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We investigate the structure, dynamics, and chemistry of the molecule-richnebula around the stellar symbiotic system R Aqr, which is significantlyaffected by the presence of a white dwarf (WD) companion. We study the effectsof the strong dynamical interaction between the AGB wind and the WD and ofphotodissociation by the WD UV radiation on the circumstellar shells.  We obtained high-quality ALMA maps of the 12CO J=2-1, J=3-2, and J=6-5 linesand of 13CO J=3-2. The maps were analyzed by means of a heuristic 3D model thatis able to reproduce the observations. In order to interpret this descriptionof the molecule-rich nebula, we performed sophisticated calculations ofhydrodynamical interaction and photoinduced chemistry.  We find that the CO-emitting gas is distributed within a relatively smallregion <~ 1.5". Its structure consists of a central dense component plusstrongly disrupted outer regions, which seem to be parts of spiral arms thatare highly focused on the orbital plane. The structure and dynamics of thesespiral arms are compatible with our hydrodynamical calculations. We argue thatthe observed nebula is the result of the dynamical interaction between the windand the gravitational attraction of the WD. We also find that UV emission fromthe hot companion efficiently photodissociates molecules except in the densestand best-shielded regions, that is, in the close surroundings of the AGB starand some shreds of the spiral arms from which the detected lines come. We canoffer a faithful description of the distribution of nebular gas in thisprototypical source, which will be a useful template for studying materialaround other tight binary systems.

Structure and dynamics of the inner nebula around the symbiotic stellar system R Aqr