Mechanical heating of protostellar discs

The magnetic field structure of a protostar truncates the accretion disc at some magnetospheric radius greater than the stellar radius. If the stellar rotation axis is not aligned with the disc rotation then the magnetic stresses at this radius transfer misaligned angular momentum and so lead to a tilt in the inner disc. If the configuration and/or strength of the stellar field changes with time then the degree of induced tilt also changes. Under appropriate circumstances such m = 1 tilting motions of the disc can propagate outward through the disc as warp waves. These waves must damp as they approach the corotation radius where their frequency matches the local Keplerian frequency. If the waves propagate sufficiently far, then the disc heating that results from the wave damping can provide a dominant contribution to a local disc heating, and so cause a disc flaring. This flaring can in turn cause changes in the disc spectral energy distribution.