  Stars on the asymptotic giant branch (AGB) produce dust in their
circumstellar shells. The nature of the dust-forming environment is influenced
by the evolution of the stars, in terms of both chemistry and density, leading
to an evolution in the nature of the dust that is produced. Carbon-rich AGB
stars are known to produce silicon carbide (SiC). Furthermore, observations of
the ~11um SiC feature show that the spectral features change in a sequence that
correlates with stellar evolution. We present new infrared spectra of amorphous
SiC and show that the ~9um feature seen in both emission and absorption, and
correlated with trends in the ~11um feature, may be due to either amorphous SiC
or to nano-crystalline diamond with a high proportion of Si substituting for C.
Furthermore, we identify SiC absorption in three ISO spectra of extreme carbon
stars, in addition to the four presented by Speck et al. (1997). An accurate
description of the sequence in the IR spectra of carbon stars requires
accounting for both SiC emission and absorption features. This level of detail
is needed to infer the role of dust in evolution of carbon stars. Previous
attempts to find a sequence in the infrared spectra of carbon stars considered
SiC emission features, while neglecting SiC absorption features, leading to an
interpretation of the sequence inadequately describes the role of dust. We show
that the evolutionary sequence in carbon star spectra is consistent with a
grain size evolution, such that dust grains get progressively smaller as the
star evolves. The evolution of the grain sizes provides a natural explanation
for the shift of the ~11um SiC feature in emission and in absorption. Further
evidence for this scenario is seen in both post-AGB star spectra and in
meteoritic studies of presolar grains.
