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Abstract

One interpretation of the activity and magnetism of late-type stars is that these both intensify with decreasing Rossby number up to a saturation level1,2,3, suggesting that stellar dynamos depend on both rotation and convective turbulence4. Some studies have claimed, however, that rotation alone suffices to parametrize this scaling adequately5,6. Here, we tackle the question of the relevance of turbulence to stellar dynamos by including evolved, post-main-sequence stars in the analysis of the rotation–activity relation. These stars rotate very slowly compared with main-sequence stars, but exhibit similar activity levels7. We show that the two evolutionary stages fall together in the rotation–activity diagram and form a single sequence in the unsaturated regime in relation only to Rossby numbers derived from stellar models, confirming earlier preliminary results that relied on a more simplistic parametrization of the convective turn-over time8,9. This mirrors recent results of fully convective M dwarfs, which likewise fall on the same rotation–activity sequence as partially convective solar-type stars10,11. Our results demonstrate that turbulence plays a crucial role in driving stellar dynamos and suggest that there is a common turbulence-related dynamo mechanism explaining the magnetic activity of all late-type stars.