High-resolution imaging of the evolving bipolar outflows in symbiotic novae: The case of the RS Ophiuchi 2021 nova outburst

Context. The recurrent and symbiotic nova RS Ophiuchi (RS Oph) underwent a new outburst phase during August 2021, about 15 years after the last event that occurred in 2006. This outburst represents the first nova event ever detected at very high energies (VHE, E > 100 GeV), and a whole set of coordinated multiwavelength observations were triggered by this event. Aims. The main goals of this work are to characterize the evolving morphology of the expanding bipolar ejecta with high accuracy and to determine the physical conditions of the surrounding medium in which they propagate. Methods. By means of high-resolution very long baseline interferometry (VLBI) radio observations, we monitored RS Oph with the European VLBI Network (EVN) and e-MERLIN at 1.6 and 5 GHz during multiple epochs from 14 to 65 days after the explosion. Results. We reveal an evolving source structure consisting of a central and compact core and two elongated bipolar outflows expanding on opposite sides of the core in the east-west direction. The ejecta angular separation with time is consistent with a linear expansion with an average projected speed of ∼7000 km s−1. We find clear evidence of a radial dependence of the density along the density enhancement on the orbital plane (DEOP), going from 1 × 107 cm−3 close to the central binary to 9 × 105 cm−3 at ∼400 AU. Conclusions. Thanks to the accurate source astrometric position provided by Gaia DR3, in this work we draw a detailed scenario of the geometry and physics of the RS Oph evolving source structure after the most recent nova event. We conclude that most of the mass lost by the red giant companion goes into the DEOP, for which we estimate a total mass of 6.4 × 10−6 M⊙, and into the circumstellar region, while only a small fraction (about one-tenth) is accreted by the white dwarf.