The 11.5 micrometer emission from carbon stars: Comparison with IR spectra of submicrometer-sized silicon carbide grains

In this paper the results of an experimental investigation of the spectra of submicrometersized silicon carbide grains are presented. The grains manufactured from two types of commercial -SiC were prepared according to the Jena IR spectroscopy program for particulates of cosmic importance. From the spectral records mass absorption coefficients have been derived. These data have been used to make a comparison of the laboratory spectra with the well-known 11.5 m emission band observed in the spectra of carbon stars, which is probably due to a transition in circumstellar SiC grains. For this aim, a simple model of an optically thin circumstellar envelope containing SiC grains has been calculated. The theoretical profile of the 11.5 m band derived by means of this model and based on the experimental mass absorption coefficients of SiC grains shows a striking similarity with the observed profile in the spectrum of the carbon star Y CVn. The total amount of SiC dust in the envelope of this star has been estimated at about 10²⁴ g.     

Presolar silicon carbide grains and their parent stars

Abstract— Carbon stars are an important source of presolar TiC, SiC, and graphite grains found in meteorites. The elemental abundances in the stellar sources of the SiC grains are inferred by using condensation calculations. These elemental abundances, together with C isotopic compositions, are used to identify possible groups of carbon stars that may have contributed SiC grains to the presolar dust cloud. The most likely parent stars of meteoritic SiC mainstream grains are N-type carbon stars and evolved subgiant CH stars. Both have s-process element abundances higher than solar and 10 < ¹²C/¹³C < 100 ratios. The J stars and giant CH stars, with solar and greater than solar abundances of s-process elements, respectively, are good candidate parents for the ‘A’ and ‘B’ SiC grains with low ¹²C/¹³C ratios. A special subgroup of CH giant stars with very large ¹²C/¹³C ratios could have parented the ‘Y’ SiC grains with ¹²C/¹³C ratios > 100. The carbon star population (e.g., N, R, J, CH groups) needed to provide the observed SiC grains is compared to the current population of carbon stars. This comparison suggests that low-metallicity CH stars may have been more abundant in the past (>4.5 Ga ago) than at present. This suggestion is also supported by condensation-chemistry modeling of the trace element patterns in the SiC grains that shows that subsolar Fe abundances may be required in the stellar sources for many SiC grains. The results of this study suggest that presolar SiC grains in meteorites can provide information about carbon stars during galactic evolution.     

Interstellar graphite in meteorites: Isotopic compositions and structural properties of single graphite grains from Murchison

Abstract— One hundred forty-three carbon grains, ranging in size from 2 to 8 μm, from two chemical and physical separates from the Murchison CM2 chondrite, were analyzed by ion microprobe mass spectrometry for their C- and N-isotopic compositions. Both separates are enriched in the exotic noble gas component Ne-E(L). Ninety grains were also analyzed for their H and O contents and 118, for Si. Thirteen grains were analyzed by micro-sampling laser Raman spectroscopy. Round grains have large C-isotopic anomalies with ¹²C/¹³C ratios ranging from 7 to 4500 (terrestrial ratio = 89). Nitrogen in these grains is also anomalous but shows much smaller deviations from the terrestrial composition, ¹⁴N/¹⁵N ratios ranging from 193 to 680 (terrestrial ratio = 272). Spherulitic aggregates and non-round compact grains have normal C-isotopic ratios but ¹⁵N excesses (up to 35%). Raman spectra of the analyzed grains indicate varying degrees of crystalline disorder of graphite with estimated in-plane crystallite dimensions varying from 18 Å (highly disordered, similar to terrestrial kerogen) to ～750 Å (well-crystallized graphite). Element contents of H, O, and Si are correlated with one another, and H and O are probably present in the form of organic molecules. On the basis of morphology, the round grains fall into two groups: grains with smooth, shell-like surfaces (“onions”) and grains that appear to be dense aggregates of small scales (“cauliflowers”). “Onions” tend to have lower trace element contents, isotopically light C (¹²C/¹³C > 89) and a high degree of crystalline order, whereas “cauliflowers” have a larger spread in trace element contents and C-isotopic ratios (they range from isotopically light to heavy) but tend to have a low degree of crystalline order. However, these differences exist only on average, and no clear distinction can be made for individual grains. A few limited conclusions can be drawn about the astrophysical origin of the carbon grains of this study. The ¹⁵N excesses in spherulitic aggregates and non-round grains can be explained as the result of ion-molecule reactions in molecular clouds. The round grains, on the other hand, must have formed in stellar atmospheres (circumstellar grains). Grains with isotopically light C must have formed in stellar environments characterized by He-burning, either in the atmosphere of Wolf-Rayet stars during the WC phase or in the He-burning, ¹²C-rich zone of a massive star, ejected by a supernova explosion. Isotopically heavy C is produced by H-burning in the CNO cycle. Possible sources for grains with heavy C are carbon stars (AGB stars during the thermally pulsing phase) or novae, but the detailed distribution of ¹²C/¹³C ratios agree neither with the distribution observed in carbon stars nor with theoretical predictions for these two types of stellar sources.     

