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

## TL;DR

This study uses SOFIA FORCAST grism spectroscopy to analyze the mineralogy of dust in post-AGB stars, revealing mixed dust compositions and processed grains, which inform models of planetary nebula formation.

## Contribution

It provides new spectroscopic evidence of mixed dust mineralogy and grain processing in post-AGB stars, challenging simple dredge-up models.

## Key findings

- Presence of both carbon-rich and oxygen-rich dust species.
- Lack of crystalline silicate emission features.
- Evidence of large, processed grains in Keplerian disks.

## 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 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 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. 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.