Dynamical simulations of magnetically channelled line-driven stellar winds - II. The effects of field-aligned rotation

Abstract

Building upon our previous magnetohydrodynamics (MHD) simulation study of magnetic channelling in radiatively driven stellar winds, we examine here the additional dynamical effects of stellar rotation in the (still) 2D axisymmetric case of an aligned dipole surface field. In addition to the magnetic confinement parameter η* introduced in Paper I, we characterize the stellar rotation in terms of a parameter W≡Vrot/Vorb (the ratio of the equatorial surface rotation speed to orbital speed), examining specifically models with moderately strong rotation W= 0.25 and 0.5, and comparing these to analogous non-rotating cases. Defining the associated Alfvén radius RA≈η1/4*R* and Kepler corotation radius RK≈W−2/3R*, we find rotation effects are weak for models with RA < RK, but can be substantial and even dominant for models with RA≳RK. In particular, by extending our simulations to magnetic confinement parameters (up to η*= 1000) that are well above those (η*= 10) considered in Paper I, we are able to study cases with RA≫RK; we find that these do indeed show clear formation of the rigid body disc predicted in previous analytic models, with however a rather complex, dynamic behaviour characterized by both episodes of downward infall and outward breakout that limit the build-up of disc mass. Overall, the results provide an intriguing glimpse into the complex interplay between rotation and magnetic confinement, and form the basis for a full MHD description of the rigid body discs expected in strongly magnetic Bp stars like σ Ori E.