We have modeled the infrared spectral energy distributions of three proto-planetary nebulae using the coal model. It is shown that the infrared emission spectra of semianthracite coal grains in thermal equilibrium adequately reproduce the 2-13 μm emission spectra of these three carbon-rich, post-asymptotic giant branch objects, including bands and continua. The spectrum of this material is also consistent with the recent detection of three bands, lying at 10.6, 11.5, and 12.2 μm, in these circumstellar environments. Moreover, a parallel is found between the spectral changes that result from the natural evolution of semianthracite toward anthracite coal and those exhibited by carbonaceous circumstellar dust during the evolution of the nebula from the protoplanetary to the planetary stage.
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Guillois et al. (1996) studied this question.