Modelling Variability in Hot-Star Winds

Abstract

I review 2-D hydrodynamical simulations of rotating hot-star winds with azimuthal structure induced by modulation of the radiative driving force near the wind base. As a first step toward examining more realistic perturbation mechanisms (e.g., nonradial pulsations, or magnetic fields), the driving modulation here is taken to arise from bright and dark spots in the stellar photosphere. These spots induce decreases or increases in wind flow speed, and as the star rotates, spiral “Co-Rotating Interaction Regions” (CIRs) form, much as in the solar wind, from interaction between fast and slow flow streams. A new feature unique to line-driven flow is a velocity-gradient kink that propagates inward from interaction fronts at a fast radiative-acoustic mode speed. The slowly evolving velocity plateaus that form behind such kinks give rise to absorption features with a slow apparent acceleration, much like the Discrete Absorption Components (DACs) often observed in UV wind lines from hot stars. In simulation models with base driving sinusoidally modulated between increase and decrease, there arise alternating spiral streams of enhanced or decreased density, associated respectively with decreased or increased flow speeds. These speed variations have substantial impact on the line profile, and so these dynamical simulation are not as successful as analogous kinematic models of corotating density streams in reproducing the “phase-bowing” of periodic absorption modulations observed in the recent IUE ‘Mega’ project.