The structure of the R Aquarii nebula.

The authors present long-slit spectra of high angular and spectral resolution of the red emission lines obtained from the extended nebula around the symbiotic Mira variable R Aqr. The deduced velocity field allows a clear-cut kinematical distinction between an inner and outer nebulosity. Each nebular component is described as a bipolar, hour-glass-like, expanding shell resulting from a radial gas flow presenting velocities which increase with increasing latitude angle of the flow vectors. The line emission, originating in a thin surface layer of each shell, is prominent in the equatorial zone but becomes progressively fainter at higher latitude angles. Due to rather low electron densities the outer shell can be detected in a narrow equatorial ring zone only. The observations support that R Aqr is a binary consisting of a mass-losing Mira variable and a compact companion accreting part of that mass through a disk. Explosions occurring on the companion at typical time intervals of 450 yr produce shells of about 10^-5M_sun;. The kinematical structure of the shells suggests the existence of a geometrically and optically thick, disk-like cloud around the companion, which is swept-up after each explosion by the ejected matter. A high concentration of the cloud material towards the orbital plane will lead to the observed bipolar flow pattern with minimum expansion rates in the equatorial zone.