Vol. 575
In section 6. Interstellar and circumstellar matter

Stellar wind in state transitions of high-mass X-ray binaries

This paper, the continuation of a major effort to produce realistic multidimensional models of mass flows in binaries and nonspherical winds, concentrates on the mass accretion in the Cyg X-1 system. The wind is computed assuming radiative acceleration by limb- and gravity-darkened mass loser using a modified CAK-type wind (where the radiative driving dependds on the velocity gradient to a power alpha, and a line strength parameter eta).

The full effects of the Roche geometry are included for both the O star and black hole accreter. In these time-dependent simulations, the authors include photoionization (assuming low optical depths for the moment, since detailed radiative transfer is still prohibitively computationally intense), and the thermal properties of the gas are followed using optically thin heating and cooling functions. They also explicitly account for XR photoionization slowing of the wind in the immediate vicinity of the gainer, thus moderating the mass accretion with time.

The main results are the parameter study in two and three dimensions, showing how the accretion region structure and dynamics depend on the assumptions about the wind driving. For increasing values of alpha, or increasing dependence on the velocity gradient for the driving force, the wind passes from a chaotic structure to an almost classic bow shock. The most dramatic variations result from changing the XR photoionization. The authors show, for example, the effect of transiting between high/soft to low/hard states. The dense gas stream in the proximity of the L1 point, which resembles the Roche lobe overflow in semi-detached binaries, completely disappears when the wind is ionized at its base, and lines are incapable of accelerating the wind. Matter consequently does not reach the escape velocity and falls back onto the primary.