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Y. Kimura, J. A. Nuth III (NASA GSFC), F. T. Ferguson (NASA GSFC & Catholic University of America)
A 21-\mum-emission feature has been observed in the shells of carbon-rich post-asymptotic giant branch (AGB) stars. The carrier of the 21-\mum feature remains unidentified, although many candidate materials have been proposed, including nanodiamond, SiS2, a derivative of SiC and nanometer-sized TiC.
In particular, TiC grains were extensively discussed after the report by von Helden (2000). Gas-phase TiC clusters less than 1 nm in diameter have been suggested as the source of the 21-\mum dust feature. The spectrum of TiC clusters recorded in the laboratory provides a good fit with the observational data. However, only negative results have been reported for both theoretical and laboratory experimental studies concerning TiC since the discovery by von Helden.
Recent measurements of fullerenes and Ti atoms recorded in our laboratory have demonstrated the presence of an infrared feature near 21 \mum. The feature observed has nearly the same shape and position as is observed for one of the most enigmatic features in post-AGB stars. In our experimental system, large-cage carbon particles, such as large fullerenes, were produced from CO gas by the Boudouard reaction. Large-cage carbon particles intermixed with Ti atoms were produced by the evaporation of a Ti-metal-wrapped carbon electrode in CO gas. The infrared spectra of large fullerenes interacting with Ti atoms show a characteristic feature at 20.3 \mum that closely corresponds to the 20.1-\mum feature observed in post-AGB stars. Both the laboratory and stellar spectra also show a small but significant peak at 19.0 \mum, which is attributed to fullerenes. We propose that the interaction between fullerenes and Ti atoms may be a plausible explanation for the 21-\mum feature seen in some post-AGB stars.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.