Dust in the Winds of Proto-planetary Nebulae: RV Tauri Stars and SRd Variables

We are conducting a SOFIA FLITECAM and FORCAST grism spectrum survey to examine the mineralogy of the circumstellar dust in a sample of post asymptotic giant branch yellow supergiants that are believed to be the precursors of planetary nebulae. Our sample contains the brightest RV Tauri and SRd stars, including those in the southern hemisphere. This program will produce the first self-consistent sampling of the entire 2.8- 37.1 micron spectral region for a broad cross-section of this class of objects. The wavelength coverage and spectral resolution of the grisms will allow us to produce accurate models of the composition and grain-size distribution of the carbon and silicate materials and to test for the presence of hydrocarbon components in the grain materials. Relationships between the chemical composition of the circumstellar dust and the elemental abundances of the central stars will be investigated. Models of the distribution of dust in the circumstellar environments of these stars can also be constrained by the data. Here we report preliminary results based on observations made during SOFIA Cycles 2 and 3.