Magnetic activity in pre-main-sequence stars

There is a much greater spread of surface activity in pre-main-sequence stars, which are otherwise similar, than in main-sequence stars. It is suggested that some pre-main-sequence stars may contain ‘fossil’ magnetic fields in their outer layers which are stronger than any dynamo field would be. If surface activity is related to the magnetic field, the stars would exhibit stronger activity than those without significant ‘fossil’ fields. Before the star reaches the main sequence, the ‘fossil’ field in the convection zone may be affected by ohmic and turbulent decay and reduce to the dynamo value. If this is the case, it can explain the smaller spread of surface activity in main-sequence stars.