2022A&A...664A..12M

Context. V1309 Sco is the only confirmed non-compact stellar merger, identified thanks to the match of its pre-outburst light curve to that of a contact binary. Therefore, anything that can be deduced from existing observations may serve as benchmark constraints for models.
Aims. We present some observational evidence to guide future hydrodynamical simulations and common envelope studies.
Methods. Using archive spectra taken at high and mid spectral resolution during the V1309 Sco outburst and late decline, together with the inferential methods we developed to study nova ejecta through panchromatic high resolution spectroscopic follow ups, we constrained the physical state, structure, dynamics, and geometry of the transient that originated in the stellar merger.
Results. We found that the emitted spectra arise from two distinct contributions: matter expelled during the 2008 outburst and circumbinary gas produced during historic mass-loss episodes. These two components are likely to exhibit orthogonal geometry, with the 2008 mass loss displaying a dust-laden bipolar ejecta produced by a time limited rapidly accelerating wind and the circumbinary gas having a donut-like shape. A central source powers them both, having produced a fluorescent light pulse, but we cannot precisely determine the time it started or its spectral energy distribution. We can, however, place its upper energy cutoff at about 54 eV and the bulk of its emission at < 20 eV. We also know that the central source turned off within months after the outburst and before the ejecta turned optically thin.