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D.A. Uzdensky, C. Litwin, A. Konigl (U. Chicago)
A large-scale stellar magnetic field that threads a surrounding accretion disk can mediate a torque that regulates the stellar rotation rate, can channel some of the inflowing matter to polar regions, and may play a role in the transfer of angular momentum from the disk to the surrounding interstellar medium, possibly in the form of a centrifugally driven wind. These processes have originally been studied in the context of X-ray pulsars but have more recently been recognized as possibly playing a central role also in young stellar objects. Motivated by these applications and guided by recent numerical simulations and by related work in solar flare research, we have begun to develop analytic and semianalytic tools for analyzing the complex phenomena involved in the magnetic star--disk interaction. In this contribution we report on preliminary results on the time evolution of the magnetic field structure brought about by the relative rotation between the disk and the star. Although the magnetic field has a negligible dynamical effect on the disk over a rotation period, its structure in the magnetosphere outside the disk can change drastically on this time scale. Specifically, as the disk rotates, the field in the magnetosphere evolves through a sequence of sheared force-free equilibria. To determine this sequence, we first model the situation where the field lines are frozen into both the star and the disk and where the medium outside the disk is perfectly conducting. Subsequently, however, we relax these assumptions and consider the effects of finite resistivity both inside and outside the disk. This research is supported in part by NASA grants NAG 5-3687 and NAG 5-1485.
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