"Just as the shapes of snowflakes provide information on the temperature and humidity of the upper atmosphere, the characteristics of presolar grains in meteorites limits the environments in the outflow of gas from stars in which they could have formed," explains Yuki Kimura of the Hokkaido team. Unfortunately, however, it has proved difficult to pin down the possible environments for the formation of grains consisting of a titanium carbide core and a surrounding graphitic carbon mantle.
Better understanding of the environment around stars in which the grains could have formed is crucial to learning more about the interstellar environment in general. That could, in turn, help clarify how stars evolve and how the materials around them become the building blocks for planets.
The structure of the grains appears to suggest that their titanium carbide core first formed and was then subsequently coated in a thick layer of carbon in more distant regions of gas outflow from stars that formed before the sun.
The team explored the conditions that might recreate the grain formation in laboratory modeling studies guided by theoretical work on grain nucleation—the formation of grains from tiny original specks. This work was augmented by experiments performed in the periods of microgravity experienced aboard sub-orbital rocket flights.
The results offered some surprises. They suggest the grains most likely formed in what the researchers call a non-classical nucleation pathway: a series of three distinct steps not predicted by conventional theories. First, carbon forms tiny, homogenous nuclei; titanium then deposits on these carbon nuclei to form carbon particles containing titanium carbide; finally, thousands of these fine particles fuse to form the grain.
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