitzerEra

IR emission spectra are calculated for dust heated by starlight, for mixtures of amorphous silicate and graphitic grains, including varying amounts of PAH particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented that are consistent with observed emission spectra, including those newly obtained by Spitzer. We find a size distribution for the PAHs giving emission band ratios consistent with the observed spectra of the Milky Way and other galaxies. Emission spectra are presented for a wide range of starlight intensities. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7.7 μm emission feature is produced mainly by PAH particles containing Umin. We present graphical procedures using Spitzer IRAC and MIPS photometry to estimate the parameters qPAH, Umin, and γ, the fraction fPDR of the dust luminosity coming from photodissociation regions with U > 100, and the total dust mass Mdust.

Fluorcalciobritholite, (Ca,REE)5[(Si,P)O4]3F, a new mineral: description and crystal chemistry

The new mineral fluorcalciobritholite, ideally Ca 3 Ce 2 (SiO 4 ) 2 (PO 4 )F, has been found at Mount Kukisvumchorr, Khibiny alkaline complex, Kola Peninsula, Russia, in veinlets which contains aggregates of orthoclase, nepheline, sodalite and biotite in association with grains of fayalite, gadolinite-(Ce), zircon, monazite-(Ce), zirconolite (“polymignite”), fluorapatite, fluorite, molybdenite, lollingite and graphite. Fluorcalciobritholite forms long-prismatic hexagonal crystals up to 0.5 x 10 mm; the main crystal form is the hexagonal prism {10–10}. The mineral is transparent, with a pale pinkish to brown colour and a white streak. The hardness (Mohs) is 5.5, and the observed density is 4.2(1) g/cm 3 . Optically, it is uniaxial (−) with ω 1.735(5), e 1.730(5). Electron microprobe gave the following empirical formula based on [Si+P+S] = 3 apfu : [Ca 2.80 (Ce 0.93 La 0.54 Nd 0.26 Y 0.18 Pr 0.08 Sm 0.03 Gd 0.03 Dy 0.02 Yb 0.02 Er 0.01 ) ∑2.12 Th 0.04 Mn 0.03 Sr 0.02 ] ∑4.99 [(Si 1.94 P 1.06 ) ∑3 O 12 ] [F 0.76 O 0.22 Cl 0.01 ] ∑0.99 (Z = 2). The IR spectrum of metamict fluorcalciobritholite from Siberia showed a marked similarity with those of hydroxylbritholite-(Ce) and hydroxylbritholite-(Y). The strongest lines of the X-ray powder pattern [ d in A ( I ) ( hkl )] are: 3.51 (45) 002, 3.15 (70) 102, 2.85 (100) 211, 121, 2.78 (60) 300. The mineral is hexagonal, space group P 6 3 / m , with a = 9.580(7), c = 6.985(4) A, V = 555.2(7) A 3 . The crystal structure was refined from single-crystal X-ray diffraction data to R F = 0.029. Fluorcalciobritholite, whose simplified formula is (Ca,REE) 5 [(Si,P)O 4 ] 3 F, differs from fluorbritholite in having Ca > ∑REE, and differs from fluorapatite in having Si > P. Its compositional field falls within the limits Ca 2.5 REE 2.5 (SiO 4 ) 2.5 (PO 4 ) 0.5 F (boundary with fluorbritholite) and Ca 3.5 REE 1.5 (SiO 4 ) 1.5 (PO 4 ) 1.5 F (boundary with fluorapatite). Both the mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names.

Dust yields in clumpy supernova shells: SN 1987A revisited

We present a study of the effects of clumping on the emergent spectral energy distribution (SED) from dusty supernova (SN) shells illuminated by a diffuse radiation source distributed throughout the medium. These models are appropriate for Type II SNe older than a few hundred days, when the energy input is dominated by γ-rays from 56 Co decay. The fully 3D radiation transport problem is solved using a Monte Carlo code, MOCASSIN, and we present a set of models aimed at investigating the sensitivity of the SEDs to various ·clumping parameters. We find that, contrary to the predictions of analytical prescriptions, the combination of an optical and infrared (IR) observational data set is sufficient to constrain dust masses even in the case where optically thick clumps are present. Using both smoothly varying and clumped grain density distributions, we obtain new estimates for the mass of dust condensed by the Type II SN 1987A by fitting the optical and IR spectrophotometric data of Wooden et al. at two epochs (day 615 and day 775). When using amorphous carbon grains, our best fits to the observational data imply that about 2.0 x 10 -4 M O of dust had condensed in the envelope of SN 1987A by day 615 and between 2.0 x 10 -4 and 4.2 x 10 -3 M ⊙ by day 775. We find that the absence of a silicate emission or absorption feature in the observed mid-IR spectra implies that no more than 15 per cent of the dust formed around SN 1987A can have been in the form of silicate particles. Our models require larger dust masses for the case of graphite grains, namely between 4.2 x 10 -4 and 6.6 x 10 -4 M ⊙ at day 615 and between 4.5 x 10 -4 and 7.5 x 10 -4 M ⊙ at day 775. From our numerical models, we derive dust masses for SN 1987A that are comparable to previous analytic clumped graphite grain mass estimates, and at least two orders of magnitude below the 0.1-0.3 M ⊙ that have been predicted to condense as dust grains in primordial core-collapse SN ejecta. This low condensation efficiency for SN 1987A is in contrast to the case of SN 2003gd, for which a dust condensation efficiency as large as 0.12 has recently been estimated.

