The Effect of Stellar Evolution on SiC Dust Grain Sizes

Stars on the asymptotic giant branch (AGB) produce dust in theircircumstellar shells. The nature of the dust-forming environment is influencedby the evolution of the stars, in terms of both chemistry and density, leadingto an evolution in the nature of the dust that is produced. Carbon-rich AGBstars are known to produce silicon carbide (SiC). Furthermore, observations ofthe ~11um SiC feature show that the spectral features change in a sequence thatcorrelates with stellar evolution. We present new infrared spectra of amorphousSiC and show that the ~9um feature seen in both emission and absorption, andcorrelated with trends in the ~11um feature, may be due to either amorphous SiCor to nano-crystalline diamond with a high proportion of Si substituting for C.Furthermore, we identify SiC absorption in three ISO spectra of extreme carbonstars, in addition to the four presented by Speck et al. (1997). An accuratedescription of the sequence in the IR spectra of carbon stars requiresaccounting for both SiC emission and absorption features. This level of detailis needed to infer the role of dust in evolution of carbon stars. Previousattempts to find a sequence in the infrared spectra of carbon stars consideredSiC emission features, while neglecting SiC absorption features, leading to aninterpretation of the sequence inadequately describes the role of dust. We showthat the evolutionary sequence in carbon star spectra is consistent with agrain size evolution, such that dust grains get progressively smaller as thestar evolves. The evolution of the grain sizes provides a natural explanationfor the shift of the ~11um SiC feature in emission and in absorption. Furtherevidence for this scenario is seen in both post-AGB star spectra and inmeteoritic studies of presolar grains.Comment: accepted by ApJ 8 figure