Wind-Roche-lobe-overflow: outflow morphology and orbital evolution

Wind-Roche-lobe-overflow (WRLOF) is a type of interaction in binary stars that is less intense than Roche-lobe-overflow and more intense than Bondi-Hoyle-Lyttleton accretion. It is an effective way to transfer mass in relatively wide binaries as one of the stars ascend to asymptotic-giant-branch (AGB) or red-giant-branch (RGB). Some gas in the slow wind can be captured and accreted onto the secondary while the rest becomes outflow from the binary and carry away both mass and angular momentum. Meanwhile, circumbinary discs could form around the binary stars or spiral structure could be found. The mass transfer, outflow and tidal friction in close binaries could also change the dynamics of the binary stars. The orbital period may decay and the binary separation may also shrink. We find that the binary separation in some close binaries shrink rapidly and will finally lead to stronger binary interaction, such as common envelope and binary merger, within the lifetime of the giant star.