The dust envelope of R Cas

The spectral energy distribution in the far infrared and the shape of a broad emission band in the spectrum of R Cas at 9–13 µm can be reproduced in a model with a dust envelope consisting of approximately half amorphous olivine (Mg_0.8Fe_1.2SiO₄) and half amorphous aluminum-oxide grains (Al₂O₃), with a small admixture of spinel grains (MgAl₂O₄). The dust envelope’s optical depth τ(50 µm) is ≈5×10^?3 [τ(1.25 µm)≈0.07 for a _gr≈0.05 µm], and its mass within r≤0.025 pc M _dust is ≈8×10^?6 M _⊙. The index α in the power-law radial dust distribution, n _d ∝(R ₊/r)^α, is ≈1.8. Over the last several thousand years, the mass-loss rate of R Cas has been decreasing as $dot M(t) propto t^{0.2} $ (where time is measured backward from the present). This probably implies that R Cas experienced a thermal helium flare several thousand years ago. If M _gas/M _dust≈200 (where M _gas is the gas mass), the mean mass-loss rate of the star is $dot M approx 6 times 10^{ - 7} M_ odot /yr$ .