Challenging the identification of nitride dust in extreme carbon star spectra

Nitride dust is predicted to form in small amounts around carbon stars, but the most likely candidate species such as aluminium nitride (AlN) have not yet been detected. Recently, α-Si₃N₄ was inferred to be the main carrier of the 8.5-12.5 μm absorption band(s) of an extreme carbon star (AFGL 5625), based on comparison with laboratory KBr dispersion spectra. However, this absorption band has also been attributed to silicon carbide (SiC) and C₃. To investigate whether or not nitride dust has truly been detected and if it is present in other extreme carbon stars, we (i) gathered new laboratory infrared (IR) absorbance spectra from a suite of nitride compounds, including Si₃N₄, using the thin film technique which provides correct relative intensities of weak and strong peaks, and (ii) compared these data to Infrared Space Observatory Short Wavelength Spectrometer (750 SWS) spectra of seven different extreme carbon stars which also show broad absorption features around ∼11 μm. The astronomical data show an apparent continuum of feature shapes ranging from full width at half-maximum (FWHM) of 2 to 2.7 μm, with peak barycenters ranging from 10.3 to 11 μm. The previous possible match between the extreme carbon star and Si₃N₄ was also based on many minor peaks longward of the 8.5-12.5 μm feature. We show that the fit to AFGL 5625 and other extreme carbon stars is unconvincing, because the reported 13-25 μm features in the observed spectrum are essentially noise. Features due to other nitrides are also not obvious. The apparent continuum of absorption features in the 8.5-12.5 μm band is better represented by previously published fits of a combination of crystalline (β polytype) and amorphous SiC, where the apparent broadening and shift of the peak barycenter are due to increasing contributions from the amorphous solid.