Impact of orbital motion on the structure and stability of adiabatic shocks in colliding wind binaries

The collision of winds from massive stars in binaries results in the formation of a double-shock structure with observed signatures from radio to X-rays. We study the structure and stability of the colliding wind region as it turns into a spiral due to orbital motion. We focus on adiabatic winds, where mixing between the two winds is expected to be restricted to the Kelvin-Helmholtz instability (KHI). Mixing of the Wolf-Rayet wind with hydrogen-rich material is important for dust formation in pinwheel nebulae such as WR 104, where the spiral structure has been resolved in infrared. We use the hydrodynamical code RAMSES with an adaptive grid. A wide range of binary systems with different wind velocities and mass loss rates are studied with 2D simulations. A specific 3D simulation is performed to model WR 104. Orbital motion leads to the formation of two distinct spiral arms where the KHI develops differently. We find that the spiral structure is destroyed when there is a large velocity gradient between the winds, unless the collimated wind is much faster. We argue that the KHI plays a major role in maintaining or not the structure. We discuss the consequences for various colliding wind binaries. When stable, there is no straightforward relationship between the spatial step of the spiral, the wind velocities, and the orbital period. Our 3D simulation of WR 104 indicates that the colder, well-mixed trailing arm has more favourable conditions for dust formation than the leading arm. The single-arm infrared spiral follows more closely the mixing map than the density map, suggesting the dust-to-gas ratio may vary between the leading and trailing density spirals. However, the density is much lower than what dust formation models require. Including radiative cooling would lead to higher densities, and also to thin shell instabilities whose impact on the large structure remains unknown.