Wind accretion in binary stars - II. Accretion rates

Smoothed particle hydrodynamics (SPH) is used to estimate accretion rates of mass, linear and angular momentum in a binary system where one component undergoes mass loss through a wind. Physical parameters are chosen such as to model the alleged binary precursors of barium stars, whose chemical peculiarities are believed to result from the accretion of the wind from a companion that was formerly on the asymptotic giant branch (AGB). The binary system modelled consists of a 3-M⊙ AGB star (losing mass at a rate 10-6 M⊙ yr-1) and a 1.5-M⊙ star on the main sequence, in a 3-au circular orbit. Three-dimensional simulations are performed for gases with polytropic indices γ = 1, 1.1 and 1.5, to bracket more realistic situations that would include radiative cooling. Mass accretion rates are found to depend on resolution, and we estimate typical values of 1-2 per cent for the γ = 1.5 case and 8 per cent for the other models. The highest resolution obtained (with 400000 particles) corresponds to an accretor of linear size ≈ 16 R⊙. Despite being (in the γ = 1.5 case) about 10 times smaller than theoretical estimates based on the Bondi-Hoyle prescription, the SPH accretion rates remain large enough to explain the pollution of barium stars. Uncertainties in the current SPH rates remain, however, owing to the simplified treatment of the wind acceleration mechanism, as well as to the absence of any cooling prescription and to the limited numerical resolution. Angular momentum transfer leads to significant spin-up of the accretor and can account for the rapid rotation of HD 165141, a barium star with a young white dwarf companion and a rotation rate unusually large among K giants. In the circular orbit modelled in this paper, hydrodynamic thrust and gravitational drag almost exactly compensate and so the net transfer of linear momentum is nearly zero. For small but finite eccentricities and the chosen set of parameters, the eccentricity tends to decrease.