Circumstellar matter in symbiotic stars

By definition, symbiotic stars exhibit simultaneously the absorption features of a cool giant and high excitation emission lines, e.g. HeII, [OIII], of an ionised nebula. Thus the presence of circumstellar material is a necessary classification criterion of these objects. It is now generally accepted that symbiotic stars are binary systems consisting of a red giant and a hot radiation source, in most cases a hot white dwarf. The strong emission lines originate in a dense nebula which is thought to be wind material lost by the cool giant and ionised by the hot companion.

Besides the compact nebula other components of circumstellar material are observed. Enhanced IR emission due to circumstellar dust is found in D-type (D for dust) symbiotic systems. Very extended ionised regions have been mapped with radio interferometers or optical imaging techniques (e.g. Taylor 1988, Solf 1988, Corradi & Schwarz 1993, Schwarz 1993). Some features of the extended structures can be associated with bipolar outflow (velocities ∼ 100 km/s) from the central, unresolved binary system.

In this paper we discuss how the geometric structure of the circumstellar environment of symbiotic systems can be clarified from an analysis of light scattering processes.

Scattering processes in the circumstellar environment of symbiotic stars

Polarisation measurements are a well known tool for studying scattering processes. Polarimetric observations of symbiotic stars in broad and narrow band filters have shown that these objects are often intrinsically polarised (e.g. Piirola 1983, Schulte-Ladbeck 1985, Schulte-Ladbeck et al. 1990).