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 mum can be reproduced in a model with a dust envelope consisting of approximately half amorphous olivine (Mg0.8Fe1.2SiO4) and half amorphous aluminum-oxide grains (Al2O3), with a small admixture of spinel grains (MgAl2O4). The dust envelope's optical depth tau (50 mum) is approximate to 5 x 10(-3) [tau (1.25 mum) approximate to 0.07 for a(gr) approximate to 0.05 mum], and its mass within r less than or equal to 0.025 pc M-dust is approximate to 8 x 10(-6) M-.. The index alpha in the power-law radial dust distribution, n(d) proportional to (R-*/r)(alpha), is approximate to 1.8. Over the last several thousand years, the mass-loss rate of R Cas has been decreasing as (M) over dot (t) proportional to 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 approximate to 200 (where M-gas is the gas mass), the mean mass-loss rate of the star is (M) over dot approximate to 6 x 10(-7)M(.)/yr. (C) 2001 MAIK "Nauka/Interperiodica".