High-resolution numerical simulations of unstable colliding stellar winds

Abstract

We investigate the hydrodynamics of the interaction of two supersonic winds in binary systems. The collision of the winds creates two shocks separated by a contact discontinuity. The overall structure depends on the momentum flux ratio eta of the winds. We use the code RAMSES with adaptive mesh refinement to study the shock structure up to smaller values of eta, higher spatial resolution and greater spatial scales than have been previously achieved. 2D and 3D simulations, neglecting orbital motion, are compared to widely-used analytic results and their applicability is discussed. In the adiabatic limit, velocity shear at the contact discontinuity triggers the Kelvin-Helmholtz instability. We quantify the amplitude of the resulting fluctuations and find that they can be significant even with a modest initial shear. Using an isothermal equation of state leads to the development of thin shell instabilities. The initial evolution and growth rates enables us to formally identify the non-linear thin shell instability (NTSI) close to the binary axis. Some analogue of the transverse acceleration instability is present further away. The NTSI produces large amplitude fluctuations and dominates the long-term behaviour. We point out the computational cost of properly following these instabilities. Our study provides a basic framework to which the results of more complex simulations, including additional physical effects, can be compared.