A model for the formation of large circumbinary disks around post AGB stars

We propose that the large, radius of ∼103 AU, circumbinary rotating disks observed around some post-asymptotic giant branch (post-AGB) binary stars are formed from slow AGB wind material that is pushed back to the center of the nebula by wide jets. We perform 2D-axisymmetrical numerical simulations of fast and wide jets that interact with the previously ejected slow AGB wind. In each system there are two oppositely launched jets, but we use the symmetry of the problem and simulate only one jet. A large circularization-flow (vortex) is formed to the side of the jet which together with the thermal pressure of the shocked jet material accelerate cold slow-wind gas back to the center from distances of ∼103–104 AU. We find for the parameters we use that up to ∼10−3M⊙ is back-flowing to the center. We conjecture that the orbital angular momentum of the disk material results from the non-axisymmetric structure of jets launched by an orbiting companion. This conjecture will have to be tested with 3D numerical codes.