Challenging the Identification of Silicon Nitride Dust in Extreme Carbon Stars

It has been well established that SiC is a dominant mineral in the condensation sequence of carbon-rich stars (or C-stars). The presence of other mineral species in interstellar dust surrounding C-stars may be indicative of exotic formation conditions for these objects. Observers have long held out hope for detecting the compound silicon nitride (Si3N4) in stellar spectra; however, previous attempts to identify Si3N4 in dust material around novae, planetary nebulae, and late-type binary and Cstars have proved to be unsuccessful (Clément et al. 2005, ApJ, 821, 985 and references therein). Clément et al. (2005) suggested that a broad, double-peaked 9-11 μm absorption feature in the ISO SWS spectra of two extreme C-stars (AFGL 2477 and AFGL 5625) is due to Si3N4. This assignment was based on the correlation of several weak observational spectral features with laboratory spectral features of α-Si3N4 in the 15-30 μm range. The broad 9-11 μm feature had been previously attributed to a mixture of SiC and interstellar silicate (Speck et al. 1997, MNRAS, 20, 431), and more recently to amorphous SiC (Speck et al. 2005, ApJ, 634, 426). We dispute the Si3N4 assignment on the basis of expected interstellar abundances, Si3N4 meteoritic isotope studies, blackbody correction methods, and spectral peak assignment as compared to noise. Speck et al. (2005) discovered another extreme carbon star (IRAS 00210+6221) that exhibits a 9-11 μm absorption feature identical to those found in AFGL 2477 and AFGL 5625. A preliminary re-analysis of the spectra of these three extreme carbon star spectra has revealed that neither AFGL 2477 nor IRAS 00210+6221 display any of the 15-30 μm features. For AFGL 5625, any features present in this range are at > 2σ level, and therefore may just be noise. We compare the observational spectra to independently acquired laboratory spectra for Si3N4, as well as other nitride minerals consistent with recently published condensation sequences (e.g., AlN, TiN), carbides, and silicides. Our thin film laboratory absorbance spectra of αand β-Si3N4 appear to give good agreement with the KBr pellet method transmission spectra of Clément et al. (2005). Based on ∗Washington University St. Louis †University of Missouri Columbia