A SOFIA/FORCAST Grism Study of the Mineralogy of Dust in the Winds of Proto-planetary Nebulae

RV Tauri stars and SRd variable stars are post-asymptotic giant branch yellow supergiants that are believed to be the immediate precursors of planetary nebulae. I will present a SOFIA/FORCAST grism spectroscopic survey of the mineralogy of the circumstellar dust in a sample of 15 RV Tauri stars and 3 SRd variable stars. The spectra do not exhibit any prominent crystalline silicate emission features. For most of the systems, our analysis suggests that the grains are relatively large and have undergone significant processing, supporting the hypothesis that the dust is confined to a Keplerian disk and that we are viewing the heavily processed, central regions of the disk from a nearly face-on orientation. Our model of each star indicates the presence of both carbon rich and oxygen rich dust species (contrary to simple dredge-up models) with a majority of the dust in the form of amorphous carbon and graphite. The oxygen rich dust is primarily in the form of amorphous silicates. This requires that either these stars result from a narrow range of masses that terminate AGB evolution just as the carbon exceeds the oxygen abundance, or that the formation process is not single star evolution. We speculate the formation process is common envelope evolution.