The generation and stability of magnetic fields in CP stars

A variety of magnetohydrodynamic mechanisms that may play a role in magnetic, chemically peculiar (mCP) stars is reviewed. These involve dynamo mechanisms in laminar flows as well as turbulent environments, and magnetic instabilities of poloidal and toroidal fields as well as combinations of the two. While the proto-stellar phase makes the survival of primordial fields difficult, the variety of magnetic field configurations on mCP stars may be an indication for that they are instability remnants, but there is no process which is clearly superior in explaining the strong fields. 1. What do we have to explain? A considerable fraction of chemically peculiar A and B stars (CP stars) show strong surface magnetic fields of about 500 G up to over 10 kG. The evolution of these fields from star formation to their current presence is unknown. This paper compiles a number of physical processes which may or may not play a role in the whole scenario of CP star magnetism. The fraction of main-sequence stars with radiative envelopes which show magnetic fields was found to increase from F stars to late B stars (Wolff 1968). The result was recently confirmed by Power et al. (2007) who found an increase of mCP star fractions with stellar mass up to 3.6M⊙ for nearby stars. A further increase towards higher masses was hidden in the small number of higher-mass stars. While the average fraction of mCP stars among normal stars of the same mass range is generally quoted as being roughly 10%, the fraction is less than 2% in the solar neighbourhood of 100 pc radius (Power et al. 2007). Why only a fraction of intermediate-mass stars shows strong magnetic fields is one of the key issues in the search for their origin. The other is that mCP stars form a distinct sub-group with slow rotation among A and B stars. There are indications of a decrease of magnetic field strength with stellar age for M > 3M⊙ (Kochukhov & Bagnulo 2006; Landstreet et al. 2007). Although the increase of stellar radius may cause the reduction at constant magnetic flux, it appears that even the flux is decreasing during the main-sequence life of mCP stars. At first glance this decay may be attributed to ohmic decay in the radiative envelope of the star. The radial profile of the magnetic diffusivity η(r)