Evolution of red Nova V4332 Sagittarii remnant

We present the multicolor BV RI photometry and BTA/SCORPIO spectroscopy for the red nova V4332 Sgr, performed in 2005–2012. We have analyzed the behavior of the nova remnant, considering our observations along with all the published observations and archival photographic photometry. The atomic and molecular emission spectra show an exponential weakening. Since 2003, the line fluxes have on the average decreased 30-fold. The continuum of the M-type giant in the spectrum has weakened abruptly between 2006 and 2008, twice in the red and by 4 times in the blue range. The variations in the spectral energy distribution of the stellar continuum after the outburst correspond to a decrease in the surface temperature of the M-type giant by 1000 K. The presence of a faint star of about 20m is possible. The star can be either a member of the system or a field star. It is found that the phenomenon of the red nova in this system is not related with the M-type star. The progenitor of the explosion could be a blue straggler, which has disappeared from the spectral energy distribution after the explosion of 1994. It is most likely that this object was a contact binary system. To explain the “cold explosion” of V4332 Sgr, we have accepted the hypothesis of merging components of a contact binary star in a hierarchical triple or multiple system. There are evidences of dynamical destruction of the outburst remnant and accretion of its matter to the M-type giant. The cause of the red nova phenomenon is thought to be a sudden explosive energy release in the center of a star having a massive envelope, and a subsequent expansion of the envelope in the conditions close to adiabatic. As a result of the explosion, thermal energy reaches the surface of the envelope a year or a few years after the outburst, whereas the envelope already has a large surface area. The cause of the explosion in the center of the star can be both a merger event of the nuclei of two stars in the contact system with a formed common massive envelope and instability in the core of a single massive star. Thus, the red novae can be heterogeneous objects at different evolutionary stages.