SOFIA/FORCAST Monitoring of the Dust Emission from R Aqr: Start of the Eclipse

We present mid-infrared (mid-IR) spectra from our continued monitoring of R Aquarii, the nearest symbiotic Mira, using the Stratospheric Observatory for Infrared Astronomy (SOFIA). New photometric and spectroscipic data were obtained with the Faint Object infraRed CAmera for the SOFIA Telescope in 2018 and 2019 after the system had started its "eclipse," during which it became two magnitudes fainter in the visual. The mid-IR flux, in particular the 10 μm silicate feature, has strengthened compared with the previous cycles. Radiative transfer models for the circumstellar dust emission were calculated for the new spectra, and recalculated for those previously obtained using more appropriate values of the near-IR magnitudes to constrain the properties of the asymptotic giant branch spectra heating the dust. The modeling shows that the luminosity dependence on pulsation phase is not affected by the onset of the eclipse, and that the increase in the mid-IR flux is due to a higher dust density. The models also confirm our earlier results that micron-size grains are present, and that no changes in the grain composition are required to explain the variations in the spectra.