The Rise of a Magnetic Flux Tube through the RadiativeEnvelope of a 9

We explore the possibility that the magnetic eld generated by a dynamo at the interface between the convective core and radiative envelope of a massive star can be transported to the surface by buoyancy. 1. Application of solar ux tube theory to massive stars Among the topics receiving attention at the present meeting has been the possible role of magnetic elds in producing some of the phenomena observed in the atmospheres and envelopes of Be stars. In the case of the Sun, the causal connection between magnetic elds and activity in the solar atmosphere and surface layers is well-established. In this paper, we examine one aspect of the conjecture that hot stars, particularly Be stars, might possess surface magnetic elds whose ultimate source is a hydromagnetic dynamo located deep within the stellar interior. It is widely thought that the magnetism of massive stars is primordial in origin. Here we assume that a magnetic eld can be generated by a dynamo that operates in the vicinity of the interface between the convective and radiative portions of the stellar interior, similar to what is believed to occur in Our primary concern is how the generated elds are transported over the considerable distance between the site of the dynamo and the surface of the star. Resistive eeects alone are incapable of accomplishing this, since simple estimates indicate that for stars more massive than a few M , the diiusion time across the radiative envelope exceeds the main sequence lifetime. We herein brieey explore the alternative possibility that the elds are transported to the surface in bril form under the action of the buoyant force. In the Sun, magnetic ux emerges from the photosphere in the form of brils or ux tubes. The eld is thought to assume this form in or near the dynamo domain at the bottom of the convective envelope. If the formation of ux tubes is a process that is not speciic to the elds contained inside the Sun, then the possibility exists that a dynamo generated eld at the interface between the convective and radiative zones in a hot star, might be conngured in a similar way. In this case, the buoyant force might likewise enable ux from deep in the interior to reach the surface in a time scale that is shorter than the evolutionary time scales.