Talk

Universal dynamo scaling in young and evolved stars

Jyri Lehtinen, Max Planck Institute for Solar System Research

We present an updated rotation-activity relation of 155 late-type stars from the Mount Wilson Calcium H&K survey. This survey contains stars with a wide range of activity levels and evolutionary stages from young main sequence stars to red giants. Evident from these data is how the giant stars show similar activity levels to the main sequence stars despite having 10 to 100 times longer rotation periods. We show that this apparent dichotomy is resolved when the stellar activity is scaled by the Rossby number. Since the internal structure of the main sequence and post-main sequences stars differ markedly, we used stellar structure model fits to derive the convective turnover times necessary for calculating Rossby numbers for the studied stars. The Rossby number scaling turns out to be the only viable way to unify the rotation-activity relation of both main sequence and evolved stars. Other proposed scaling relations, that do not include direct a dependence on the characteristics of turbulence, results in clearly distinct branches for these two stellar populations and fail to explain their differences. This suggests that there exists a universal activity scaling, and hence a single underlying dynamo process, for all late-type stars across different evolutionary phases, but that this process has to include turbulence as one of its main ingredients along with stellar rotation.