Magnetic braking in convective stars

Abstract

Magnetic braking causes the spin-down of single stars as they evolve on the main sequence. Models of magnetic braking can also explain the evolution of close binary systems, including cataclysmic variables. The well-known period gap in the orbital period distribution of cataclysmic variable systems indicates that magnetic braking must be significantly disrupted in secondaries that are fully convective. However, activity studies show that fully convective stars are some of the most active stars observed in young open clusters. There is therefore conflicting evidence about what happens to magnetic activity in fully convective stars. Results from spectro-polarimetric studies of cool stars have found that the field morphologies and field strengths are dependent on spectral type and rotation rate. While rapidly rotating stars with radiative cores show strong, complex magnetic fields, they have relatively weak dipole components. Fully convective stars that are rapidly rotating also possess strong magnetic fields, but their configurations are much simpler; often close to dipole fields. How this change in field geometry affects the stellar wind is the focus of several ongoing modelling efforts. Initial results suggest that rapidly rotating active dwarfs drive much stronger winds, about two orders of magnitude larger than those on the Sun.