Continuum-driven shocks in active galactic nuclei

In this paper, we extend the study of instabilities in flows driven by the radiation pressure of an ionizing continuum to flows that are not plane par allel. It is well known that the plane-parallel instability leads eventuall y to the formation of continuum-driven shocks backed by a sonic transition. If these structures are thin, we find that they are unstable to a corrugat ion mode, and evolve to form sharp-peaked triangular profiles. Once this ha s occurred, the thin-shock approximation is no longer valid. We study the further development of the shocks by numerical hydrodynamic si mulations. The flow tends to break up into numerous discrete bow-shaped com ponents. The speed of these components through the upstream material is alm ost constant. As a result, the maximal velocity of radiatively driven shock s through the upstream gas may be determined by instabilities rather than b y other physical effects. Interactions between gas in the wings of neighbou ring bowshocks can, however, form subsequent generations of bowshocks that are faster and more acute than their predecessors. One likely location where continuum-driven shocks may occur is in the broad -line regions of active nuclei. We discuss the application of our results t o such flows.