A Knee-Point in the Rotation-Activity Scaling of Late-type Stars with a Connection to Dynamo Transitions. (arXiv:2007.00040v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Lehtinen_J/0/1/0/all/0/1">Jyri J. Lehtinen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kapyla_M/0/1/0/all/0/1">Maarit J. Käpylä</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Olspert_N/0/1/0/all/0/1">Nigul Olspert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Spada_F/0/1/0/all/0/1">Federico Spada</a>
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.
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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