Transient accretion disc-like envelope in the symbiotic binary BF Cygni during its 2006 - 2015 optical outburst

摘要

The optical light of the symbiotic binary BF Cygni during its last eruptionafter 2006 shows orbital variations because of an eclipse of the outburstingcompact object. The first orbital minimum is deeper than the following ones.Moreover, the Balmer profiles of this system acquired additional satellitecomponents indicating bipolar collimated outflow at one time between the firstand second orbital minima. This behaviour is interpreted in the framework ofthe model of collimated stellar wind from the outbursting object. It issupposed that one extended disc-like envelope covering the accretion disc ofthe compact object and collimating its stellar wind forms in the period betweenthe first and second minima. The uneclipsed part of this envelope isresponsible for the decrease of the depth of the orbital minimum. Thecalculated $UBVR_{C}I_{C}$ fluxes of this uneclipsed part are in agreement withthe observed residual of the depths of the first and second orbital minima. Theparameters of the envelope require that it is the main emitting region of theline H$\alpha$ but the H$\alpha$ profile is less determined from its rotationand mostly from other mechanisms. It is concluded that the envelope is atransient nebular region and its destruction determines the increase of thedepth of the orbital minimum with fading of the optical light.