Polar caps on active stars: magnetic flux emergence and transport

In recent years, Zeeman Doppler imaging of rapidly rotating solar-like stars has shown high-latitude magnetic flux patterns of intermingled magnetic polarities. Such high-latitude intermingling of positive and negative flux is inconsistent with our present understanding of how magnetic flux emerges on the Sun and is transported poleward. To determine how these patterns may arise, magnetic flux transport simulations are carried out. These simulations follow the evolution of the radial magnetic field at the surface of the star as new magnetic bipoles emerge and are advected poleward by the surface effects of differential rotation, meridional flow and supergranular diffusion. To produce intermingling of flux at high latitudes, key parameters such as the emergence profiles and transport coefficients are varied from presently used solar values. In doing so, it is found that, in order to explain the high-latitude intermingling, at least two of these parameters must be changed. First, the emergence profile must be extended to higher latitudes (lambda=50degrees-70degrees), and secondly the value of the meridional flow must be increased by around a factor of 10 (similar to100 m s(-1)). The results show that the observed intermingling of high-latitude flux can only occur through a flux emergence and transport process that is significantly different from that which occurs on the Sun. Observable features produced by both of these changes are considered, and the significance of the simulations to future observing programmes discussed. Finally the emergence profile and transport coefficients that best fit the observations of the young active star AB Dor (period=0.514 d) are put forward.