Wind Roche-Lobe Overflow: A New Mass Transfer Mode for Mira-type Binaries

We investigate mass transfer in binaries consisting of a mass-losing Mira variable and an accreting compact companion. Although these systems are generally assumed to interact via Bondi-Hoyle-Littleton (BHL) wind accretion, this mechanism cannot adequately explain the observations of the nearest and best-studied symbiotic binary, Mira. We propose a new mass-transfer mode, "wind Roche-Lobe Overflow" (wind RLOF), which we demonstrate with 3-D numerical hydrodynamic simulations. We show that this mode occurs when the wind acceleration zone lies close to, or is a significant fraction of, the Roche-lobe radius. The circumstellar outflows which result tend to be highly aspherical and strongly focused towards the binary orbital plane. The mass-transfer rates are also 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. barium stars, bipolar planetary nebulae, and la supernovae.