The majority of massive stars (more than 8 times our Sun) are born in binary and multiple-star systems. When these stars evolve during their lifetimes, they can interact with their companions. When large amount of mass is quickly transferred from one system to another, an instability in this system can occur: one of the components can spiral into the gaseous envelope of its companion, forming a Common Envelope. The evolution of the binaries during this phase is necessary to form many exciting systems in astrophysics, including sources of gravitational waves.
Despite its importance, there are several unanswered questions in this field: What systems enter CEE? What happens during CEE? How CEE remnants evolve? This problem is difficult to tackle from theoretical and computational perspectives, as it involves different scales (e.g. a red supergiant donor with a neutron star accretor) and timescales (millions of years of evolution vs. dynamical interactions lasting just days). The problem can be broken into smaller pieces, and studied separately. However, to put back all these parts together and test these results, observations are needed.
Recently, a new type of astrophysical transients called luminous red novae (LRNe) has emerged as direct observational evidence of the dynamical ejection of the CE in binaries, followed by a stellar merger. Their progenitor systems and slow brightening precursor emission can be identified in archival and multi-wavelength time-domain data. Their outbursts can be observed to obtain new information about the energetic outbursts. Finally, their late-time evolution allows us to study the dust formed quickly after the merger has happened. These multiple phases have the potential to provide a complete observational evolution of binary systems entering CEE.
The aim of this project is to study the different stages of CEE in massive binaries using observations of extragalactic LRNe. The observational sample will contain transients within 15 Mpc from massive binary progenitors with HST archival data. The project will develop novel transient selection strategies to identify a fraction of these LRNe even years before their main outburst, and study the extensive mass loss leading to coalescence. The study will provide observational evidence of the physical processes that occur before, during, and after the ejection of the CE in massive binary systems, the characteristics of their progenitors, and their rate in our Local Universe.
