Challenging the Carbon Star Dust Condensation Sequence: Anarchist C Stars

There have been several investigations of the evolution of the mid-infrared (IR) dust features in carbon star spectra based on IRAS LRS data, but these studies are somewhat contradictory. In order to understand these differences in interpretations and to develop an understanding of the carbon star dust sequence, we have reexamined 26 IRAS LRS spectra of carbon stars that have also been observed spectroscopically by ISO SWS. The low resolution and narrow wavelength coverage of the IRAS LRS data hinder determination of the effect of molecular absorptions in these spectra. This has led to incorrect estimations of the continuum levels in these spectra, which has a huge effect on the continuum-divided and continuum-subtracted spectra used to analyze trends in the shape, strength, and position of the mid-IR features. The higher resolution and broader wavelength coverage of the ISO data allow more accurate fitting of the underlying continuum. We have reassessed the trends in shape, strength, and position of the ~11 μm silicon carbide (SiC) feature and the apparent emergence of the ~9 μm feature. We find that there are no correlations between the spectral parameters. We also investigate whether any of these parameters correlate with the strength of the molecular bands; no correlation was found. Moreover, we show that the apparent 9 μm feature is probably an artifact. We discuss the implications of this study in terms of both a carbon star condensation sequence and the application of this study to the larger IRAS data set.