A SOFIA Survey of RV Tauri Stars and SRd Variables

The path from Main Sequence (MS) star to planetary nebula (PN) is a brief and turbulent period in the life of a star. RV Tauri stars and yellow semi-regular (SRd) variables are both classes of post-asymptotic giant branch (AGB) stars that lie along the horizontal track on the H-R diagram linking AGB stars to PNe and are thought to be the immediate precursors of protoplanetary nebulae (PPNe). RV Tauri stars are characterized by semi-regular, bimodal variability (possibly resulting from interaction with a binary companion), a high mass-loss rate, and, often, a prominent IR excess. SRd variables are similar to RV Tauri stars in many respects but are probably single star systems, as indicated by the absence regular pulsations.Here we present near- and mid-IR spectra on a diverse sample of RV Tauri and SRd variables from our on-going observing campaign with the FLITECAM and FORCAST instruments on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). With this rich data set we produce the first self-consistent sampling of these objects from the near- to mid-IR. We model the emission to determine the source of the IR-excess, identifying the dust species present and quantifying fundamental dust properties, such as dust mass and grain size distribution. These parameters help to determine the physical properties of the circumstellar environment and to constrain models of post-AGB mass loss.