Abstract
Observations suggest that stars lose appreciable angular momentum prior to reaching the main sequence. Two principal spin-down mechanisms have been proposed. One is removal of angular momentum by magnetized winds or jets; the other is transfer of angular momentum from the star to its accretion disk through the effects of magnetic fields. In the latter case, spin evolution occurs due to both mass accretion along field lines and torques resulting from coupling of the stellar magnetic field to the disk. In this paper we study the latter torques in the context of a magnetic field varying in time. We find that magnetic variability reduces the efficiency with which the field can wind up, somewhat widening the region of magnetic coupling. Nonetheless, the steady state result—that magnetic torques can be applied only within a thin annulus around the corotation radius—is little changed for what we believe to be realistic physical conditions. These results are generally applicable to disk accretion onto magnetized bodies.
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