Models for IRAS Observations of Circumstellar Dust Shells around Late-type Stars

M. Rowan-Robinson, T. D. Lock, D. W. Walker and S. Harris
Theoretical Astronomy Unit
Queen Mary College
Mile End Road
London E1 4NS
U. K.

Abstract

IRAS observations of circumstellar dust shells around late-type stars are compared with the models of Rowan-Robinson and Harris. IRAS data at 12 and 25 microns 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 approximately 10¹⁷ to 10¹⁸ cm from the star. A value of 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 100 microns agree reasonably with the predictions of the models, supporting the claim of Rowan-Robinson and Harris that the grain absorption efficiency Q_v is proportional to frequency from wavelength 1 to 100 microns, and hence that the grains are highly amorphous.

For M stars the IRAS data at 60 to 100 microns require that the grain absorption efficiency Q_v be proportional to frequency between 20-30 and 100 microns. 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 100 micron 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 R Scl has an excess at 60 and 100 microns 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 that terminated about 100 years ago is also a possible interpretation.

M. Rowan-Robinson, T. D. Lock, D. W. Walker, and S. Harris, "Models for IRAS Observations of Circumstellar Dust Shells around Late-type Stars" Monthly Notices of the Royal Astronomical Society, Vol. 222, pages 273-286, 1986.