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

摘要

The recurrent nova T CrB has entered in 2015 a phase of unprecedented highactivity. To trace something equivalent, it is necessary to go back to 1938,before the last nova eruption in 1946. The 2015 super-active state ischaracterized by: a large increase in the mean brightness (Delta B =0.72 magover the uderlying secular trend), vanishing of the orbital modulation from theB-band lightcurve, and appearance of strong and high ionization emission lines,on top of a nebular continuum that overwhelms at optical wavelengths theabsoption spectrum of the M giant. Among the emission lines, HeII 4686 attainsa flux in excess of Hgamma, the full set of OIII and NIII lines involved in theBowen fluorescence mechanism are strong and varying in intensity in phase withHeII 4686, and OIV and [NeV] are present. A large increase in the radiationoutput from the hot source is reponsible for a large expansion in the ionizedfraction of the M giant wind. The wind is completely ionized in the directionto the observer. A high electron density is supported by the weakness offorbidden lines and by the large amplitude and short time scale of thereprocessing by the nebular material of the highly variable photo-ionizationinput from the hot source. During the super-active state the nebula is varyingto and from ionization-bounded and density-bounded conditions, and theaugmented irradiation of the cool giant has changed the spectral type of itsside facing the hot source from M3III to M2III, i.e. an increase of ~80 K ineffective temperature.