Models for IRAS observations of circumstellar dust shells around late-type stars

IRAS observations of circumstellar dust shells around late-type stars are compared with the models of Rowan-Robinson & Harris. IRAS data at 12 and 25μm agree well with previous observations and with the models. A discussion of the environment of late-type stars shows that we should expect the outflow to be halted at a distance r₂∼10¹⁷ to 10¹⁸ cm from the star. A value for the ratio of the inner to outer radii of the dust shell, r₁/r₂=0.001, has therefore been adopted for most of the theoretical models discussed. For carbon stars the IRAS data at 60 and 100μm agree reasonably with the predictions of the models, supporting the claim of Rowan-Robinson & Harris that the grain absorption efficiency Q_v is proportional to v from λ = 1−100μm, and hence that the grains are highly amorphous. For M stars the IRAS data at 60 and 100μm require that the grain absorption efficiency $$Q_\nu \propto\nu$$ between 20–30 and 100μm. The inference is that the silicate grains formed in the atmospheres of oxygen-rich stars have a highly amorphous structure. A few stars show excess 100μm radiation, which probably arises from a shell of dust at the zone where the wind from the star runs into the interstellar gas. The carbon star RScl has an excess at 60 and 100μm consistent with being a 125 K blackbody. The location and mass of the dust implied by this emission are consistent with being an Oort cometary cloud, though an ejection which terminated about 100 yr ago is also a possible interpretation.