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1. An in situ investigation on the origins and processing of circumstellar oxide and silicate grains in carbonaceous chondrites

   https://doi.org/10.1111/maps.13418  

   Zega, Thomas J. ; Haenecour, Pierre ; Floss, Christine ( November 2019 , Meteoritics & Planetary Science)

   We report on the isotopic, chemical, and structural properties of four O‐rich presolar grains identified in situ in the Adelaide ungrouped C2, LaPaZ Icefield (LAP) 031117CO3.0, and Dominion Range (DOM) 08006CO3.0 chondrites. All four grains have oxygen‐isotopic compositions consistent with origins in the circumstellar envelopes (CSE) of low‐mass O‐rich stars evolved along the red‐giant and asymptotic‐giant branch (RGB,AGB, respectively) of stellar evolution. Transmission electron microscope (TEM) analyses, enabled by focused‐ion‐beam scanning electron microscope extraction, show that the grain from Adelaide is a single‐crystal Mg‐Al spinel, and comparison with equilibrium thermodynamic predictions constrains its condensation to 1500 K assuming a total pressure ≤10⁻³ atm in its hostCSE. In comparison,TEManalysis of two grains identified in theLAP031117 chondrite exhibits different microstructures. GrainLAP‐81 is composed of olivine containing a Ca‐rich and a Ca‐poor domain, both of which show distinct orientations, suggesting changing thermodynamic conditions in the hostCSEthat cannot be precisely constrained.LAP‐104 contains a polycrystalline assemblage of ferromagnesian silicates similar to previous reports of nanocrystalline presolar Fe‐rich silicates that formed under nonequilibrium conditions. Lastly,TEMshows that the grain extracted fromDOM08006 is a polycrystalline assemblage of Cr‐bearing spinel. The grains occur in different orientations, likely reflecting mechanical assembly in their hostCSE. The O‐isotopic and Cr‐rich compositions appear to point toward nonequilibrium condensation. The spinel is surrounded by an isotopically solar pyroxene lacking long‐range atomic order and could have served as a nucleation site for its condensation in the interstellar medium or the inner solar protoplanetary disk.

    

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

   https://doi.org/10.1111/maps.13477  

   Haenecour, Pierre ; Floss, Christine ; Brearley, Adrian J. ; Zega, Thomas J. ( May 2020 , Meteoritics & Planetary Science)

   Our 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 of¹⁷O‐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.

    

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3. Coordinated chemical and microstructural analyses of presolar silicate grains from AGB/RGB stars and supernovae in the CO3.0 chondrite Dominion Range 08006

   https://doi.org/10.1111/maps.13811  

   Seifert, Laura B. ; Haenecour, Pierre ; Zega, Thomas J. ( April 2022 , Meteoritics & Planetary Science)

   We report the structural and chemical analyses of six presolar silicate grains identified in situ in the CO3.0 carbonaceous chondrite Dominion Range (DOM) 08006. Two of the grains have O‐isotopic compositions consistent with origins in the circumstellar envelopes of low‐mass (<2M_☉) asymptotic giant branch (AGB)/red giant branch (RGB) stars, although without Mg‐isotopic data, origins in supernovae (SNe) cannot be ruled out. The other four grains have O‐isotopic compositions consistent with origins in the ejecta of type‐II SNe. Transmission electron microscopy analyses reveal that all grains are crystalline (single crystal or polycrystalline) and have varied compositions. The analyzed AGB/RGB grains include an Fe‐rich crystalline olivine with an Fe‐sulfide inclusion and a chemically zoned olivine grain that also contains an Fe‐oxide rim. The grains derived from SNe include two polycrystalline assemblages with structures that overlap with both olivine and pyroxene, an assemblage composed of both a single crystal of forsterite and polycrystalline forsterite, and an orthopyroxene grain with an embedded Fe‐sulfide crystal. The thermodynamic origins of both AGB/RGB and SN grains are also diverse. The structure and compositions of two grains are consistent with equilibrium thermodynamic predictions of condensation, whereas four are not, suggesting formation through nonequilibrium or multistep processes. Our observations of presolar silicate grains suggest that the circumstellar envelopes of AGB/RGB stars and the ejecta of SNe can produce grains with comparable structures and compositions.

    

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4. Formation of Interstellar C60 From Silicon Carbide Circumstellar Grains

   https://doi.org/10.15278/isms.2019.WA01  

   Bernal, J ; Haenecour, P. ; Howe, J. ; Zega, T.J. ; . Amari, S. ; Ziurys, L.M. ( July 2019 , The Astrophysical journal)

   We have conducted laboratory experiments with analog crystalline silicon carbide (SiC) grains using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The 3C polytype of SiC was used—the type commonly produced in the envelopes of asymptotic giant branch (AGB) stars. We rapidly heated small (∼50 nm) synthetic SiC crystals under vacuum to ∼1300 K and bombarded them with 150 keV Xe ions. TEM imaging and EELS spectroscopic mapping show that such heating and bombardment leaches silicon from the SiC surface, creating layered graphitic sheets. Surface defects in the crystals were found to distort the six-membered rings characteristic of graphite, creating hemispherical structures with diameters matching that of C60. Such nonplanar features require the formation of five-membered rings. We also identified a circumstellar grain, preserved inside the Murchison meteorite, that contains the remnant of an SiC core almost fully encased by graphite, contradicting long-standing thermodynamic predictions of material condensation. Our combined laboratory data suggest that C60 can undergo facile formation from shock heating and ion bombardment of circumstellar SiC grains. Such heating/bombardment could occur in the protoplanetary nebula phase, accounting for the observation of C60 in these objects, in planetary nebulae (PNs) and other interstellar sources receiving PN ejecta. The synthesis of C60 in astronomical sources poses challenges, as the assembly of 60 pure carbon atoms in an H-rich environment is difficult. The formation of C60 from the surface decomposition of SiC grains is a viable mechanism that could readily occur in the heterogeneous, hydrogen-dominated gas of evolved circumstellar shells. 

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5. The recurrent nova V3890 Sgr: a near-infrared and optical study of the red giant component and its environment

   https://doi.org/10.1093/mnras/stac2199  

   Kaminsky, B. ; Evans, A. ; Pavlenko, Ya V. ; Woodward, C. E. ; Banerjee, D. P. K. ; Gehrz, R. D. ; Walter, F. ; Starrfield, S. ; Ilyin, I. ; Strassmeier, K. G. ; et al ( September 2022 , Monthly Notices of the Royal Astronomical Society)

   We present an analysis of the red giant component of the recurrent nova V3890 Sgr, using data obtained before and after its 2019 eruption. Its effective temperature is Teff = 3050 ± 200 K for log g = 0.7, although there are modest changes in Teff. There is an overabundance of both carbon (0.20 ± 0.05 dex) and sodium (1.0 ± 0.3 dex) relative to their solar values, possibly the result of ejecta from the 1990 nova eruption being entrained into the red giant photosphere. We find 12C/13C =25 ± 2, a value similar to that found in red giants in other recurrent novae. The interpretation of the quiescent spectrum in the 5–38$\, \mu$m region requires the presence of photospheric SiO absorption and cool (∼400 K) dust in the red giant environment. The spectrum in the region of the Na i D lines is complex, and includes at least six interstellar components, together with likely evidence for interaction between ejecta from the 2019 eruption and material accumulated in the plane of the binary. Three recurrent novae with giant secondaries have been shown to have environments with different dust content, but photospheres with similar 12C/13C ratios. The SiO fundamental bands most likely have a photospheric origin in the all three stars.

    

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