The response of an accretion disc to an inclined dipole with application to AA Tauri

We compute the warping of a disc induced by an inclined dipole. We consider a magnetised star surrounded by a thin Keplerian diamagnetic disc with an inner edge that corotates with the star. We suppose the stellar field is a dipole with an axis that is slightly misaligned with the stellar rotation a xis. The rotation axes of the disc material orbiting at large distances fro m the star and that of the star are supposed to coincide. The misalignment of the magnetic and rotation axes results in the magnetic pressure not bein g the same on the upper and lower surfaces of the disc. The resultant net v ertical force produces a warp which appears stationary in a frame corotatin g with the star. We find that, if viscosity is large enough (alpha similar to 0.01-0.1) to damp bending waves as they propagate away, a smoothly varyi ng warp of the inner region of the disc is produced. The amplitude of the w arp can easily be on the order of ten percent of the disc inner radius for reasonably small misalignment angles (less than 30 degrees). Viscous dampin g also introduces a phase shift between the warp and the forcing torque, wh ich results in the locations of maximum elevation above the disc forming a trailing spiral pattern. We apply these results to recent: observations of AA Tau, and show that the variability of its light curve, which occurs with a period comparable to the expected stellar rotation period, could be due to obscuration produced by a warp configuration of the type we obtain.