Understanding Mass Transfer in Wind-Interacting Binaries: SPH Models of ``Wind Roche-lobe Overflow''

We investigate mass transfer in wind-interacting binaries consisting of a mass-losing, Mira variable and an accreting, compact companion. Mass transfer in such systems is generally assumed to occur via Bondi-Hoyle-Littleton (BHL) accretion. However, the accretion rates and outflow geometries that result from this mass-transfer mechanism do not adequately explain the observations of the nearest and best studied symbiotic binary, Mira, or, for example, the formation of chemically polluted stars, e.g. barium stars. We propose a new mass-transfer mode, ``wind Roche-Lobe Overflow (wind RLOF)'', which we investigate with numerical hydrodynamic simulations. This mode occurs when the wind acceleration zone lies close to, or is a significant fraction of, the Roche-lobe radius. We show that the circumstellar outflows which result from wind RLOF tend to be highly aspherical and strongly focused towards the binary orbital plane. The subsequent mass-transfer rates are at least an order of magnitude greater than the analogous BHL values. We discuss the implications of these results for symbiotic binaries and other related systems, e.g. bipolar (proto)-planetary nebulae, supernovae Ia and Wolf-Rayet binaries.