Wind accretion by a binary stellar system and disc formation

Abstract

I calculate the specific angular momentum of mass accreted by a binary system embedded in the dense wind of a mass-losing asymptotic giant branch star. The accretion flow is of the Bondi-Hoyle-Lyttleton type. For most of the space of the relevant parameters the flow is basically an isothermal high Mach number accretion flow. I find that when the orbital plane of the accreting binary system and the orbital plane of the triple system are not parallel to each other, the accreted mass on to one or two of the binary system components has high specific angular momentum. For a large fraction of triple-star systems, accretion discs will be formed around one or two of the stars in the binary system, provided that the mass ratio of the two stars in the accreting binary system is ?0.5. Such discs may blow jets which shape the descendant planetary nebula (PN). The axis of jets will be almost parallel to the orbital plane of the triple-star system. One jet is blown outward relative to the wind, while the other jet passes near the mass-losing star, and is more likely to be slowed down or deflected. I find that during the final asymptotic giant branch phase, when the mass-loss rate is very high, an accretion disc may form for orbital separation between the accreting binary systems and the mass-losing star of up to ∼400-800 au. I discuss the implications for the shape of the descendant PN, and list several PN which may have been shaped by an accreting binary-star system, i.e. by a triple-star system.