The Effect of Stellar Evolution on SiC Dust Grain Sizes

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 ~11 (mu)m 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 ~9 (mu)m feature seen in both emission and absorption, and correlated with trends in the ~11 (mu)m feature, may be due to either amorphous SiC or nanocrystalline 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 and coworkers. 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 the 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 that 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 ~11 (mu)m 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.