Luminous red novae: observing the ejection of the binary's common envelope in real time
Common envelope evolution is one of the most challenging phases in binary stellar evolution. The discovery of gravitational waves represented a true "renaissance" in this field, which already delivered great theoretical progress within the last few years. However, there are still many uncertainties that require an observational approach. Within the last decade, a new type of astrophysical transients named luminous red novae (LRNe) have emerged as a promising observational approach to witness the elusive phase of common envelope evolution (and ejection) in close binary stars. In my talk, I will describe the LRN AT2018bwo discovered in NGC45 and the characteristics of its massive progenitor 10-13 years before the dynamical ejection. Detailed binary evolution models suggest a 12-16 Msun primary that initiated Roche-lobe overflow a few centuries ago. Using the common envelope formalism we estimate that only 0.15-0.5 Msun of envelope was ejected during the dynamical spiral-in of the secondary, implying a merger event. The agreement between this value and the mass derived by modelling the characteristics of the transient itself reveals a promising observational approach to study failed common envelope ejections for massive progenitors.
