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One interpretation of the activity and magnetism of late-type stars is that these both intensify with decreasing Rossby number up to a saturation level(1-3), suggesting that stellar dynamos depend on both rotation and convective turbulence(4). Some studies have claimed, however, that rotation alone suffices to parametrize this scaling adequately(5,6). 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 levels(7). 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 time(8,9). This mirrors recent results of fully convective M dwarfs, which likewise fall on the same rotation-activity sequence as partially convective solar-type stars(10,11). 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.
Uniform analysis of both main-sequence and evolved, post-main-sequence stars shows that a common, turbulence-dependent, dynamo mechanism operates throughout these stages of stellar evolution.
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- 1 Active
01/01/2020 → 30/04/2024
Project: EU: ERC grants