Dynamo generated toroidal magnetic fields in rapidly rotating stars

¹ School of Mathematics, University of Manchester, Oxford Road, Manchester, M13 9PL, UK e-mail: moss@ma.man.ac.uk
² Department of Physics, Moscow University, 119992 Moscow, Russia e-mail: sokoloff@dds.srcc.msu.su

Received: 26 November 2008
Accepted: 16 February 2009

Abstract

Aims. Results presented in the recent observational paper by Petit et al. (2008, MNRAS, 388, 80) suggest that, for solar-like stars, the large-scale surface toroidal fields become strong for rotational periods less than about 12 d. We discuss this observation in the context of 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 rotation law: (a) a bifurcation resulting in a sudden increase of the near-surface toroidal field, with all other details of the model being kept unaltered; (b) a new type of surface boundary condition at the stellar surface, forced by internal reorganization of magnetic field with increasing 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 of parameters, although we cannot conclusively eliminate a possible role for the latter mechanism. We conclude that any such bifurcation most plausibly is associated with some reorganization in the stellar hydrodynamics as the stellar rotation rate increases. The simplest suggestion, i.e. a transition from a solar-like rotation law (with spoke-like angular velocity contours through the bulk of the convection zone) to a law with quasi-cylindrical isorotation contours as thought to be appropriate to rapidly rotating stars, seems to reproduce the observed phenomenology reasonably well.

Conclusions. While we appreciate the manifold uncertainties in the available theory and observations, we thus suggest that the bifurcation deduced by Petit et al. (2008) is an observational manifestation of the transition between solar-like and quasi-cylindrical rotation laws, occuring near a rotational period of 12 d.