Interacting Giants and Compact Stars

Abstract: This thesis is based on four papers dealing with various aspects of interactions in binary stars. Interactions between stars occur at nearly all stages of their evolution and can take many forms. For example, stars may lose material to a binary companion, merge, interact with groups of other stars in star clusters and explode in binary systems, among other interactions.The first paper in this thesis, Bobrick et al. (2017) (Paper I), models how white dwarfs interact with neutron stars as they spiral into contact due to gravitational wave emission. Through the use of hydrodynamic simulations with the Oil-on-Water code, we investigated the process of mass transfer in such binaries. We found that early phases of interactions in these systems lead to significant loss of angular momentum, driving systems to merge more often than previously expected. The third paper in the thesis, Bobrick et al. (2021a) (Paper III), describes the subsequent evolution of the white dwarf-neutron star binaries containing a massive white dwarf after they merge. In this case, the white dwarf gets shredded into a disc, reaching high temperatures leading to nuclear reactions. These nuclear reactions in the disc produce nickel-56 that gets ejected with the rest of the material from the vicinity of the neutron star. As the ejected material expands, the radioactive nickel-56 heats the material, causing it to glow and become observable as a supernova-like transient event. We used hydrodynamic simulations based on the Water code and a nuclear processing code Torch to study nucleosynthesis in the disc, and a supernova spectral synthesis code SuperNu to model how these events may be observed. Unlike papers I and III, which dealt with compact objects, papers II and IV focussed on interactions involving giant stars. In the second paper, Vos et al. (2020) (Paper II), we modelled how mass transfer between red giants and main-sequence stars can give rise to subdwarf B stars. These subdwarf B stars are remnant cores of the red giants that ignited helium while losing mass. By performing a population study based on detailed stellar structure code MESA, we found that the orbits of such subdwarf B binaries bear imprints of the chemical history of our Galaxy. The fact that the Milky Way had changed its metal content over time allowed us to explain the orbital periods of the known subdwarf B binaries. In our fourth study, Bobrick et al. (2021b) (Paper IV), we investigated the formation history of Betelgeuse, which is a red supergiant visible to the naked eye. It has been recently realised that Betelgeuse is likely an outcome of a merger between two stars that were ejected from their birth environment. To test this scenario, we used the FewBody code together with a Monte Carlo-based model of dynamical interactions in the Milky Way star clusters and synthesised a population of stars which may lead to the formation of Betelgeuse. We have confirmed that a stellar merger is indeed a likely mechanism behind the formation of Betelgeuse.