Abbott wave-triggered runaway in line-driven winds from stars and accretion disks

Line-driven winds from stars and accretion disks are accelerated by scattering in numerous line transitions. The wind is believed to adopt a unique critical solution out of the infinite variety of shallow and steep solutions. We study the inherent dynamics of the transition toward the critical wind. A new runaway wind mechanism is analyzed in terms of radiative acoustic (Abbott) waves, which are responsible for shaping the wind velocity law and fixing the mass loss. Three different flow types result depending on the location of perturbations. First, if the shallow solution is perturbed sufficiently far downstream, a single critical point forms in the flow, which is a barrier for Abbott waves, and the solution tends to the critical one. Second, if the shallow solution is perturbed upstream from this critical point, mass overloading results, and the critical point is shifted inward. This wind exhibits a broad, stationary region of decelerating flow, and its velocity law has kinks. Third, for perturbations even further upstream, the overloaded wind becomes time-dependent and develops shocks and dense shells.