How Odd is Betelgeuse?

Sammanfattning: The role of massive stars (those with masses greater than eight solar masses) in the chemical enrichment of galaxies and renewed star formation forms an important field of fundamental research in modern astrophysics. The complex nature of massive star evolution produces formidable challenges in furthering our understanding of the processes driving massive star evolution. Our ever-changing picture is further complicated by the relatively recent realisation that the vast majority of massive stars are formed in multiple star systems. In close binaries, interactions between the stars can lead to an exchange of matter, dramatically altering the structure and evolution of the component stars and the binary system as a whole. Depending on the initial binary configuration, this transfer of mass may be stable or unstable, producing results varying from wide binaries with massive secondaries, to common envelopes with tight binaries or even stellar mergers. A fraction of binary systems will be disrupted when the primary star explodes as a supernova, producing a runaway secondary star. Alpha-Orionis, commonly known as Betelgeuse, is believed to be the product of such a scenario. We synthesise a realistic population of binary systems containing massive stars and simulate the evolution of these systems accounting for mass transfer through Roche lobe overflow. We analyse the probability for each system to be disrupted after the supernova explosion of the primary for a range of supernova kicks and determine which fraction of these systems will produce a secondary star in the estimated mass range of Betelgeuse. We also investigate which mass transfer channel is most likely to produce a Betelgeuse-like star. We find that ~ 14% and ~ 25% of binaries will produce a secondary in the mass ranges 13-18 M_sun and 11-20 M_sun, respectively, allowing for the large uncertainties in determining the mass of Betelgeuse. Of these Betelgeuse candidates, approximately 75% emerge from systems following stable mass transfer. We show the dependence of the fraction of bound systems on the magnitude and direction of the supernova kick velocity and the pre-supernova orbital elements of the binary. We establish the requirement for substantial supernova kicks in the disruption of close binaries and the generation of runaway velocities consistent with that of Betelgeuse. We also briefly discuss mechanisms other than post-supernova ejections which lead to runaway massive stars and how these might apply to Betelgeuse-like stars.