In low-mass binary systems, mass transfer is likely to occur via a slow and dense stellar wind when one of the stars is in the asymptotic giant branch (AGB) phase. Observations show that many binaries that have undergone AGB mass transfer have orbital periods of 1-10 yr, at odds with the predictions of binary population synthesis models. In this paper we investigate the mass-accretion efficiency and angular-momentum loss via wind mass transfer in AGB binary systems and we use these quantities to predict the evolution of the orbit. To do so, we perform 3D hydrodynamical simulations of the stellar wind lost by an AGB star in the time-dependent gravitational potential of a binary system, using the AMUSE framework. We approximate the thermal evolution of the gas by imposing a simple effective cooling balance and we vary the orbital separation and the velocity of the stellar wind. We find that for wind velocities larger than the relative orbital velocity of the system the flow is described by the Bondi-Hoyle-Lyttleton approximation and the angularmomentum loss is modest, which leads to an expansion of the orbit. On the other hand, for low wind velocities an accretion disk is formed around the companion and the accretion efficiency as well as the angular-momentum loss are enhanced, implying that the orbit will shrink. We find that the transfer of angular momentum from the binary orbit to the outflowing gas occurs within a few orbital separations from the center of mass of the binary. Our results suggest that the orbital evolution of AGB binaries can be predicted as a function of the ratio of the terminal wind velocity to the relative orbital velocity of the system, v ∞ /v orb . Our results can provide insight into the puzzling orbital periods of post-AGB binaries and also suggest that the number of stars entering into the common-envelope phase will increase. The latter can have significant implications for the expected formation rates of the end products of low-mass binary evolution, such as cataclysmic binaries, type Ia supernova and double white-dwarf mergers.

Wind mass transfer in binary systems with asymptotic giant branch (AGB) donor stars plays a fundamental role in the formation of a variety of objects, including barium stars and carbon-enhanced metal-poor (CEMP) stars. In an attempt to better understand the properties of these systems, we carry out a comprehensive set of smoothed-particle hydrodynamics (SPH) simulations of wind-losing AGB stars in binaries for a variety of binary mass ratios, orbital separations, initial wind velocities, and rotation rates of the donor star. The initial parameters of the simulated systems are chosen to match the expected progenitors of CEMP stars. We find that the strength of interaction between the wind and the stars depends on the ratio of wind velocity to orbital velocity (v∞/vorb) and on the binary mass ratio. Strong interaction occurs for close systems and comparable mass ratios, and gives rise to a complex morphology of the outflow and substantial angular-momentum loss, which leads to a shrinking of the orbit. As the orbital separation increases and the mass of the companion star decreases, the morphology of the outflow and the angular-momentum loss become more similar to the spherically symmetric wind case. We also explore the effects of tidal interaction and find that for orbital separations up to 7−10 AU, depending on mass ratio, spin-orbit coupling of the donor star occurs at some point during the AGB phase. If the initial wind velocity is relatively low, we find that corotation of the donor star results in a modified outflow morphology that resembles wind Roche-lobe overflow. In this case the mass-accretion efficiency and angular-momentum loss differ from those found for a non-rotating donor. Finally, we provide relations for the mass-accretion efficiency and angular-momentum loss as a function of v∞/vorb and the binary mass ratio that can be easily implemented in a population synthesis code to study populations of barium stars, CEMP stars, and other products of interaction in AGB binaries, such as cataclysmic binaries and type Ia supernovae.

Carbon-enhanced metal-poor stars, CH stars, barium stars and extrinsic S stars, among other classes of chemically peculiar stars, are thought to be the products of the interaction of low-and intermediate-mass binaries which occurred when the most evolved star was in the asymptotic giant branch (AGB) phase. Binary evolution models predict that because of the large sizes of AGB stars, if the initial orbital periods of such systems are shorter than a few thousand days, their orbits should have circularised due to tidal effects. However, observations of the progeny of AGB binary stars show that many of these objects have substantial eccentricities, up to e ≈ 0.9. In this work we explore the impact of wind mass transfer on the orbital parameters of AGB binary stars by performing numerical simulations in which the AGB wind is modelled using a hydrodynamical code and the dynamics of the stars is evolved using an N-body code. We find that in most models the effect of wind mass transfer will contribute to the circularisation of the orbit, but on longer timescales than tidal circularisation if e 0.4. We also find that for relatively low initial wind velocities and pseudo-synchronisation of the donor star, a structure resembling wind Roche-lobe overflow is observed as the stars approach periastron. In this case, the interaction between the gas and the star is stronger than when the initial wind velocity is high and the orbit shrinks while the eccentricity decreases. In one of our models wind interaction is found to pump the eccentricity of the orbit on a similar timescale as tidal circularisation. However, since the orbit of this model is shrinking tidal effects will become stronger during the evolution of the system. Although our study is based on a small sample of models, it offers some insight into the orbital evolution of eccentric binary stars interacting via winds. A larger grid of numerical models for different binary parameters is needed to test if a regime exists where hydrodynamical eccentricity pumping can effectively counteract tidal circularisation, and if this can explain the puzzling eccentricities of the descendants of AGB binaries.