We compare the spectral sequence of 3 m spectra of the annealed
QCC with observed spectra of PPNe categorized in class B by
Geballe ([1997]) and Tokunaga ([1997]). Based on the
similarity between the laboratory spectra and observed ones
particularly in the spectral morphology and the feature to continuum
ratio, we propose a scenario that carbonaceous dust is formed as
condensates similar to QCC samples in the PPN phase, and suffers
spectroscopic transformation when driven outward from the star,
transversing a warm region of the dust shell. Further observational
and laboratory works would confirm this hypothetical view by utilizing
high spatial resolution spectroscopy in observations and UV excited
QCC emission spectroscopy in laboratory, for instance.
Acknowledgements
We thank the referee, W.W. Duley for his helpful comments that improved the manuscript. We are also grateful to A.T. Tokunaga and T.R. Geballe who kindly provided the spectra of PPNe. MG is supported by the JSPS fellowship.
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