Magnetic fields in massive stars. I. Dynamo models

Motivated by mounting evidence for the presence of magnetic fields in the a tmospheres of "normal" early-type main-sequence stars, we investigate the v arious possible modes of dynamo action in their convective core. Working wi thin the framework of mean field electrodynamics, we compute alpha (2) and alpha (2)Omega dynamo models and demonstrate that the transition from the f ormer class to the latter occurs smoothly as internal differential rotation is increased. Our models also include a magnetic diffusivity contrast betw een the core and radiative envelope. The primary challenge facing such mode ls is to somehow bring the magnetic field generated in the deep interior to the stellar surface. We investigate the degree to which thermally driven m eridional circulation can act as a suitable transport agent. In all models with strong core-to-envelope magnetic diffusivity contrast-presumably close st 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 like ly not attained in the interior of early-type main-sequence stars. Dynamo a ction then remains highly probable, but an alternate mechanism must be soug ht to carry the magnetic field to the surface.