On magnetic braking of late-type stars

Abstract

The paper reports on a preliminary study of the variation with angular velocity of the rate of braking of a late-type star. The strength of the dynamo-built field <b>B</b> will normally increase with rotation, so that the surface where the wind speed becomes Alfvénic moves further out from the star and the angular momentum transported per unit mass is correspondingly higher. The stronger field tends to trap more gas within a ‘dead zone’ which does not contribute to the braking, but the extent of the dead zone is limited by the pressure required to balance the higher centrifugal force. These effects are illustrated by a simply parametrized field model. Braking rates are estimated for the two cases with the coronal base density independent of <it>B</it> and proportional to <it>B</it>, respectively.