On the Steady Nature of Line‐Driven Disk Winds

We perform an analytic investigation of the stability of line-driven diskwinds, independent of hydrodynamic simulations. Our motive is to determinewhether or not line-driven disk winds can account for the wide/broad UVresonance absorption lines seen in cataclysmic variables (CVs) andquasi-stellar objects (QSOs). In both CVs and QSOs observations generallyindicate that the absorption arising in the outflowing winds has a steadyvelocity structure on time scales exceeding years (for CVs) and decades (forQSOs). However, published results from hydrodynamic simulations of line-drivendisk winds are mixed, with some researchers claiming that the models areinherently unsteady, while other models produce steady winds. The analyticinvestigation presented here shows that if the accretion disk is steady, thenthe line-driven disk wind emanating from it can also be steady. In particular,we show that a gravitational force initially increasing along the windstreamline, which is characteristic of disk winds, does not imply an unsteadywind. The steady nature of line-driven disk winds is consistent with the 1Dstreamline disk-wind models of Murray and collaborators and the 2.5Dtime-dependent models of Pereyra and collaborators. This paper emphasizes theunderlying physics behind the steady nature of line-driven disk winds usingmathematically simple models that mimic the disk environmen