ON SILICON CARBIDE GRAINS AS THE CARRIER OF THE 21 mm EMISSION FEATURE IN POST–ASYMPTOTIC GIANT BRANCH STARS

The mysterious 21 mm emission feature seen in 12 proto–planetary nebulae remains unidentified since its first detection in 1989. Over a dozen candidate materials have been proposed within the past decade, but none of them have received general acceptance. Very recently, silicon carbide (SiC) grains with impurities were suggested to be the carrier of this enigmatic feature, based on recent laboratory data that doped SiC grains exhibit a resonance at ∼21 mm. This proposal gains strength from the fact that SiC is a common dust species in carbon-rich circumstellar envelopes. However, SiC dust has a strong vibrational band at ∼11.3 mm. We show in this Letter that in order to be consistent with the observed flux ratios of the 11.3 mm feature to the 21 mm feature, the band strength of the 21 mm resonance has to be very strong, too strong to be consistent with current laboratory measurements. But this does not yet readily rule out the SiC hypothesis since recent experimental results have demonstrated that the 21 mm resonance of doped SiC becomes stronger as the C impurity increases. Further laboratory measurements of SiC dust with high fractions of C impurity are urgently needed to test the hypothesis of SiC as the carrier of the 21 mm feature. Subject headings: circumstellar matter — dust, extinction — infrared: stars — stars: AGB and post-AGB — stars: individual (HD 56126)