Is the 21 Micron Feature Observed in Some Post-AGB Stars Caused by the Interaction between Ti Atoms and Fullerenes?

Recent measurements of fullerenes and Ti atoms recorded in our laboratory have demonstrated the presence of an infrared feature near 21 μm. The feature observed has nearly the same shape and position as is observed for one of the most enigmatic features in post-asymptotic giant branch (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 μm that closely corresponds to the 20.1 μm feature observed in post-AGB stars. Both the laboratory and stellar spectra also show a small but significant peak at 19.0 μm, which is attributed to fullerenes. Here we propose that the interaction between fullerenes and Ti atoms may be a plausible explanation for the 21 μm feature seen in some post-AGB stars.