Evolution of the magnetic fields of main-sequence CP-stars. I.

Abstract

A study of the average surface magnetic fields Bs shows that the damping time for the magnetic field of chemically peculiar stars is τ ~ 108 −109 years, which is consistent with the assumption of ohmic dissipation. Besides ohmic losses, there are evidently no other causes of the magnetic field decay, such as differential rotation, meridional circulation, turbulence, etc. An earlier idea that the average surface magnetic field of CP-stars varies as the reciprocal of the cube of the increasing radius of the evolving star, as it should for a dipole magnetic field configuration, is confirmed. There are signs, however, of a simultaneous gradual increase in the magnetic field beginning with the ZAMS and extending up to the time a star leaves the main sequence. Most likely there is a gradual simplification and ordering of the structure of the surface magnetic field because of the instability of fine structures. CP-stars appear to be divided into two groups, with normal and extreme values of Bs. Most are in the normal group, with the extreme group forming a small part of the whole. They may have developed from the most highly magnetized protostellar clouds.