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arXiv · 2007.00040

A Knee-Point in the Rotation-Activity Scaling of Late-type Stars with a Connection to Dynamo Transitions

Jyri J. Lehtinen·Maarit J. Käpylä·Nigul Olspert·Federico Spada
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Abstract

The magnetic activity of late-type stars is correlated with their rotation rates. Up to a certain limit, stars with smaller Rossby numbers, defined as the rotation period divided by the convective turnover time, have higher activity. A more detailed look at this rotation-activity relation reveals that, rather than being a simple power law relation, the activity scaling has a shallower slope for the low-Rossby stars than for the high-Rossby ones. We find that, for the chromospheric CaII H&K activity, this scaling relation is well modelled by a broken two-piece power law. Furthermore, the knee-point of the relation coincides with the axisymmetry to non-axisymmetry transition seen in both the spot activity and surface magnetic field configuration of active stars. We interpret this knee-point as a dynamo transition between dominating axi- and non-axisymmetric dynamo regimes with a different dependence on rotation and discuss this hypothesis in the light of current numerical dynamo models.

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Jyri J. Lehtinen, Maarit J. Käpylä, Nigul Olspert, Federico Spada. 2020-06-30. A Knee-Point in the Rotation-Activity Scaling of Late-type Stars with a Connection to Dynamo Transitions. https://doi.org/10.3847/1538-4357%2Fabe621

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