Accretion discs around magnetic stars

Accretion discs are commonplace around magnetic stars. They can be found in newly formed stars and in interacting binaries. The effects of the stellar magnetic field on these discs are manifold. For example it may act as a 'seed field' for the initiation of the magneto-rotational instability and thus drive viscosity. However this thesis explores the more direct effect of the stellar magnetic field on the global structure of circumstellar accretion discs, and how this structure may in turn influence the time dependent behaviour of the disc. This work is done through a combination of analytic and numerical techniques, by modelling a torque which represents the effect of the magnetic field.;Some young stellar objects exhibit disc outbursts, known as FU Orionis events, which are separated by recurrence times much longer than would be expected from the standard disc instability model. However, it is shown here that a moderate stellar magnetic field may truncate the accretion disc, and so stabilise the inner accretion disc against outbursts. This forces outbursts to begin in the outer disc where higher trigger densities must be reached, and so extends the recurrence time. This result is of great interest since planet formation may be strongly influenced by the structure of the protoplanetary disc.;The dwarf nova WZ Sagittae is unusual in that it also has very long recurrence times. In addition, a series of short echos have been observed to follow the main outburst. It has therefore been suggested that this object is surrounded by an accretion disc which is magnetically truncated in a similar manner to those around the FU Orionis stars. It is shown that such a truncated disc may, when placed within the binary Roche potential, exhibit echo outbursts.