SOFIA FORCAST Grism Study of the Mineralogy of Dust in the Winds of Proto-planetary Nebulae: RV Tauri Stars and SRd Variables

Abstract

We present a SOFIA FORCAST grism spectroscopic survey to examine the mineralogy of the circumstellar dust
in a sample of post-asymptotic giant branch (post-AGB) yellow supergiants that are believed to be the precursors
of planetary nebulae. Our mineralogical model of each star indicates the presence of both carbon-rich and oxygenrich 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. 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. These results help to determine the physical properties of the post-AGB circumstellar
environment and to constrain models of post-AGB mass loss and planetary nebula formation