2003ApJ...587..771C

The carrier of the 21 µm band observed in post-asymptotic giant branch (post-AGB) stars is examined. We analyze the infrared spectra of the TiC clusters measured by von Helden et al. in 2000 and determine the absorption efficiency Q in the 21 µm band. Using Q, we estimate the Ti/Si abundance ratios needed to realize the flux ratios of the 21 and 11 µm emission observed in the infrared spectra of the post-AGB stars exhibiting both 21 and 11 µm emission. In view of the nature of the TiC condensation by which TiC grains are quickly mantled by graphite, we calculate the emission spectra of the graphite-coated TiC grains and other possible types of core-mantle grains and compare with the observed spectra. Both the abundance and condensation considerations strongly suggest that TiC is an implausible carrier of the observed infrared 21 µm feature around carbon-rich post-AGB stars.