Hydrodynamical simulations of corotating interaction regions and discrete absorption components in rotating O-star winds

We present two-dimensional hydrodynamical simulations of corotating interaction regions (CIRs) in the wind from a rotating O star, together with resulting synthetic line profiles showing discrete absorption components (DACs). For computational tractability, we use a local, Sobolev treatment of the radiative force, which suppresses the small-scale instability intrinsic to line driving but still allows us to model the dynamics of large-scale wind structure. As a first step toward modeling the wind response to large-scale base perturbations (e.g., from surface magnetic fields or nonradial pulsations), the structure here is explicitly induced by localized increases or decreases in the radiative force, as would result from a bright or dark ''star spot'' near the star's equator. We find that bright spots with enhanced driving generate high-density, low-speed streams, while dark spots generate low-density, high-speed streams. CIRs form where fast material collides with slow material; e.g., at the leading (trailing) edge of a stream from a dark (bright) spot, often steepening into shocks. The unperturbed supersonic wind obliquely impacts the high-density CIR and sends back a nonlinear signal that takes the form of a sharp propagating discontinuity (''kink'' or ''plateau'') in the radial velocity gradient. In the wind's comoving frame, these features propagate inward at the fast characteristic speed derived by Abbott for radiatively modified acoustic waves, but because this is generally only slightly less than the outward wind speed, the features evolve only slowly outward in the star's frame. We find that these slow kinks, rather than the CIRs themselves, are more likely to result in DACs in the absorption troughs of unsaturated P Cygni line profiles. Because the hydrodynamic structure settles to a steady state in a frame corotating with the star, the more tightly spiraled kinks sweep by an observer on a longer timescale than material moving with the wind itself. This is in general accord with observations showing slow apparent accelerations for DACs.