The Effect of Magnetic Field Tilt and Divergence on the Mass Flux and Flow Speed in a Line‐driven Stellar Wind

We carry out an extended analytic study of how the tilt andfaster-than-radial expansion from a magnetic field affect the mass flux andflow speed of a line-driven stellar wind. A key motivation is to reconcileresults of numerical MHD simulations with previous analyses that had predictednon-spherical expansion would lead to a strong speed enhancement. By includingfinite-disk correction effects, a dynamically more consistent form for thenon-spherical expansion, and a moderate value of the line-driving power index$\alpha$, we infer more modest speed enhancements that are in good quantitativeagreement with MHD simulations, and also are more consistent with observationalresults. Our analysis also explains simulation results that show thelatitudinal variation of the surface mass flux scales with the square of thecosine of the local tilt angle between the magnetic field and the radialdirection. Finally, we present a perturbation analysis of the effects of afinite gas pressure on the wind mass loss rate and flow speed in both sphericaland magnetic wind models, showing that these scale with the ratio of the soundspeed to surface escape speed, $a/v_{esc}$, and are typically 10-20% comparedto an idealized, zero-gas-pressure model.