Vol. 580
In section 6. Interstellar and circumstellar matter

Post-outburst spectra of a stellar-merger remnant of V1309 Scorpii: from a twin of V838 Monocerotis to a clone of V4332 Sagittarii

The authors report continuing observations of the only dynamically verified merger event ever observed. This paper extends the analysis to the period after the initial ejection and dust formation event, presenting exquisite optical and near-infrared high-resolution X-shooter spectra and NLTE modeling of the state of the expelled gas. The system is surrounded by a cold, stratified envelope composed mainly of molecular gas (principally oxides, e.g. VO, CrO, RbO, water) expanding at velocities up to about 300 km/s. The few strong atomic lines, e.g. K I, showed early P Cyg profiles, but the absorption has gradually disappeared because the material has dispersed but without deceleration. An intriguing possibility is that some heavy-element nucleosynthesis, especially Cr and Ni, may accompany the merger and leave an isotopic trace anomaly in Galactic abundances. The authors show how the spectroscopic evolution of this system has passed through similar stages to two other Galactic merger candidates, V4332 Sgr and V838 Mon, neither of which was caught in the act at the moment of the binary fusion. This is a remarkable system that illuminates the entire class of so-called red novae.