Common dynamo scaling in late-type main sequence and evolved stars

Both the magnetic activity and magnetic field strengths of late-type stars are observed to scale up with stellar rotation up to a certain saturation level. We show strong evidence from chromospheric Ca II H&K line emission observations that the same rotation-activity relation is shared by both main sequence stars and evolved giants. This indicates that the dynamos of both young and evolved stars follow the same underlying principles, irrespective of the stellar internal structure. The common scaling relation can only be found in relation to the Rossby number, i.e. the ratio of rotation period to the convective turnover time, meaning that this shared dynamo must be governed by turbulent convection. A more detailed analysis of the shape of the rotation-activity relation reveals a localised break in its slope at mid-activity levels. This knee point lies close to the threshold where large scale non-axisymmetric activity appears on fast rotating stars. It may thus be connected to a transition between axisymmetric and non-axisymmetric dynamo modes in slow and fast rotating stars.