The 2015 super-active state of recurrent nova T CrB and the long term evolution after the 1946 outburst

The recurrent nova T CrB has entered in 2015 a phase of unprecedented high activity. To trace something equivalent, it is necessary to go back to 1938, before the last nova eruption in 1946. The 2015 super-active state is characterized by: a large increase in the mean brightness (Delta B=0.72 mag over the underlying secular trend), vanishing of the orbital modulation from the B-band lightcurve, and appearance of strong and high ionization emission lines, on top of a nebular continuum that overwhelms at optical wavelengths the absorption spectrum of the M giant. Among the emission lines, Hell 4686 attains a flux in excess of H gamma, the full set of OIll and NIII lines involved in the Bowen fluorescence mechanism are strong and varying in intensity in phase with Hell 4686, and OIV and [NeV] are present. A large increase in the radiation output from the hot source is responsible for a large expansion in the ionized fraction of the M giant wind. The wind is completely ionized in the direction to the observer. A high electron density is supported by the weakness of forbidden lines and by the large amplitude and short time scale of the reprocessing by the nebular material of the highly variable photo-ionization input from the hot source. During the super active state the nebula is varying to and from ionization-bounded and density-bounded conditions, and the augmented irradiation of the cool giant has changed the spectral type of its side facing the WD from M3III to M2III, i.e. an increase of similar to 80 K in effective temperature. (C) 2016 Elsevier B.V. All rights reserved.