# 3. Another Example: colliding winds (2D)¶

## 3.1. Introduction¶

This example is taken from Stevens et al. (1992), section 4.1, modelling the colliding winds of the binary system V444 Cyg. This eclipsing binary system contains a Wolf-Rayet (WR) star (of spectral type WN5) with a an O6 companion. We assume that the WR star is the primary because it is more evolved than the O6 star.

The properties of the stars (taken from Stevens et al.) are as follows for the primary (WR):

• mass-loss rate $$\dot{M} = 1.4\times10^{-5} \,\mathrm{M}_\odot \, \mathrm{yr}^{-1}$$
• wind speed $$v_\infty = 2000 \,\mathrm{km\,s}^{-1}$$
• radius $$R_\star = 2.9 \,\mathrm{R}_\odot$$
• temperature $$T_\mathrm{eff}=35$$ kK

and for the secondary (O6):

• mass-loss rate $$\dot{M} = 10^{-6} \,\mathrm{M}_\odot \, \mathrm{yr}^{-1}$$
• wind speed $$v_\infty = 2000 \,\mathrm{km\,s}^{-1}$$
• radius $$R_\star = 10 \,\mathrm{R}_\odot$$
• temperature $$T_\mathrm{eff}=40$$ kK

The system has an orbital period of 4.2 days, although in this 2D simulation we ignore the orbital motion and treat the wind-wind interaction as though the stars were at rest in an inertial frame with cylindrical symmetry along the line connecting the stars. The separation of the stars is taken as $$2.8\times10^{12}$$ cm.

The shocked wind in the wind-collision region should be quite strongly radiative, and this has a big effect on the hydrodynamics of the wind-collision region. In this example the simulation domain has dimensions $$z\in[-1.024,1.024]\times10^{13}$$ cm, $$R\in[0,1.024]\times10^{13}$$ cm, resolved by $$640\times320$$ grid cells, and 2 refined levels centred on the origin (i.e. 3 levels in total). The finest grid contains the two stars.

## 3.2. Running the simulation¶

An animation of the simulation can be viewed here. This simulation took about 3 hours to run using 4 MPI processes on an Intel NUC. The equivalent 3D simulation would take about 10 thousand core-hours.