The nature of the silicon carbide in carbon star outflows

We present 7.5-13.5 mu m UKIRT CGS3 spectra of 32 definite or candidate carbon stars. In addition to the extreme carbon star AFGL 3068, the only carbon star previously known to show the 11-mu m silicon carbide (SiC) feature in absorption, we have discovered three further examples of sources that show SiC in net absorption, namely IRAS 02408 + 5458, AFGL 2477 and AFGL 5625. We investigate the mineralogy of carbon star SiC and its relationship to meteoritic dust by using a chi(2)-minimization routine to fit the observed SiC features, and laboratory optical constants that have been published for a variety of SiC samples. With the exception of R For, all of the observed SiC features are best fitted by alpha-SiC grains, Excluding V414 Per, all of the sources with 8-13 mu m colour temperatures >1200K (corresponding to mass-loss rates at the bottom end of the range) are best fitted by alpha-SiC in pure emission, whereas all but one of the sources with 8-13 mu m colour temperatures < 1200 K (corresponding to higher mass-loss rates) are best fitted using self-absorbed alpha-SiC emission, The four sources whose SiC features are in net absorption (and which have the lowest 8-13 mu m colour temperatures and therefore presumably the highest mass-loss rates) are also well fitted by self-absorbed alpha-SiC emission, but with higher optical depths. Given that beta-SiC is the form most commonly found in meteorites, we have searched for evidence of beta-SiC in the circumstellar shells of all these carbon stars. However, our observations provide no unambiguous evidence for the presence of beta-SiC around these stars, with all of the observed SiC features being best explained in terms of alpha-SiC grains, The self-absorption that we find in the observed SiC emission features has not previously been taken into account in radiative transfer modelling, and so the amount of SIC present in the outflows has probably been underestimated in the past.