Pre-main sequence dynamos and relic magnetic fields of solar-type stars

Abstract

We argue that contemporary magnetic fields in the radiative cores of solar-type stars are relics of hydromagnetic dynamos operating over the pre-main sequence epoch when the core formed. Simulations of differential rotation and dynamos of a 1 M⊙ star are performed for a sequence of evolutionary stages of the core-growth epoch. A big difference in the dynamo-generated field between the initial and final stages of the core formation is predicted. An axisymmetric and cyclic field similar to the present day Sun is excited in a star with a mature radiative zone. An infant Sun with its core just starting to grow generates a nonaxisymmetric global field which is steady in a rotating frame. A qualitative explanation for the nonaxisymmetry is suggested. A model for the field capturing by the growing core is designed. The internal field is most efficiently captured over the initial phase when the core grows most rapidly. A nonaxisymmetric global relic field with an amplitude somewhat below 1 G is predicted for a main-sequence star of solar mass. An interpretation of the active longitude phenomenon as observed on the Sun and stars in terms of the nonaxisymmetric relic field is suggested.