2001ApJ...558..165E

Using a series of radiative transfer models of circumstellar dust shells, we explore the physical origins of the variety of shapes of the 10 µm silicate feature seen in the spectra of oxygen-rich circumstellar dust shells. In order to match the full range of observed spectral shapes, the models explore four parameters: the relative abundance of amorphous alumina and amorphous silicates, the inner dust shell radius, the optical depth, and the geometric thickness of the shell. Optically thin shells dominated by amorphous silicate grains reproduce the classic narrow silicate feature at 10 µm. Increasing the optical depth of the shell produces spectral features at 10 µm with stronger components at 11 µm, but to match the [12]-[25] IRAS colors, these optically thick shells must be geometrically thin (i.e., have a truncated outer radius). Spectra with broad, low-contrast emission features peaking at wavelengths longer than ∼11 µm originate from optically thin shells composed of amorphous alumina. These findings provide a physical basis for the silicate dust sequence defined by Sloan & Price. We suggest that the [12]-[25] color is an indicator of geometric shell thickness. Thin shells can only arise if the star ejects mass and forms dust in a noncontinuous process.