Modelling decretion discs in Be/X-ray binaries using Smoothed Particle Hydrodynamics

Abstract

Be stars are dynamic systems even when completely isolated. Variabilities in the
emission of these stars across all wavelengths are observed with periods ranging from a few minutes to many decades. Be stars are generally accepted to be surrounded by a decretion disc that gives rise to a number of these observable variabilities. The fact that this disc must be formed of material that is ejected from the surface of the star requires a complex mechanism that is still not fully understood. Be stars coupled with a compact object companion in a Be/X-ray binary have exhibited a large variety of complex behaviour. As Be/X-ray binaries are also the largest population of high mass X-ray binaries, they are a valuable resource for the investigation of the extreme physics of compact objects. In this thesis, a code that implements the computational method of smoothed particle hydrodynamics (SPH) is used to model Be/X-ray binaries. SPH simulations have a resolution that is dependent on the density of individual regions. Thus, SPH is suited to modelling the circumstellar discs of Be stars because there is a considerable range between the maximum and minimum densities. Such a model can describe the properties of the disc in detail during binary interactions and allows a comparison to observable Be/X-ray binaries. This thesis builds upon the previous work that has been done to investigate the properties of the circumstellar discs of Be stars. It begins with an investigation of a broad range of disc properties. The same simulations are then shown to agree with observationally determined relationships between the Be star’s disc size, the orbital period and the semi-major axis of the binary. The Be/neutron star binary SXP 5.05 is then targeted with simulations attempting to reproduce the observations of the Iband and X-ray flux during a large optical outburst that occurred in 2013. It is found that the outburst can be replicated by a sudden and considerable increase in the mass ejection of the Be star. Be/X-ray binaries with different compact object companions are then modelled. The simulations agree with previous suggestions that Be stars with black hole companions have smaller discs and are fainter X-ray sources. Finally, the observational implications gleaned from these simulations and the possibilities of future work are considered.

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ORCID for Wynn Ho: orcid.org/0000-0002-6089-6836

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