Trace-element concentrations in single circumstellar silicon carbide grains from the Murchison meteorite

Abstract— Concentrations of the trace elements Mg, Al, Ca, Ti, V, Fe, Sr, Y, Zr, Ba and Ce were determined by ion microprobe mass spectrometry in 60 individual silicon carbide (SiC) grains (in addition, Nb and Nd were determined in 20 of them), from separate KJH (size range 3.4–5.9 μm) of the Murchison carbonaceous meteorite, whose C-, N- and Si-isotopic compositions have been measured before (Hoppe et al., 1994) and provide evidence that these grains are of stellar origin. The selected SiC grains represent all previously recognized subgroups: mainstream (20 < ¹²C/¹³C < 120; 200 < ¹⁴N/¹⁵N; Si isotopes on slope 1.34 line), grains A (¹²C/¹³C < 3.5), grains B (3.5 < ¹²C/¹³C < 10), grains X (¹⁵N excesses, large ²⁸Si excesses) and grains Y (150 < ¹²C/¹³C < 260; Si isotopes on slope 0.35 line). Data on these grains are compared with measurements on fine-grained SiC fractions. Trace-element patterns reflect both the condensation behavior of individual elements and the source composition of the stellar atmospheres. A detailed discussion of the condensation of trace elements in SiC from C-rich stellar atmospheres is given in a companion paper by Lodders and Fegley (1995). Elements such as Mg, Al, Ca, Fe and Sr are depleted because their compounds are more volatile than SiC. Elements whose compounds are believed to be more refractory than SiC can also be depleted due to condensation and removal prior to SiC condensation. Among the refractory elements, however, the heavy elements from Y to Ce (and Nd) are systematically enriched relative to Ti and V, indicating enrichments by up to a factor of 14 of the s-process elements relative to elements lighter than Fe. Such enrichments are expected if N-type carbon stars (thermally pulsing AGB stars) are the main source of circumstellar SiC grains. Large grains are less enriched than small grains, possibly because they are from different AGB stars. The trace-element patterns of subgroups such as groups A and B and grains X can at least qualitatively be understood if grains A and B come from J-type carbon stars (known to be lacking in s-process enhancements shown by N-type carbon stars) or carbon stars that had not experienced much dredge-up of He-shell material and if grains X come from supernovae. However, a remaining puzzle is how stars become carbon stars without much accompanying dredge-up of s-process elements.     

The formation of condensed carbon particles in the atmospheres of white dwarfs

The existence of condensed carbon in the form of liquid droplets and graphite grains is found in white dwarf atmospheres with parametersg=10⁸ cm s^–2, H/He10^–3, andT _eff6000 K on the basis of model atmospheres techniques. It is shown that the condensation layers are dynamically stable and, consequently, that white dwarfs cannot supply the condensed particles to the interstellar medium. Possible observable effects are considered.     

