Continuum and molecular emission from the inner regions of the symbiotic system R Aquarii

Context . Symbiotic systems often include an asymptotic giant branch (AGB) star and a hot compact companion, such as a white dwarf, that are in close interaction. Due to the intense ultraviolet emission from the hot companion, the molecular content of circumstellar envelopes in the symbiotic systems is poor. As a result, the less abundant molecules have not been previously studied in detail in this kind of object. Aims . R Aqr is the closest and best-studied symbiotic system. Our aim is to study the inner regions of R Aqr based on ALMA observations of the continuum and line emission. Methods . We present very sensitive ALMA maps of the continuum emission at 1.3 and 0.45 mm. We also obtain the spatial distribution of the recombination line H30 α with a high and moderate angular resolution, and it is compared with the emission of the continuum at 1.3 mm. High-resolution maps of several molecules are obtained in the three observed ALMA bands. We study the molecular emissions using a simplified model to explain the brightness distributions seen in the central position of our maps. Results . We find that the low-resolution continuum map at 1.3 mm shows the emission of the radio photosphere of the AGB star, its surroundings, and the structure of the bipolar jet launched by the companion. The high-resolution continuum map at 1.3 mm shows the innermost part of the jet, probably revealing the position of the secondary, and suggests mass transfer from the AGB star to the white dwarf. The brightness distribution of H30 α is similar but not coincident with the continuum emission, and it basically probes the region where the jet is formed. The brightness distributions of the studied molecular lines show a variety of shapes. The emissions of the abundant molecules, CO and SiO, are relatively extended since they can survive far from the AGB star in spite of the intense ultraviolet emission from the white dwarf. On the contrary, less abundant molecules only survive in regions close to the AGB star, where shielding is stronger. From our best-fit model for these weak species, we find that the expanding shell is ~10 14 cm in size, which means that these less abundant species are confined to the intra-orbital regions.