The expanding wind shells of very luminous and cool stars ("Red Giants")
are cold and dense enough as to allow the formation of microscopic
solid particles ("dust grains"), consisting mainly of silicates or
carbon (soot). Due to their very large opacity, dust grains are efficient
absorbers of radiative energy and momentum. Although only about 1% of
the total mass is in the form of dust, it dominates the dynamics of the
circumstellar shell once it has formed. For the highest mass loss rates,
the central stars are completely obscured at visible wavelengths
("IR-stars"). Observationally, such objects are only detectable in the
infrared due to emission originating from the cold circumstellar dust
component. At radio wavelengths, the gas component radiates characteristic
emission lines of different molecules (e.g. CO, OH). Modern infrared
observations, in particular by the
Infrared Space Observatory ISO, hold
the key to clarify several hitherto unsolved problems and open questions
of present day stellar astrophysics.
A brief summary of the work on dusty AGB winds done by the
"Stellar Physics" group at the AIP may be found in the documents
Circumstellar Dust Shells and
The puzzle of the circumstellar CO shells by M. Steffen &
D. Schönberner.