Line‐driven Ablation and Wind Tilting by External Irradiation

The directional variation of the velocity gradient in a supersonic, radiatively accelerated flow gives an effectively anisotropic character to the line-scattering process. This leads to surprising consequences in source geometries that are more complex than isolated nonrotating stars. In this paper we explore the wind dynamics from a planar slab atmosphere that is irradiated by an external oblique source, within the framework of standard Castor, Abbott, & Klein (CAK) wind theory. We show that the presence of externally incident radiation can be surprisingly effective at tilting the flow away from the vertical. Even more surprising is our conclusion that such illumination should often enhance the mass loss and can even induce outflow from a surface with no intrinsic radiation source. We examine the physical causes of such "line-driven ablation" and discuss the potential implications for modeling line-driven flows in massive-star binary and accretion-disk systems.