Moving-Mesh Hydrodynamic Simulations of Colliding Wind Binaries

While theories for stellar evolution are well developed for single stars, the majority of massive stars are found in binary systems, which gives rise to different evolutionary pathways, phenomena and fates for these stars. Colliding wind binaries, systems in which both massive stars have strong stellar winds, are ideal laboratories for exploring the complex hydrodynamics in wind interacting regions, given the extensive and detailed multi-wavelength studies from X-rays to radio of the high and low energy emission produced by processes such as particle acceleration and dust formation. High resolution, multi-dimensional simulations of colliding wind binaries can provide insight towards the specific mass configurations, instabilities, and hydrodynamics of these complicated shocked wind colliding regions. In this paper, we begin with a description of the observations and physics of stellar winds, in particular the line-driven winds that characterize the stars in colliding wind binaries. We then present hydrodynamic simulations of stellar winds and colliding wind binaries using the moving-mesh magnetohydrodynamics simulation code AREPO to generate predictions for single and binary systems with immense stellar winds. We demonstrate that the two and three-dimensional AREPO simulations of a stellar outflow from a single star are in good agreement with analytical solutions for a spherically symmetric, stationary stellar wind. Having established that the code captures the relevant physics for a single stellar outflow, we apply the same numerical methods to colliding wind binaries. Our colliding wind binary simulations are able to achieve an accurate contact discontinuity location to within 1.9% of the analytical solution, an adiabatic strong-shocked density, temperature and velocity, accurate to within 2%, 8.6% and 34%, respectively, and an isothermal shocked density accurate to within 22%. Modeling these colliding wind binary systems is challenging due to the large dynamical range in fluid properties required to resolve shocks and instabilities while maintaining well-defined boundaries of the stars. While this has been previously explored with adaptive mesh refinement and smooth particle hydrodynamics studies, applying AREPO's unique moving-mesh numerical methods to colliding wind binaries lays the groundwork for future, complementary modeling, including detailed radiative cooling, shock stability, dust formation, X-ray and radio emission, and binary evolution.