Hydrodynamic Simulations of the Interaction between an AGB Star and a Main Sequence Companion in Eccentric Orbits

The Rotten Egg Nebula has at its core a binary composed of a Mira star and an A-type companion at a separation >10 au. It has been hypothesized to have formed by strong binary interactions between the Mira and a companion in an eccentric orbit during periastron passage ~800 years ago. We have performed hydrodynamic simulations of an asymptotic giant branch star interacting with companions with a range of masses in orbits with a range of initial eccentricities and periastron separations. For reasonable values of the eccentricity, we find that Roche lobe overflow can take place only if the periods are <<100 years. Moreover, mass transfer causes the system to enter a common envelope phase within several orbits. Since the central star of the Rotten Egg nebula is an AGB star, we conclude that such a common envelope phase must have lead to a merger, so the observed companion must have been a tertiary companion of a binary that merged at the time of nebula ejection. Based on the mass and timescale of the simulated disc formed around the companion before the common envelope phase, we analytically estimate the properties of jets that could be launched. Allowing for super-Eddington accretion rates, we find that jets similar to those observed are plausible, provided that the putative lost companion was relatively massive.Comment: accepted for publication in MNRA

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

Empirical thesis.Bibliography: pages 139-144.1. Introduction -- 2. Our "codes" and the generation of the initial stellar model -- 3. The effect of the initial separation on common envelope simulations -- 4. The effect of the primary rotation on the common envelope simulations -- 5. The effect of massive primaries on the common envelope interaction -- 6. Energy conservation in ENZO -- 7. Conclusions and future work -- References.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.Mode of access: World wide web1 online resource (xiv, 144 pages) colour illustration