SPHERE/ZIMPOL observations of the symbiotic system R: I. Imaging of the stellar binary and the innermost jet clouds

International audienceContext. R Aqr is a symbiotic binary system consisting of a mira variable, a hot companion with a spectacular jet outflow, and an extended emission line nebula. Because of its proximity to the Sun, this object has been studied in much detail with many types of high resolution imaging and interferometric techniques. We have used R Aqr as test target for the visual camera subsystem ZIMPOL, which is part of the new extreme adaptive optics (AO) instrument SPHERE at the Very Large Telescope (VLT). Aims. We describe SPHERE /ZIMPOL test observations of the R Aqr system taken in H alpha and other filters in order to demonstrate the exceptional performance of this high resolution instrument. We compare our observations with data from the Hubble Space Telescope (HST) and illustrate the complementarity of the two instruments. We use our data for a detailed characterization of the inner jet region of R Aqr. Methods. We analyze the high resolution approximate to 25 mas images from SPHERE /ZIMPOL and determine from the H alpha emission the position, size, geometric structure, and line fluxes of the jet source and the clouds in the innermost region <2 `' (< 400 AU) of R Aqr. The data are compared to simultaneous HST line filter observations. The H alpha fluxes and the measured sizes of the clouds yield H alpha emissivities for many clouds from which one can derive the mean density, mass, recombination time scale, and other cloud parameters. Results. Our H alpha data resolve for the first time the R Aqr binary and we measure for the jet source a relative position 45 mas West (position angle ‐89.5 degrees) of the mira. The central jet source is the strongest H alpha component with a flux of about 2.5 x 10(‐12) erg cm(‐2) s(‐1). North east and south west from the central source there are many clouds with very diverse structures. Within 0.5 `' (100 AU) we see in the SW a string of bright clouds arranged in a zig‐zag pattern and, further out, at 1 `'‐2 `', fainter and more extended bubbles. In the N and NE we see a bright, very elongated filamentary structure between 0.2 `'‐0.7 `' and faint perpendicular `` wisps'' further out. Some jet clouds are also detected in the ZIMPOL [O I] and He I filters, as well as in the HST‐WFC3 line filters for H alpha, [O III], [N II], and [O I]. We determine jet cloud parameters and find a very well defined correlation N‐e proportional to r(‐1.3) between cloud density and distance to the central binary. Densities are very high with typical values of N‐e approximate to 3 x 10(5) cm(‐3) for the ``outer'' clouds around 300 AU, N‐e approximate to 3 x 10(6) cm(‐3) for the ``inner'' clouds around 50 AU, and even higher for the central jet source. The high Ne of the clouds implies short recombination or variability timescales of a year or shorter. Conclusions. H alpha high resolution data provide a lot of diagnostic information for the ionized jet gas in R Aqr. Future H alpha observations will provide the orientation of the orbital plane of the binary and allow detailed hydrodynamical investigations of this jet outflow and its interaction with the wind of the red giant companion