Magnetic Fields in Massive Stars. I. Dynamo Models

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Abstract
Motivated by mounting evidence for the presence of magnetic fields in the atmospheres of "normal" early-type main-sequence stars, we investigate the various possible modes of dynamo action in their convective core. Working within the framework of mean field electrodynamics, we compute α2 and α2Ω dynamo models and demonstrate that the transition from the former class to the latter occurs smoothly as internal differential rotation is increased. Our models also include a magnetic diffusivity contrast between the core and radiative envelope. The primary challenge facing such models is to somehow bring the magnetic field generated in the deep interior to the stellar surface. We investigate the degree to which thermally driven meridional circulation can act as a suitable transport agent. In all models with strong core-to-envelope magnetic diffusivity contrast—presumably closest to reality— whenever circulation is strong enough to carry a significant magnetic flux, it is also strong enough to prevent dynamo action. Estimates of typical meridional circulation speeds indicate that this regime is likely not attained in the interior of early-type main-sequence stars. Dynamo action then remains highly probable, but an alternate mechanism must be sought to carry the magnetic field to the surface.