Stellar mergers in the era of large surveys

Most stars in our Universe live in binaries. Unstable mass transfer from one star to another can lead to the formation of a common envelope: a shared gaseous shell where both stars orbit. The end of this phase is marked by the quick spiral-in of the secondary star towards its companion, leading to violent interactions between the components. The whole, or part of the binary's common envelope may get ejected, and the binary may even completely merge. This last phase has been observed as astrophysical transients called luminous red novae (LRNe), allowing us to study the progenitor stars, the energetics of the outburst and the properties of the ejected material. In my talk I will present the methodology and results of our automated search for LRNe in the ZTF and BlackGEM surveys. I also will summarise the results of our photometric and spectroscopic follow-up campaign for three recent LRNe, associated with massive stellar progenitors. While the transients have quickly faded in the optical bands, their infrared signatures reveal the formation of cold dust shells, which are reprocessing the light of the newly formed stars. These are ideal targets for future IR surveys