Revisiting V1309 Sco 2008 outburst spectra. Observational evidence for theoretical modeling of stellar mergers

TL;DR

This study analyzes high-resolution spectra of the 2008 V1309 Sco stellar merger to understand its ejecta and circumbinary gas, providing observational constraints for future hydrodynamical models of stellar mergers.

Contribution

It offers detailed observational evidence on the physical state, structure, and geometry of the merger ejecta and circumbinary material, informing theoretical modeling efforts.

Findings

Ejected matter from the outburst shows bipolar, dust-laden geometry.

Circumbinary gas has a donut-like shape from historic mass loss.

Central source had an energy cutoff around 54 eV and turned off months after outburst.

Abstract

CONTEXT: V1309 Sco is the only certain noncompact stellar merger, due to its indisputable preoutburst light curve matching that of a contact binary of almost equal mass stars. Therefore, anything that can be deduced from the existing observations serves as benchmark constraints for models. AIMS: We present some observational evidences 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 constrain the physical state, structure, dynamics and geometry of the transient 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 likely have 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 from the outburst and before the ejecta turned optically thin.