Circumbinary disk formation through AGB star winds

Circumbinary disks are commonly observed around post-asymptotic giant branch (post-AGB) star binaries, yet their formation and especially their long-term evolution remain unclear. We investigate this process using smoothed-particle hydrodynamics simulations of AGB star wind-binary interactions across different wind velocities, binary eccentricities, and mass ratios. When the wind is fast, or the companion is relatively low-mass, corresponding to the Bondi-Hoyle regime, the outflow remains largely unbound and forms a spiral density pattern. In contrast, slower winds and more massive companions lead to wind Roche-lobe overflow (WRLOF), where a circumsingle disk forms around the companion and efficiently transfers angular momentum to the outflow, producing a circumbinary disk. We perform simulations for over 600 binary orbital periods and find that the resulting disk properties depend sensitively on binary parameters, with higher eccentricities producing more extended and eccentric circumbinary disks, while lower mass companions reduce the disk density and growth rate. We further find that angular momentum transport within the circumbinary disk is dominated by spiral structures and shocks generated by the binary-wind interaction, corresponding to effective stresses comparable to or larger than the imposed viscosity. These results show that wind-binary interactions can naturally generate diverse circumbinary disk morphologies observed in AGB and post-AGB star systems.