Comparison of the Raman spectra of ion irradiated soot and collected extraterrestrial carbon

We use a low pressure flame to produce soot by-products as possible analogues of the carbonaceous dust present in diverse astrophysical environments, such as circumstellar shells, diffuse interstellar medium, planetary disks, as well as in our own Solar System. Several soot samples, displaying an initial chemical diversity from aromatic to aliphatic dominated material, are irradiated with 200-400 keV H⁺, He⁺, and Ar⁺⁺ ions, with fluences comprised between 10¹⁴ and 10¹⁶ ions/cm², to simulate expected radiation induced modification on extraterrestrial carbon. The evolution of the samples is monitored using Raman spectroscopy, before, during, and after irradiation. A detailed analysis of the first- and second-order Raman spectra is performed, using a fitting combination of Lorentzian and/or Gaussian-shaped bands. Upon irradiation, the samples evolve toward an amorphous carbon phase. The results suggest that the observed variations are more related to vacancy formation than ionization processes. A comparison with Raman spectra of extraterrestrial organic matter and other irradiation experiments of astrophysically relevant carbonaceous materials is presented. The results are consistent with previous experiments showing mostly amorphization of various carbonaceous materials. Irradiated soots have Raman spectra similar to those of some meteorites, IDPs, and Comet Wild 2 grains collected by the Stardust mission. Since the early-Sun expected irradiation fluxes sufficient for amorphization are compatible with accretion timescales, our results support the idea that insoluble organic matter (IOM) observed in primitive meteorites has experienced irradiation-induced amorphization prior to the accretion of the parent bodies, emphasizing the important role played by early solar nebula processing.     

Raman spectroscopic investigation of two grains from comet 81P/Wild 2: Information that can be obtained beyond the presence of sp2‐bonded carbon

Abstract– Raman analyses were performed of individual micrometer‐sized fragments of material returned to Earth by the NASA Stardust mission to comet 81P/Wild 2. The studied fragments originated from grains (C2054,0,35,91,0 and C2092,6,80,51,0) of two different penetration tracks that occurred in two different silica aerogel collector cells. All fragments of both particles have Raman spectra characteristic of amorphous sp²‐bonded carbon that are in general agreement with the results published in previous Stardust particle studies. The present study, however, does not focus on the discussion of specific details of the D and G band parameters, but rather reports on additional information that can be obtained from returned Stardust samples via Raman spectroscopy. Most notably, the Raman spectra show that all analyzed fragments of the particles were contaminated with the capture medium (i.e., aerogel). The silica aerogel is laced with organic aliphatic and aromatic hydrocarbon impurities that resulted in strong bands in the ～ 2900 Δcm^?1 spectral range (C‐H stretching modes). Aerogel bands are also found in the 1000–1600 Δcm^?1 spectral range, where they overlap with the bands of the amorphous sp²‐bonded carbon. The peaks associated with the aerogel contamination differ between the two grains that originated from two different aerogel cells. In addition to the bands due to aerogel contamination and the always present sp²‐bonded carbon bands, fragments of particle C2092,6,80,51,0 also show Raman peaks for pyrrhotite and Fa₃₀Fo₇₀ olivine. Complete (up to 4000 Δcm^?1) raw and baseline‐corrected Raman spectra of the Stardust particles are shown and discussed in detail.     

Interstellar Grains in Primitive Meteorites: Diamond,Silicon Carbide,and Graphite

Abstract— Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O > 1), and apparently are “stardust” formed in stellar atmospheres. Microdiamonds, of median size ～ 10 Å, are most abundant (～ 400–1800 ppm) but least understood. They contain anomalous noble gases including Xe-HL, which shows the signature of the r- and p-processes and thus apparently is derived from supernovae. Silicon carbide, of grain size 0.2–10 μm and abundance ～ 6 ppm, shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars of 1–3 solar masses. Some grains appear to be ≥10⁹ a older than the Solar System. Graphite spherules, of grain size 0.8–7 μm and abundance <2 ppm, contain highly anomalous C and noble gases, as well as large amounts of fossil ²⁶Mg from the decay of extinct ²⁶Al. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars.     

