Slowly, slowly in the wind: 3D hydrodynamical simulations of wind mass transfer and angular-momentum loss in AGB binary systems

Abstract

Wind mass transfer in binary systems with AGB donor stars plays a fundamental role in the formation of a variety of objects, including barium stars and CEMP stars. We carry out a comprehensive set of 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 both the wind-velocity-to-orbital-velocity ratio ($v_\infty/v_\mathrm{orb}$) and 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, as well as 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 a relation for both the mass-accretion efficiency and angular-momentum loss as a function of $v_\infty/v_\mathrm{orb}$ 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.