2019MNRAS.489..891H

We investigate kinematics of mass-loss from the vicinity of the second Lagrange point L2 with applications to merging binary stars, common envelope evolution, and the associated transient brightenings. For ballistic particle trajectories, we characterize initial velocities and positional offsets from L2 that lead to unbound outflow, fall back followed by a formation of a decretion disc, collision with the binary surface, or a hydrodynamic shock close to the binary, where some particle trajectories loop and self-intersect. The latter two final states occur only when the trajectories are initiated with offset from L2 or with velocity vector different from corotation with the binary. We find that competition between the time-dependent and steeply radially decreasing tidal torques from the binary, Coriolis force, and initial conditions lead to a non-trivial distribution of outcomes in the vicinity of L2. Specifically, even for initial velocities slower than corotation, we find that a set of initial position offsets leads to unbound outflows. Our results will aid in the interpretation of the morphology of mass-loss streams in hydrodynamic simulations.