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

TL;DR

This paper identifies a knee-point in the rotation-activity relation of late-type stars, linking it to a transition in magnetic dynamo regimes from axisymmetric to non-axisymmetric configurations, supported by observational and numerical evidence.

Contribution

It reveals a broken power law in the rotation-activity relation and connects the knee-point to a dynamo transition in stellar magnetic behavior.

Findings

The rotation-activity relation follows a broken power law.

The knee-point coincides with a magnetic dynamo transition.

Surface magnetic field configurations change at the knee-point.

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.