Accretion disc–stellar magnetosphere interaction: field line inflation and the effect on the spin-down torque |
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| Authors: | Vasso Agapitou John C B Papaloizou |
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| Institution: | Astronomy Unit, School of Mathematical Sciences, Queen Mary and Westfield College, London E1 4NS |
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| Abstract: | We calculate the structure of a force-free magnetosphere which is assumed to corotate with a central star and which interacts with an embedded differentially rotating accretion disc. The magnetic and rotation axes are aligned, and the stellar field is assumed to be a dipole. We concentrate on the case when the amount of field line twisting through the disc–magnetosphere interaction is large , and consider different outer boundary conditions. In general the field line twisting produces field line inflation (e.g. Bardou & Heyvaerts), and in some cases with large twisting many field lines can become open. We calculate the spin-down torque acting between the star and the disc, and we find that it decreases significantly for cases with large field line twisting. This suggests that the oscillating torques observed for some accreting neutron stars could be caused by the magnetosphere varying between states with low and high field line inflation. Calculations of the spin evolution of T Tauri stars may also have to be revised in the light of the significant effect that field line twisting has on the magnetic torque resulting from star–disc interactions. |
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| Keywords: | accretion accretion discs stars: magnetic fields stars: neutron stars: pre‐main‐sequence stars: rotation |
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