Magnetic fields in massive stars: dynamics and origin

Evidence continues to accumulate in favour of the presence of magnetic fields on the surfaces of massive stars. Some authors hypothesize that such fields originate in a dynamo in the convective core, with buoyancy bringing flux tubes to the surface. Here we show that, when realistic stellar models are used, this ‘core dynamo hypothesis’ encounters a serious difficulty: in order for surface fields to originate in a core dynamo, the core must create magnetic fields that are much stronger than equipartition values. As an alternative hypothesis for the origin of magnetic fields in OB stars, we suggest that a dynamo is operating in shear-unstable gas in the radiative stellar envelope. Using a recently developed code for the evolution of rotating stars, we find that in a 10-M⊙ zero-age main sequence (ZAMS) star with a typical rotation period, more than 90 per cent of the volume of the star is subject to shear instability. We have recently proposed that dynamo operation in shear-unstable gas helps to explain certain properties of mass loss in cool giants. Here, in the context of hot stars, we show that fields which originate in the shear-unstable regions of the envelope can reach the surface without violating the constraints of equipartition. Such fields rise to the surface on time-scales which are short compared to main-sequence lifetimes.