Wind accretion in binary stars - I. Mass accretion ratio

Three-dimensional hydrodynamic calculations are performed in order to investigate mass transfer in a close binary system, in which one component undergoes mass loss through a wind. The mass ratio is assumed to be unity. The radius of the mass-losing star is taken to be about a quarter of the separation between the two stars. Calculations are performed for gases with a ratio of specific heats gamma = 1.01 and 5/3. Mass loss is assumed to be thermally driven so that the other parameter is the sound speed of the gas on the mass-losing star. Here, we focus our attention on two features: flow patterns and mass accretion ratio, which we define as the ratio of the mass accretion rate onto the companion, (M) over dot(acc), to the mass loss rate from the mass-losing primary star, (M) over dot(loss). We characterize the flow by the mean normal velocity of the wind on the critical Roche surface of the mass-losing star, V-R. When V-R < 0.4 AOmega where A and Omega are the separation between the two stars and the angular orbital frequency of the binary, respectively, we obtain Roche-lobe over-flow (RLOF), while for V-R > 0.7 AOmega we observe wind accretion. We find very complex flow patterns in between these two extreme cases. We derive an empirical formula of the mass accretion ratio as 0.18 x 10(-0.75VR/AOmega) in the low velocity regime and 0.05 (V-R/AOmega)(-4) in the high velocity regime.