The nature of silicon carbide: astronomical observations VS meteoritic evidence

Details The nature of silicon carbide: astronomical observations VS meteoritic evidence The nature of silicon carbide: astronomical observations VS meteoritic evidence

Sep 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..703s&link_type=abstract

Meteoritics, vol. 32, pages 703-712

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3

Scientific paper

We have investigated the 7.5-13.5 μm spectra of 30 definite or candidate carbon stars. We discuss the discrepancies between properties of SiC grains found in meteorites and the spectral properties of dust emitting in red giant winds, where most of the meteoritic grains are believed to have formed. We have investigated the nature of carbon star silicon carbide and its relationship to meteoritic SiC dust, by using a c2-minimisation routine to fit the observed SiC features with laboratory optical constants that have been published for a variety of SiC samples. All but one of the observed astronomical SiC features are best fitted by a-SiC grains. All but one of the sources with 8-13 μm colour temperatures > 1200K (corresponding to mass-loss rates at the bottom end of the range) are best fitted by a-SiC in pure emission, whereas all but one of the sources with 8-13 μm colour temperatures < 1200K (corresponding to higher mass-loss rates) are best fitted using self-absorbed a-SiC emission. The four sources whose SiC features are in net absorption (and which have the lowest 8-13 μm colour temperatures and therefore presumably the highest mass-loss rates) are also well fitted by self-absorbed a-SiC emission, but with higher optical depths. Given that -SiC is the form most commonly found in meteorites, we have searched for evidence of -SiC in the circumstellar shells of all these stars. However, our observations provide no unequivocal evidence for the presence of -SiC around these stars. Other discrepancies between meteoritic SiC grains and astronomical spectra are discussed. The self-absorption that we find in the observed Si C emission features has not previously been taken into account in radiative transfer modelling and so the amount of SiC present in the outflows may have been underestimated in the past.

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