Determining misorientation of graphite grains from the angular dependence of X-ray emission spectra

Angular-resolved X-ray absorption spectra were measured for pyrolytic graphite samples of various quality. A new approach to determining the misorientation of graphite grains in polycrystalline samples is proposed, which is based on calculations of the angular dependence of the relative intensity of a peak corresponding to the π* state for a normal distribution of grains. The experimental values are used to construct theoretical angular dependences using partial densities of the π* and σ* states determined from the nonempirical calculations for graphite.

The Kalguty complex deposit, the Gorny Altai: Mineralogical and geochemical characteristics and fluid regime of ore formation

The results of detailed mineralogical, geochemical, and thermobarogeochemical studies of the Kalguty complex greisen deposit are presented. The chemical compositions of ore veins, greisens, and other geological bodies have been determined. A wide range of chemical elements from Li to U (48 elements, including noble metals and REE) has been determined in ore minerals. Graphite in association with quartz and sulfides was identified in ore veins for the first time. Graphite is enriched in a light carbon isotope. The δ13C value varies from −26.3 ± 0.4 to −26.6 ± 0.3‰. High Au, Ag, Hg, Te, Sb, Bi, Cu, Pb, Zn, Fe, and S contents were detected in graphite grains with a microprobe. The graphite content increases regularly with depth; the spatial correlation of graphite with W, Mo, Cu, Au, Pt, Pd, and other metals is established. The highest Au, Ag, Pt, Pd, and Os contents are characteristic of minor intrusions of albitized granite porphyry (γπ2J1vk), intramineral dikes of hydrothermally altered kalgutite (γπ3J1vk), ore veins, their greisen selvages, and autonomous ore-bearing greisen bodies of the Mo stock type. Gold occurs in native form, while silver is contained largely in sulfides and sulfosalts. High PGE contents are characteristic of pyrite, wolframite, and molybdenite. The major components of fluid inclusions in quartz (H2O, CO2, CO, and H2) have been studied, as well as hydrocarbons (CH4, C2H6, C3H8, C4H10, C5H12, C6H14, C2H2, and C2H4) contained therein. Two-phase fluid inclusions are predominant, while single-and three-phase inclusions are less abundant. The homogenization temperatures of primary and secondary inclusions are 290–340 and 140–160°C, respectively. The concentration of dissolved salts (NaCl and KCl) in two-phase inclusions amounts to 11.6–14.0%. The H2O and CO2 contents decrease with depth, whereas the CO, H2, and HC concentrations increase in the same direction. Graphite is regarded as a product of reactions with participation of fluid (gas) components. The ore mineralization was formed under contrasting conditions related to the oxidation of a primary reduced deep metalliferous fluid.

Pre-solar grains from supernovae and novae

AbstractPre-solar grains from supernova ejecta – silicon carbide of type X, Si3N4 and low-density graphite – are characterized by Si isotopic anomalies (mainly 28Si excesses), low 14N/15N, high 26Al/27 Al ratios, and occasionally by excesses in 44Ca (from 44Ti decay). Overall isotopic features of these SiC and graphite grains can be explained by mixing of inner Si-rich zones and the outer C-and He-rich zones, but supernova models require fine tuning to account for 14N/15N and 29Si/28Si ratios of the grains. Isotopic ratios of Zr, Mo and Ba in SiC X grains may be explained by a neutron burst model. Some of the pre-solar nanodiamonds require a supernova origin to explain measured xenon isotopic ratios. Only a few nova grain candidates, with low 12C/13C, 14N/15N, and high 26Al/27 Al ratios, have been identified.

They Came from the Deep in the Supernova: The Origin of TiC and Metal Subgrains in Presolar Graphite Grains

A new formation scenario for TiC and Fe-Ni metal inclusions in presolar graphite grains of supernova origin is described. The mineralogy and chemistry require condensation of Fe-Ni titanides from Fe-, Ni-, and Ti-rich gaseous ejecta, subsequent carburization to make TiC and metal, and encapsulation into graphite. Titanides only condense if Si is depleted relative to heavier elements, which requires α-rich freeze-out and a deep mass cut for the supernova ejecta. This Si-poor core material must remain unmixed with other supernova zones until the titanides condense. This can be accomplished by transport of core ejecta in bipolar jets through the major expanding supernova zone ejecta. If the jets stall in regions dominated by C-rich ejecta such as the C-He zone, where graphite condenses, thermochemically favored in situ carburization of the titanides—either before or during encapsulation into condensing graphite—leads to a TiC-and-metal composite. This scenario agrees with theoretical models and observations of asymmetric core collapse in supernovae that are associated with bipolar jets loaded with iron-peak elements.