Intrinsic properties of carbon stars. II. Spectra,colours, and HR diagram of cool carbon stars

On the basis of the effective temperature scale proposed previously for cool carbon stars (Paper I), other intrinsic properties of them are examined in detail. It is shown that the major spectroscopic properties of cool carbon stars, including those of molecular bands due to polyatomic species (SiC₂, HCN, C₂H₂ etc.), can most consistently be understood on the basis of our new effective temperature scale and the theoretical prediction of chemical equilibrium. Various photometric indices of cool carbon stars also appear to be well correlated with the new effective temperatures. Furthermore, as effective temperatures of some 30 carbon stars are now obtained, the calibration of any photometric index is straightforward, and some examples of such a calibration are given. In general, colour index-effective temperature calibrations for carbon stars are quite different from those for K-M giant stars. It is found that the intrinsic (R —I)₀ colour is nearly the same for N-irregular variables in spite of a considerable spread in effective temperatures, and this fact is used to estimate the interstellar reddening of carbon stars. An observational HR diagram of red giant stars, including carbon stars as well as K-M giant stars, is obtained on the basis of our colour index-effective temperature calibrations and the best estimations of luminosities. It is shown that carbon stars and M giant stars are sharply divided in the HR diagram by a nearly vertical line at aboutT _eff = 3200 K (logT _eff = 3.50) and the carbon stars occupy the upper right region of M giant stars (except for some high luminosity, high temperature J-type stars in the Magellanic Clouds; also Mira variables are not considered). Such an observational HR diagram of red giant stars shows rather a poor agreement with the current stellar evolution models. Especially, a more efficient mixing process in red giant stars, as compared with those ever proposed, is required to explain the formation of carbon stars.     

Populations of nanodiamond grains in meteorites from the data on isotopic composition and content of nitrogen

The present study has shown that the dependence of the isotopic composition of nitrogen on the N/C ratio, revealed from the data for bulk samples of meteoritic nanodiamond, can be obtained within the framework of the following model of the composition of populations of nanodiamond grains: (a) initial nanodiamond, i.e., the nanodiamond in the protoplanetary cloud before the accretion of the meteorite parent bodies, was composed mainly of grains of two populations (denoted as C_N and C_F), the ratio of which changed in meteorites depending on the degree of hydrothermal metamorphism; (b) only the grains of one of these populations (C_N) contain volume-bound nitrogen with δ¹⁵N = ?350‰; (c) the grains of both populations contain surface-bound nitrogen (δ¹⁵N ≡ 0). The calculations revealed the following properties of population grains in this model. (1) The grains of the C_N and C_F populations are most likely the same in isotopic composition of carbon and heterogeneous in distribution of its isotopes: the central part of grains is enriched with the δ¹²C isotope relative to the remainder of the grain. While the value of δ¹³C is ?37.3 ± 1.1‰ for carbon in the central part, it is ?32.8 ± 1.5‰ for the whole volume of the grains. (2) The noble gases of the HL component, specifically Xe-HL, are anomalous in isotopic composition and are most likely contained in the third population of nanodiamond grains (denoted as C_HL), the mass fraction of which is negligible relative to that for other grain populations. Only the grains of the C_HL population have an undoubtedly presolar origin, while the grains of the other nanodiamond populations could have formed at the early stages of the evolution of the protoplanetary cloud material before the accretion of the meteoritic parent bodies.     

The nature of silicon carbide: Astronomical observations versus meteoritic evidence

Abstract— We have investigated the 7.5–13.5 μm spectra of 30 definite or candidate carbon stars. We discuss the discrepancies between properties of SiC grains found in meteorites and the spectral properties of dust emitting in red giant winds, where most of the meteoritic grains are believed to have formed. We have investigated the nature of carbon star SiC and its relationship to meteoritic SiC dust, by using a X²-minimisation routine to fit the observed SiC features with laboratory optical constants that have been published for a variety of SiC samples. All but one of the observed astronomical SiC features are best fitted by α-SiC grains. All but one of the sources with 8–13 μm colour temperatures >1200 K (corresponding to mass-loss rates at the bottom end of the range) are best fitted by α-SiC in pure emission; whereas, all but one of the sources with 8–13 μm colour temperatures <1200 K (corresponding to higher mass-loss rates) are best fitted using self-absorbed α-SiC emission. The four sources whose SiC features are in net absorption (and which have the lowest 8–13 μm colour temperatures and, therefore, presumably the highest mass-loss rates) are also well fitted by self-absorbed α-SiC emission but with higher optical depths. Given that β-SiC is the form most commonly found in meteorites, we have searched for evidence of β-SiC in the circumstellar shells of all these stars. However, our observations provide no unequivocal evidence for the presence of β-SiC around these stars. Other discrepancies between meteoritic SiC grains and astronomical spectra are discussed. The self-absorption that we find in the observed SiC emission features has not previously been taken into account in radiative transfer modeling and so the amount of SiC present in the outflows may have been underestimated in the past.     

