The impact of initial conditions on simulations of the common envelope binary interaction

In this work we analyse various aspects of the common envelope (CE) interaction between two stars via numerical simulations. The common envelope (CE) interaction is a short phase of the interaction between two stars (a primary and a companion) in a binary system characterised by the dense cores of the two objects orbiting inside their merged envelopes. During this phase, orbital energy and angular momentum are transferred to the gas of the envelope, that can become unbound from the potential well of the system, leaving behind a close binary. Unfortunately, due to its short duration, the CE phase is not readily observed (only one case has been observed until now) and numerical simulations are a major way to investigate its physics. However, to this time, numerical studies have failed to fully reproduce the observed post-CE parameters, that is, close binaries with separations generally shorter than ≃ 5 R⊙ and where all the envelope has been expelled, yielding instead rather large final separations and never expelling the whole envelope. Since the CE interaction has been analysed in multiple works, but without explicitly taking inconsideration the effect of the single physical parameters, in this PhD we tried to do so. One of the main topics we investigated during this PhD work has been the effect of large initial binary separations on the CE interaction. We performed a simulation with the binary components initially placed at the maximum possible distance that would guarantee the system to end in a CE.The main outcomes of this work show that a larger initial separation does not dramatically affect the CE interaction. The final separations obtained in this way are slightly larger with respect to an identical system where only the separation is reduced in such a way that the CE begins at the beginning of the simulation. The amount of mass unbound from the potential well of the binary is also slightly larger. Another important part of this work has been the study of the effects of rotation on the CE interaction. To achieve this goal we spun up the original star we used for the study on larger separations, after investigating the possibility and reliability to create a more accurate stellar model. The results of this investigation show that initial rotation of the primary star has negligible effects on the outputs of the CE interaction. The third effect we worked on is the variation of the final separation and unbound mass in function of the mass of the primary. We therefore performed a set of simulations with a more massive primary star and set of companions with different masses. This simple study showed that for the same companion’s mass a more massive primary generates a closer binary at the end of the CE interaction, in the range of observations, yielding however less unbound mass. Additionally, during the work we encountered a numerical problem with ENZO, which showed poor conservation of energy in our simulations. We therefore had to devote part of this PhD work to investigate the issue and find a solution for it.