Detection of Silicon Nitride Particles in Extreme Carbon Stars^*

On the basis of spectra taken by the Infrared Space Observatory and laboratory measurements, we present the first spectroscopic evidence for the presence of silicon nitride (Si₃N₄) grains in the circumstellar shells of a number of extreme carbon stars. Laboratory measurements of α, β, and amorphous Si₃N₄ are compared with spectra of the extreme carbon stars IRAS 21318+5631 and AFGL 2477. Both stars show an absorption feature near the well-known 11 μm band of silicon carbide (SiC) but peaking at and extending to considerably shorter wavelengths. We demonstrate that the position and the band profile of the observed features coincide well with the main features of laboratory Si₃N₄ spectra, especially in the case of IRAS 21318+5631. While this band alternatively may be explained by SiC grains plus interstellar silicate absorption, in the way worked out by Speck and coworkers, there is further strong evidence for Si₃N₄ grains in IRAS 21318+5631 and possible evidence for Si₃N₄ grains in AFGL 2477 provided by weaker absorption features in the wavelength region longward of the main feature.