Hydrodynamical Simulations of Corotating Interaction Regions and Discrete Absorption Components in Rotating O-Star Winds

We present two-dimensional hydrodynamical simulations of corotating stream structure in the wind from a rotating O star, together with resulting synthetic line profiles showing discrete absorption components (DACs). An azimuthal variation is induced by a local increase or decrease in the radiative driving force, as would arise from a bright or dark “star spot” in the equatorial plane. Since much of the emergent wind structure seems independent of the exact method of perturbation, we expect similar morphology in winds perturbed by localized magnetic fields or nonradial pulsations, as well as by either rotationally-modulated structure or transient mass ejections. We find that bright spots with enhanced driving generate high-density, low-speed streams, while dark spots generate low-density, high-speed streams. Corotating interaction regions (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 which takes the form of a sharp propagating discontinuity (“kink” or “plateau”) in the radial velocity gradient. These features travel inward in the co-moving frame at the radiative-acoustic characteristic speed, and thus slowly outward in the star’s frame. We find that these slow kinks, rather than the CIRs themselves, are more likely to result in high-opacity 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 time scale than material moving with the wind itself. This is in general accord with observations showing slow apparent accelerations for DACs. Subject headings: circumstellar matter – hydrodynamics – radiative transfer – stars: early-type – stars: mass loss – stars: rotation