RELATING STELLAR CYCLE PERIODS TO DYNAMO CALCULATIONS

Stellar magnetic activity in slowly rotating stars is often cyclic, wi th the period of the magnetic cycle depending critically on the rotati on rate and the convective turnover time of the star. Here we show tha t the interpretation of this law from dynamo models is not a simple ta sk. It is demonstrated that the period is (unsurprisingly) sensitive t o the precise type of non-linearity employed. Moreover the calculation of the wave-speed of plane-wave solutions does not (as was previously supposed) give an indication of the magnetic period in a more realist ic dynamo model, as the changes in length-scale of solutions are not e asily captured by this approach. Progress can be made, however, by con sidering a realistic two-dimensional model, in which the radial length -scale of waves is included. We show that it is possible in this case to derive a more robust relation between cycle period and dynamo numbe r. For all the non-linearities considered in the most realistic model, the magnetic cycle period is a decreasing function of \D\ (the amplit ude of the dynamo number). However, discriminating between different n onlinearities is difficult in this case and care must therefore be tak en before advancing explanations for the magnetic periods of stars.