TEM studies of a circumstellar rock

Reported here is the discovery of crystals of titanium carbide in a grain of silicon carbide which formed as a circumstellar dust particle in the atmosphere of a carbon-rich star. Just as in the case of terrestrial rocks, whose assemblage of minerals gives us clues to the composition and conditions of the environment in which they formed, the titanium carbide crystals and their textural relationship to the silicon carbide give us important clues to the nature of the stellar atmosphere in which they formed. From microscopic studies of the relationships between the atomic planes of the silicon carbide and the titanium carbide, we can show that the titanium carbide cannot have existed as already-formed crystals in a gas around which silicon carbide subsequently condensed. An alternative possibility is that both minerals grew quickly and simultaneously from condensing gas in the rapidly cooling and expanding stellar atmosphere. Other microscopic features of the silicon carbide, such as abundant atomic layer disorder and crystal twinning, similarly suggest rapid grain growth. However, another possibility is that the titanium carbide grew inside of the silicon carbide by diffusion of titanium atoms. Our calculations suggest that this scenario is less likely, given the relatively short times (a year or less) for which stellar condensates can be expected to be exposed to temperatures high enough to make diffusion sufficiently rapid.