SOFIA/FORCAST Monitoring of the Dust Emission from R Aqr: Start of the
  Eclipse
  We present mid-infrared 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 (FORCAST) 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 mu-m
silicate feature, have strengthened compared with the previous cycles.
Radiative transfer models for the circumstellar dust emission were calculated
for the new spectra, and re-calculated for those previously obtained using more
appropriate values of the near-IR magnitudes to constrain the properties of the
AGB 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.
