TEM analyses of unusual presolar silicon carbide: Insights into the range of circumstellar dust condensation conditions

TL;DR

This study uses TEM to analyze presolar SiC grains from stars and supernovae, revealing complex histories and diverse condensation conditions influenced by environmental factors beyond temperature.

Contribution

It provides detailed TEM analyses of unusual presolar SiC grains, showing complex structures and heterogeneities, and highlights the wide range of conditions under which SiC can condense in stellar environments.

Findings

Presence of non-3C, 2H crystal domains in grains

Evidence of elemental heterogeneities and subgrains

SiC condensation occurs under diverse environmental 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 {\mu}-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 greater than or equal 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 {\mu}-Raman studies, about 10% of SiC grains may have infrared (IR) spectral features that are influenced by crystal defects, porosity, and/or subgrains. Future sub-diffraction limited IR measurements of complex SiC grains might shed further light on the relative contributions of each of these features to the shape and position of the characteristic IR 11-{\mu}m SiC feature and thus improve the interpretation of IR spectra of AGB stars like those that produced the presolar SiC grains.