Wind Accretion in Separated Binaries

Extract

In the framework of Euler flows, I present simulations of wind accretion in well separated binaries for three different cases: Cygnus X-3, a high mass X-ray binary (see e.g. Schmutz et al. 1996), ζ Aurigae, a system showing strong atmospheric eclipse effects (see e.g. Griffin et al. 1990), and the generic barium star model of Theuns and Jorissen (1993), which is recomputed here with a different numerical method (a MUSCL-type integrator, incorporated in our AMRCART code). These simulations represent a first, small sample out of the large parameter space of wind accretion in separated binaries (see Table 1).

On the orbital scale, the vorticity of the flow is due to the orbital motion of the stars. It is particularly large for the case where the wind velocity is comparable to the orbital speed of the stars (see Fig. 1). In the vicinity of the accreting star, the vorticity is determined by the accretion process. With two strong vortices above and below the accretor (with respect to the orbital plane), the flow resembles the flow out of a bath tub. On a smaller scale, one finds even more vorticity: The flow is highly turbulent. Such turbulence and the shear flows in the large scale vortices cannot be well captured by an Euler flow model. A next step of improvement would be the use of an appropriate MHD turbulence model.