Presolar grains as tracers of circumstellar and interstellar environments

Presolar grains as tracers of circumstellar and interstellar environments 

Circumstellar condensates carry information about the environments in which they formed and about the environments through which they passed on the way to being incorporated into meteorites and IDPs. To date, studies to extract this information have been done primarily on presolar silicon carbide and graphite (Bematowicz et al., 2006). 

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al., 

These two types are the lowest-temperature forms of silicon carbide. The temperatures of formation are broadly consistent with expectations for conditions in circumstellar outflows. 

Silicon carbide grains are known to contain subgrains of titanium carbide. Equilibrium thermodynamics predicts that titanium carbide will condense before silicon carbide (Fig. 5.13). The titanium carbide grains were apparently accreted by the growing silicon carbide grains and were enclosed as the silicon carbide grains continued to grow. 

---