The research in this project addressed open questions concerning the common envelope evolution (CEE), which is a cataclysmic phase in the evolution of binary stars and which enables evolutionary outcomes unachievable in other ways. A strong, recent driver to better understand CEE comes from the observations of merging black holes and neutron stars with gravitational waves. A new observational probe of CEE are transient brightenings called Luminous red novae. In this project, we performed computer simulations to comprehensively examine different phases of CEE with a particular emphasis on the accompanying transients.
CEE can begin in several ways. By identifying and studying each of these ways we aim to assess which binaries will go through CEE. Perhaps the most common way to start CEE is when the transfer of mass between two stars in an orbiting binary becomes unstable. An important ingredient needed to assess the mass transfer (in)stability in stellar evolution calculations is the relation between the size of the star (or rather by how much it overfills its so-called Roche lobe) and the mass transfer rate. Furthermore, loss of mass from one of the stars will alter its internal structure, which serves as a starting point for the subsequent more dynamical evolution. Another way to start CEE is when a star on an eccentric orbit grazes its companion. Each grazing ejects a little bit of mass and tightens the orbit.
The transient brightening associated with the dynamical phase of CEE can reveal new information about the progenitor binary system, the mechanism of its instability, and the surrounding medium. To successfully model these transients, we need incorporate a number of physical effects such as recombination and ionization of various elements, formation of dust and molecules, and transport of radiation inside this environment.
Finally, dynamical interactions inside CEE eventually get weaker and the system evolution slows down. Further evolution includes interplay between the binary orbit and the remaining, expanding gas. A number of different phenomena can play a role in this late phase, in particular magnetic fields and the possibility of collimated bipolar outflows. Our goal here was to perform dedicated multi-dimensional simulations of this phase and link them to preceding dynamical evolution and observed phenomena.
