Dynamo generated toroidal magnetic fields in rapidly rotating stars

摘要

Aims. Results presented in the recent observational paper by Petit et al. (2008, MNRAS, 388, 80) suggest that, for solar-like stars, thelarge-scale surface toroidal fields become strong for rotational periods less than about 12 d. We discuss this observation in the contextof stellar dynamo theory, as a manifestation of a bifurcation in dynamo regimes occurring around this rotation period.Methods. Working in the context of mean field theory, we first consider two options for such a bifurcation for a given stellar rotationlaw: (a) a bifurcation resulting in a sudden increase of the near-surface toroidal field, with all other details of the model being keptunaltered; (b) a new type of surface boundary condition at the stellar surface, forced by internal reorganization of magnetic field withincreasing rotation rate, which causes a more-or-less abrupt increase in large-scale surface toroidal field.Results. Neither of these options seem to provide anything like the reported behaviour of surface toroidal field for plausible choices ofparameters, although we-cannot conclusively eliminate a possible role for the latter mechanism. We conclude that any such bifurcationmost plausibly is associated with some reorganization in the stellar hydrodynamics as the stellar rotation rate increases. The simplestsuggestion, i.e. a transition from a solar-like rotation law (with spoke-like angular velocity contours through the bulk of the convectionzone) to a law with quasi-cylindrical isorotation contours as thought to be appropriate to rapidly rotating stars, seems to reproduce theobserved phenomenology reasonably well.Conclusions. While we appreciate the manifold uncertainties in the available theory and observations, we thus suggest that thebifurcation deduced by Petit et al. (2008) is an observational manifestation of the transition between solar-like and quasi-cylindricalrotation laws, occuring near a rotational period of 12 d.