Interacting Giants and Compact Stars

This doctoral thesis is based on four papers dealing with interactions involving giant and compact stars. In papers I and III, we study how and when neutron stars can draw mass from white dwarfs and produce long-lived ultra-compact X-ray binaries, or else merge and produce a luminous supernova-like event. We found that white dwarf-neutron star binaries merge more often than previously thought and modelled the nucleosynthesis and supernova manifestations of such mergers. In paper II, we showed that composite subdwarf B binaries bear the imprints of Galactic chemical evolution. Such binaries are produced when red giants lose their envelopes to companions and ignite helium in their cores. As we showed, accounting for how the Galaxy changed its metal content over time is needed to explain the orbital of such binaries. In paper IV, we studied how Betelgeuse, the 10th brightest star in the sky, may likely form as an outcome of a merger of two stars, previously ejected from a nearby stellar cluster in the Orion Nebula.

In this doctoral thesis, we model how giant and compact stars interact with their companions. Compact stars, such as white dwarfs or neutron stars, are endpoints of stellar evolution. In comparison, red giants may form when stars like the Sun run out of their nuclear fuel and expand to the size comparable to Earth orbit. We used computer simulations to model how compact and giant stars may lose mass or merge with their companions. For compact white dwarf neutron star binaries, we also modelled nuclear burning occurring during such mergers and the bright optical events produced by such mergers. In the case of interacting giants, we showed that some of the outcomes of such interactions called subdwarf B stars bear the imprints of how our Galaxy changed its chemical composition over time. Similarly, we modelled how the massive giant star Betelgeuse has likely formed through a merger of two stars previously ejected from the nearby Orion Nebula cluster.