TEM Analyses of Unusual Presolar Silicon Carbide: Insights into the Range of Circumstellar Dust Condensation Conditions

Abstract

Presolar silicon carbide (SiC) grains in meteoritic samples can help constrain circumstellar condensation processes and conditions in C-rich stars and core-collapse supernovae. This study presents our findings on eight presolar SiC grains from AGB stars (four mainstream and one Y grain) and core-collapse supernovae (three X grains), chosen on the basis of µ-Raman spectral features that were indicative of their having unusual non-3C polytypes and/or high degrees of crystal disorder. Analytical transmission electron microscopy (TEM), which provides elemental compositional and structural information, shows evidence for complex histories for the grains.Our TEM results confirm the presence of non-3C,2H crystal domains. Minor element heterogeneities and/or subgrains were observed in all grains analyzed for their compositions. The C/O ratios inferred for the parent stars varied from 0.98 to ≥1.03. Our data show that SiC condensation can occur under a wide range of conditions, in which environmental factors other than temperature (e.g., pressure, gas composition, heterogeneous nucleation on pre-condensed phases) play a significant role. Based on previous µ-Raman studies, ~10% of SiC grains may have infrared (IR) spectral features that are influenced by crystal defects, porosity, and/or

TEM analyses of in situ presolar grains from unequilibrated ordinary chondrite LL3.0 Semarkona

“…Although the gas composition only changes slowly in AGB star envelopes, the observation of complex Al and N zonation textures in a presolar SiC grain (G619) from an AGB star (Singerling et al, 2021) illustrates that conditions clearly do change on the timescales that SiC grains, at least, form. To summarize, grains that are stoichiometric, crystalline, Mg-rich (silicates only), and homogeneous are consistent with formation under equilibrium conditions, whereas those that are non-stoichiometric, amorphous/weakly nanocrystalline, Fe-rich (silicates only), and heterogeneous likely formed under non-equilibrium conditions.…”

“…The higher Al abundance in the region of high stacking fault density could indicate a change in gas composition during condensation, but might also have resulted from gradual diffusion of the Al to energetically favorable defect sites (e.g., the stacking fault boundaries) over the lifetime of the grain. Indeed, Al-rich SiC in association with stacking fault-rich regions was observed in a grain from an AGB star (G312) by Singerling et al (2021).…”

Optical Properties of C$-$rich ($^{12}$C, SiC and FeC) Dust Layered Structure of Massive Stars

“…Furthermore, Kodolányi et al (2018) used transmission electron microscopy (TEM) to study micron-sized (0.99×0.66-2.23×1.64 µm 2 ) SiC from the SN dusts, and there are enough heavy elements 12 C, 28 Si to condense into SiC dust in the SNR. In addition, the sub-grain with high 56 Fe content and large 28 Si loss may be 56 Fe metal, while the sub-grain with 28 Si enrichment or small 28 Si loss may be 56 Fe silicide (Singerling et al 2021). Here, we believe that this type of 56 Fe metal tends to be 56 Fe and FeC.…”

“…We use the open−source CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization, version 6.0) software to acquire the initial structure of C−rich dust (Wang et al 2010;Su et al 2017;Yin et al 2020). Then we predict stable or metastable nanostructures based on the chemical composition given by CALYPSO interfaced with Vienna Ab−initio Simulation Package (VASP).…”

“…Nozawa et al (2003) showed that the newly formed dust is mainly graphite or amorphous carbon. In addition, when the radius is 0.1 µm, the condensation temperature of C-rich dust is ∼ (1300−1350) K, and the formation temperature of Fe 3 C is between 1200 K and 1500 K (Singerling et al 2021). Dwek et al (2010) found via Spitzer satellite that silicate dust are best suited for the evolution of (8−30) µm spectrum and time data.…”