WIND ACCRETION IN BINARY STARS .2. ACCRETION RATES

Smoothed particle hydrodynamics (SPH) is used to estimate accretion ra tes 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 accretio n of the wind from a companion that was formerly on the asymptotic gia nt branch (AGB). The binary system modelled consists of a 3-M(.) AGB s tar (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 simulat ions are performed for gases with polytropic indices gamma=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 gamma=1.5 case and 8 per cent for the other models. The highest resolution obtained (wit h 400 000 particles) corresponds to an accretor of linear size approxi mate to 16 R(.). Despite being (in the gamma = 1.5 case) about 10 time s smaller than theoretical estimates based on the Bondi-Hoyle prescrip tion, the SPM accretion rates remain large enough to explain the pollu tion of barium stars. Uncertainties in the current SPH rates remain, h owever, owing to the simplified treatment of the wind acceleration mec hanism, as well as to the absence of any cooling prescription and to t he limited numerical resolution. Angular momentum transfer leads to si gnificant spin-up of the accretor and can account for the rapid rotati on 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 alm ost exactly compensate and so the net transfer of linear momentum is n early zero. For small but finite eccentricities and the chosen set of parameters, the eccentricity tends to decrease.