Origin of the stable magnetic fields of A-stars, white dwarfs and magnetars

Henk Spruit (MPA)

Some stars of spectral type A (the `Ap stars') are observed to have a strong (300 to 30000 Gauss) static and large-scale magnetic field, of a chiefly dipolar shape. In one explanation this field is being continually regenerated by a dynamo process in the star's convective core. In another, the field is a remnant of the star's formation, a `fossil field'. These hypotheses have been competing since the discovery of magnetic stars half a century ago. The dynamo hypothesis has difficulty explaining the high strength of the fields observed. The main weakness of the fossil-field theory has been the lack of known field configurations which are stable enough to survive in a star for the whole of its lifetime. We demonstrate here the existence and formation of such stable magnetic fields, thus providing a solid basis for the fossil-field hypothesis. The results also apply to magnetic white dwarfs and a group of highly magnetised neutron stars known as magnetars.