Published online by Cambridge University Press: 12 April 2016
One of the defining characteristics of symbiotic stars is the presence of nebular emission lines, which indicates the existence of a circumstellar nebula. Since the excitation and ionization of atomic emission lines depend on the temperature and density of the emission region, line observations (optical or ultraviolet) therefore only selectively probe part of the circumstellar nebule. Radio observations being less sensitive to “hot spots”, reflect the global character of the ionized region and are more suitable for determining the density structure of the nebula. Recent advances in the technique of aperture synthesis allow observations to be made with high resolution (˜0ʺ.1) and direct mapping of nebular structure is now a possibility. Since the circumstellar nebula is most likely to have originated from the stellar components of the symbiotic system, a better understanding of the nebula may provide important clues to the nature of symbiotic stars.