Wind accretion in binary stars — II. Accretion rates

Abstract

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 formerly on the asymptotic giant branch (AGB). The binary system modelled consists of a 3M ⊙ AGB star (losing mass at a rate 10 −6 M ⊙ y −1 ) and a 1.5M ⊙ 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% for the γ = 1.5 case and 8% for the other models. The highest resolution obtained (with 400k particles) corresponds to an accretor of linear size ≈ 16R ⊙ . Despite being (in the γ = 1.5 case) about ten 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, due 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.

To Ba or not to Ba: Enrichment ins-process elements in binary systems with WD companions of various masses

“…The same holds true for DR Dra, whose absence of s-process overabundance is slightly more puzzling, however. The long period of DR Dra (904 d) suggests that the rapid rotation responsible for the chromospheric activity be due to spin accretion (Theuns et al 1996) rather than to tidal coupling, which requires a closer system (Fekel et al 1993). Thus there has been some binary interaction in the recent past of DR Dra.…”

BINSTAR: a new binary stellar evolution code

“…Theuns et al 1996) suggest that values of α BH ten times less the standard value could be used for slow winds. However, a better approach would consist of incorporating a more physically realistic prescription based on the results from hydrodynamical simulations.…”

AGB stars in binaries and their progeny

“…Jeffries & Stevens (1996) suggest that the K dwarfs in these wide systems were spun up by the accretion of the wind from their companion, when the latter was a mass-losing AGB star. The possibility of accreting a substantial amount of spin from the com pan ion 's wind has since been confirmed by Smooth Particle Hydrodynamics simulations (Theuns et al 1996). A clear signature that mass transfer has been operative in 2RE J0357+283 is again provided by the detection of excess barium in that star (Jeffries & Smalley 1996).…”

“…This system moreover hosts the hottest WD (T rv 35000 K) among barium systems (Fekel et al 1993). The cooling time scale of the WD is about 10 My, shorter than the magnetic braking time scale of the giant star (about 100 My; see Theuns et al 1996). The mass transfer thus occurred recently enough for the magnetic braking having not yet slowed down the giant star in a substantial way.…”

“…The pollution of the companion occurs at this phase, either through wind accretion (Boffin & Jorissen 1988;Han et al 1995;Theuns et al 1996;Karakas et al 1998) or through Roche lobe overflow (RLOF) (Iben & Tutukov 1985;Webbink 1986). The efficiency of wind accretion seems to be sufficient to account for the longest periods observed among PRS (about 30 y, see Fig.…”