Resolving the mass transfer in the symbiotic recurrent nova T Coronae Borealis

Abstract

T Coronae Borealis (T CrB) is a symbiotic recurrent nova with an 80-year recurrence interval whose next eruption is imminent. We aim to resolve the accretion mechanism of the binary system governing the mass transfer during its super-active phase. Using phase-resolved high-resolution spectroscopy, we analyze the zoo of spectral-line profiles arising from the symbiotic activity. We perform Doppler tomography of selected emission lines to resolve the system's gaseous components and their different velocity regimes. We find evidence of enhanced accretion through Roche lobe overflow during the super-active phase, as traced by the oxygen, helium, and hydrogen lines. The accretion disc is found to be fully viscously evolved and extends up to its maximal radius. By mapping the kinematics of lines probing different excitation energies, we can identify distinct interaction sites. These include the bright spot at the stream impact on the accretion disc outer radius, the irradiation at the red-giant facing side, the stream-disc overflow, the accretion disk wind, and an expanding bipolar nebula. The bipolar jet emerged at the rise of the super-active phase and underwent an acceleration phase of about five years. The temporal evolution of the lines supports the scenario where the departure from quiescence started in the disc, likely triggered by a disc instability similar to what occurs in dwarf novae outburst, leading to an increased mass accretion and causing important irradiation of the giant that has further enhanced the mass-transfer rate during the super-active phase. Conclusions. Symbiotic recurrent novae, such as T CrB, are governed by similar mass-transfer mechanisms as found in cataclysmic variables despite their different orbital properties (longer orbital periods imposing larger accretion discs) and evolutionary pathways.