SOFIA/FORCAST Observations of R Aqr: Monitoring the Dust Emission

We present mid-infrared spectra of the symbiotic Mira, R Aqr obtained with the Faint Object infraRed CAmera instrument on the Stratospheric Observatory for Infrared Astronomy in 2016 and 2017. These data, supplemented with Infrared Space Observatory observations from 20 yr earlier, allow us to study the variation of the silicate dust emission over short and long timescales. Radiative transfer models for the dust emission from the circumstellar shell indicate that changes that occur within a pulsation period are driven mainly by the variation properties of the asymptotic giant branch star, and do not require any change in the grain composition. The models show that the longer-term changes are due to variations in the dust density, probably modulated by the orbital separation of the binary companion stars. The models also suggest the presence of micron-size dust grains at all epochs.