Minimum luminosity that will eject to infinity a particle initially in circular orbit at x_esc. Plot compares solutions of the full e.o.m. (solid green curve, Eqs. (1), (2)) with solutions of the simplified equations that neglect the effects of radiation drag ([ND], thin red line, Eqs. (7), (8)). Initial luminosity is k₁ = L₁/L_Edd = 0, stellar radius X = 5. The Newtonian limit is also shown ([NL], dot-dashed line), as well as the radius of the Eddington capture sphere (ECS). See text for details.