Magnetic Braking by a Stellar Wind--I

TLDR

The stellar wind theory models a rotating magnetic star with an isothermal corona and aligned magnetic and rotation axes. A critical surface C, defined by Alfvénic points, separates a wind zone where the dipolar field enforces co‑rotation from a dead zone where magnetic pressure blocks expansion, and beyond C the flow drags the field and conserves angular momentum, while a low coronal temperature drives a centrifugal wind that is again bounded by C.

Abstract

The stellar wind theory is formulated for a rotating magnetic star, surrounded by an isothermal corona, and with the magnetic and rotation axes parallel. A critical surface C is defined by the Alfvénic points on each field-line, at which the wind-speed just equals the local Alfvén speed. The limiting field-line, with its Alfvenic point on the equator, separates the wind zone (which includes the polar regions) from the dead zone, in which outward expansion is prevented by the magnetic pressure. Within C , the roughly dipolar field is strong enough to force the flow to follow the field, and to keep the gas approximately co-rotating with the star. Beyond C , the gas flow drags the field to follow the flow, and each element approximately conserves its angular momentum. If the coronal temperature is too low for a thermal wind, the magnetically-controlled centrifugal forces drive a centrifugal wind, so that there is again a surface C limiting the zone of co-rotation.