T CrB: Overview of the accretion history, Roche-lobe filling, orbital solution, and radiative modeling

Abstract

Context. Expectations for an imminent new outburst of the recurrent symbiotic nova T CrB are mounting, initiated by the discovery in 2015 of a new enhanced mass-transfer phase (SAP), which is reminiscent of the one preceding the last recorded outburst in 1946.Aims. We aim to derive a robust estimate of the most important parameters describing the physical nature of T CrB, trace the accretion history onto its white dwarf, and account for the unexpected delay in the occurrence of the new outburst. In particular, the SAP prior to 1946 was brighter and followed by a nova eruption within six months from its conclusion. This time the 2015–2023 SAP has been fainter and although two years have passed since the end of this phase, no new eruption has taken place.Methods. Between 2005–2025, the period covering the SAP and the preceding quiescence, we collected a massive amount of photometric and spectroscopic observations at optical wavelengths. We analyzed these data together with the abundant ultraviolet (UV) observations available in the archive of the Swift satellite.Results. Guided by the results of the orbital solution and, in particular, by the radiative modeling process we employed on the whole set of available data, we derived for T CrB a binary period of 227.5528 days, along with an inclination of 61° and masses of 1.35 M_{⊙ and 0.93 M⊙ for the white dwarf and the M3III companions, respectively, making the mass transfer dynamically stable. The red giant completely fills its Roche lobe and at Vrot sin i = 4.75 ± 0.26 km s^{−1, it is rotating much more slowly than the 16 km s−1 co-rotation value. The ∼20° azimuth of the hot spot, implied by the hump shaping the optical light curve in quiescence, fixes the outer radius of the disk to ∼58 R⊙. This is the same as the canonical value expected from disk theory. In quiescence, the disk is cold and mostly neutral. The SAP was caused by an inside-out collapse of the disk, during which the mean accretion rate onto the WD was ∼28× larger than in quiescence. The SAP ended in late April 2023, but from May 2024, the mass flow has intensively resumed at disk inner radii, while the collapse wave reached the outer portions of the disk. The consequent revamp in the mass accretion could fill the gap inherited by the fainter 2015–2023 SAP and eventually lead the WD accreted shell to ignition.}}