Wind mass transfer in S-type symbiotic binaries III. Confirmation of a wind focusing in EG Andromedae from the nebular [O III] λ5007 line

Context. The structure of the wind from the cool giants in symbiotic binaries carries important information for understanding the wind mass transfer to their white dwarf companions, its fuelling, and thus the path towards different phases of symbiotic-star evolution. Aims. In this paper, we indicate a non-spherical distribution of the neutral wind zone around the red giant (RG) in the symbiotic binary star, EG And. We concentrate in particular on the wind focusing towards the orbital plane and its asymmetry alongside the orbital motion of the RG. Methods. We achieved this aim by analysing the periodic orbital variations of fluxes and radial velocities of individual components of the H alpha and [OIII] lambda 5007 lines observed on our high-cadence medium (R similar to 11 000) and high-resolution (R similar to 38 000) spectra. Results. The asymmetric shaping of the neutral wind zone at the near-orbital-plane region is indicated by: (i) the asymmetric course of the H alpha core emission fluxes along the orbit; (ii) the presence of their secondary maximum around the orbital phase =0.1, which is possibly caused by the refraction effect; and (iii) the properties of the H alpha broad wing emission originating by Raman scattering on H-0 atoms. The wind is substantially compressed from polar directions to the orbital plane as constrained by the location of the [OIII] lambda 5007 line emission zones in the vicinity of the RG at/around its poles. The corresponding mass-loss rate from the polar regions of less than or similar to 10(-8)M(circle dot) yr(-1) is a factor of greater than or similar to 10 lower than the average rate of approximate to 10(-7)M(circle dot) yr(-1) derived from nebular emission of the ionised wind from the RG. Furthermore, it is two orders of magnitude lower than that measured in the near-orbital-plane region from Rayleigh scattering. Conclusions. The startling properties of the nebular [OIII] lambda 5007 line in EG And provides an independent indication of the wind focusing towards the orbital plane - the key to understanding the efficient wind mass transfer in symbiotic binary stars.