Optical properties of the carbon dust grains in the envelopes around asymptotic giant branch stars

We have investigated the optical properties of the carbon dust grains in the envelopes around carbon-rich asymptotic giant branch stars, paying close attention to the infrared observations of the stars and the laboratory-measured optical data of the candidate dust grain materials. We have compared the radiative transfer model results with the observed spectral energy distributions of the stars including IRAS Point Source Catalog and IRAS Low Resolution Spectrograph data. We have deduced an opacity function of amorphous carbon dust grains from model fitting with infrared carbon stars. From the opacity function, we have derived the optical constants of the AMC grains. The optical constants satisfy the Kramers–Kronig relation and produce the opacity function that fits the observations of infrared carbon stars better than previous works in the wide wavelength range 1–1000 μm. We have used simple mixtures of the AMC and silicon carbide grains for modelling. We have compared the contributions that AMC and SiC grains make to the opacity for the cases of simple mixtures of them and spherical core–mantle type grains consisting of a SiC core and an AMC mantle .     

Electron energy deposition in carbon dioxide

The processes by which energetic electrons lose energy in a weakly ionized gas of carbon dioxide are discussed and a consistent set of electron impact cross-sections is compiled. Calculations of the excitations, ionizations and neutral particle heating produced by the absorption of electrons in carbon dioxide gas are carried out for fractional ionizations ranging from 10^?2 to 10^?6.     

Sulfur in presolar silicon carbide grains from asymptotic giant branch stars

We studied 14 presolar SiC mainstream grains for C‐, Si‐, and S‐isotopic compositions and S elemental abundances. Ten grains have low levels of S contamination and CI chondrite‐normalized S/Si ratios between 2 × 10^?5 and 2 × 10^?4. All grains have S‐isotopic compositions compatible within 2σ of solar values. Their mean S isotope composition deviates from solar by at most a few percent, and is consistent with values observed for the carbon star IRC+10216, believed to be a representative source star of the grains, and the interstellar medium. The isotopic data are also consistent with stellar model predictions of low‐mass asymptotic giant branch (AGB) stars. In a δ³³S versus δ³⁴S plot the data fit along a line with a slope of 1.8 ± 0.7, suggesting imprints from galactic chemical evolution. The observed S abundances are lower than expected from equilibrium condensation of CaS in solid solution with SiC under pressure and temperature conditions inferred from the abundances of more refractory elements in SiC. Calcium to S abundance ratios are generally above unity, contrary to expectations for stoichiometric CaS solution in the grains, possibly due to condensation of CaC₂ into SiC. We observed a correlation between Mg and S abundances suggesting solid solution of MgS in SiC. The low abundances of S in mainstream grains support the view that the significantly higher abundances of excess ³²S found in some Type AB SiC grains are the result of in situ decay of radioactive ³²Si from born‐again AGB stars that condensed into AB grains.     

Stable isotope analysis of carbon and nitrogen in angrites

Angrites are a small group of ancient basaltic achondrites, notable for their unusual chemistry and extreme volatile depletion. No comprehensive study of indigenous light elements currently exists for the group. Measurement of the abundances and isotopic composition of carbon and nitrogen could provide information pertaining to the evolution of the angrite parent body. Bulk‐sample stepped combustion analyses of five angrites and a glass separate from D'Orbigny were combined with earlier data and acid dissolution experiments of carbonates found in D'Orbigny to compile an inventory of indigenous carbon and nitrogen. Indigenous carbon combusted between 700 °C and 1200 °C, with abundances of 10–140 ppm and a mass‐weighted δ¹³C of ?25 to ?20‰ with the exception of D'Orbigny (δ¹³C approximately ?5‰). Nitrogen was released at 850–1200 ºC, 1–20 ppm with a δ¹⁵N ?3‰ to +4‰; again, D'Orbigny (δ¹⁵N approximately +20 to +25‰) was an exception. We interpret these components as largely indigenous and decoupled; the carbon in graphitic or amorphous form, while the nitrogen is present as a dissolved component in the silicates. No relationship with the textural sub‐classification of angrites is apparent. We suggest that the angrite parent body contains a reservoir of reduced carbon and thus may have undergone a change in redox conditions, although the timing and mechanism for this remain unclear.     

Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5‐1.5 μm

Abstract— We report isotopic abundances for C, N, Mg‐Al, Si, Ca‐Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg‐Al in 18 grains, Si in 87 grains, Ca‐Ti in 25 grains, and Fe in 8 grains. These X grains have ¹²C/¹³C ratios between 18 and 6800, ¹⁴N/¹⁵n ratios from 13 to 200, δ²⁹Si/²⁸Si between ?750 and +60%₀, δ³⁰Si/²⁸Si from ?770 to ?10%₀, and ⁵⁴Fe/⁵⁶Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic ²⁶Mg (from the radioactive decay of ²⁶Al, half‐life ? 7 times 10⁵ years) and radiogenic ⁴⁴Ca (from the radioactive decay of ⁴⁴Ti, half‐life = 60 years). While all X grains but one have radiogenic ²⁶Mg, only ～20% of them have detectable amounts of radiogenic ⁴⁴Ca. Initial ²⁶Al/²⁷Al ratios of up to 0.36 and initial ⁴⁴Ti/⁴⁸Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf‐Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct ⁴⁴Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for ²⁶Al/²⁷Al ratios, except for the highest ²⁶Al/²⁷Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed ²⁹Si/³⁰Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured ¹⁴n/¹⁵n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 M_odot; type II SN if rotational mixing, preferential trapping of N, or both from ¹⁵n‐rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial ²⁶Al/²⁷Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium‐44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti‐bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain‐size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.     

Stardust in Antarctic micrometeorites

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

Optical spectra of amorophous carbon grains

Extinction coefficient measurement of amorphous carbon grain, produced by hydrocarbon buring, have been carried out at room temperature in the spectral range from 1900 Å to 2.5 . The classical KBr pellet technique has been used to obtain quantitative data. Corrections to accout for matrix distorsion of the spectrum have been determined experimentally by measuring the spectrum of the same grains deposited on a thin quartz plate.     

A NanoSIMS and Auger Nanoprobe investigation of an isotopically primitive interplanetary dust particle from the 55P/Tempel‐Tuttle targeted stratospheric dust collector

Abstract– An IDP nicknamed Andric, from a stratospheric dust collector targeted to collect dust from comet 55P/Tempel‐Tuttle, contains five distinct presolar silicate and/or oxide grains in 14 ultramicrotome slices analyzed, for an estimated abundance of approximately 700 ppm in this IDP. Three of the grains are ¹⁷O‐enriched and probably formed in low‐mass red giant or asymptotic giant branch (AGB) stars; the other two grains exhibit ¹⁸O enrichments and may have a supernova origin. Carbon and N isotopic analyses show that Andric also exhibits significant variations in its N isotopic composition, with numerous discrete ¹⁵N‐rich hotspots and more diffuse regions that are also isotopically anomalous. Three ¹⁵N‐rich hotspots also have statistically significant ¹³C enrichments. Auger elemental analysis shows that these isotopically anomalous areas consist largely of carbonaceous matter and that the anomalies may be hosted by a variety of components. In addition, there is evidence for dilution of the isotopically heavy components with an isotopically normal endmember; this may have occurred either as a result of extraterrestrial alteration or during atmospheric entry. Isotopically primitive IDPs such as Andric share many characteristics with primitive meteorites such as the CR chondrites, which also contain isotopically anomalous carbonaceous matter and abundant presolar silicate and oxide grains. Although comets are one likely source for the origin of primitive IDPs, the presence of similar characteristics in meteorites thought to come from the asteroid belt suggests that other origins are also possible. Indeed the distinction between cometary and asteroidal sources is somewhat blurred by recent observations of icy comet‐like planetesimals in the outer asteroid belt.     

Isotopic composition of carbon and nitrogen in ureilitic fragments of the Almahata Sitta meteorite

This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon‐rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ¹³C from ?7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ¹³C = ca ?25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ¹⁵N values of ?53 to ?94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen‐rich and isotopically heavier. Thus, although the parent asteroid 2008TC₃ was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ¹³C and δ¹⁵N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.