Guilt by association: The 13 micron dust emission feature and its correlation to other gas and dust features

A study of all full-scan spectra of optically thin oxygen-rich circumstellar dust shells in the database produced by the Short Wavelength Spectrometer on ISO reveals that the strength of several infrared spectral features correlates with the strength of the 13 mum dust feature. These correlated features include dust features at 19.8 and 28.1 mum and the bands produced by warm carbon dioxide molecules (the strongest of which are at 13.9, 15.0, and 16.2 mum). The database does not provide any evidence for a correlation of the 13 mum feature with a dust feature at 32 mum, and it is more likely that a weak emission feature at 16.8 mum arises from carbon dioxide gas rather than dust. The correlated dust features at 13, 20, and 28 mum tend to be stronger with respect to the total dust emission in semiregular and irregular variables associated with the asymptotic giant branch than in Mira variables or supergiants. This family of dust features also tends to be stronger in systems with lower infrared excesses and thus lower mass-loss rates. We hypothesize that the dust features arise from crystalline forms of alumina (13 mum) and silicates (20 and 28 mum).