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

TL;DR

This paper investigates a transient, disc-like envelope around the compact object in the symbiotic binary BF Cygni during its 2006-2015 outburst, explaining orbital variations and bipolar outflows through a collimated stellar wind model.

Contribution

It introduces a model of a transient, disc-like envelope forming between orbital minima, explaining observed spectral and photometric variations during the outburst.

Findings

The envelope causes orbital minimum depth variations.

The envelope's flux matches observed residuals.

The envelope is a transient nebular region that dissipates over time.

Abstract

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