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How magnetism of solar-type stars evolves ?

Quentin Noraz; Allan Sacha Brun; Antoine Strugarek

The solar magnetic field is generated and sustained through an internal dynamo. In stars, this process is determined by the combined action of turbulent convective motions and the differential rotation profile. It can sometimes lead to magnetic cyclic variabilities, like in the Sun with the 11 years cycle. Traces of magnetic cycles have been detected for other solar-like stars as well, ranging from a few years to a few tens of years. How are these cycles controlled?
During their life, the rotation of stars is subject to complex evolution. Recent 3D numerical simulations of solar-like stars show that different regimes of differential rotation can be characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby number (slow rotators). If this regime occurs during the stellar spin-down of the main sequence, and in general for slow rotators, we may wonder how the magnetic generation through dynamo process will be impacted. In particular, can slowly rotating stars have magnetic cycles?

We present a numerical multi-D study with the STELEM and ASH codes to understand the magnetic field generation of solar-like stars under various differential rotation regimes, and focus on the existence of magnetic cycles.

We find in self-consistent 3D simulations that short cycles are favoured for small Rossby numbers (fast rotators), and long cycles for intermediate (solar-like) Rossby numbers. Slow rotators (high Rossby numbers) are found to produce only steady dynamo with no cyclic activity. However we find that specific mean-field models can produce magnetic cycles with anti-solar differential rotation only if the alpha effect is fine tuned for this purpose. It is still unclear today whether this latter regime can be achieved self-consistently in global 3D simulations.

We then conclude that slow rotating stars in the anti-solar differential rotation regime can sustain magnetic cycles only for very specific dynamo processes. A detection of magnetic cycles for such stars would therefore be a tremendous constrain on deciphering what type of dynamo is actually acting in solar-like stars, and thus on how their magnetism can evolve. This problematic is particularly relevant in the context of the PLATO mission, which will provide new constraints, in particular on the differential rotation and the magnetic activity taking place in these stars.

A presentation linked to a pre-recorded talk for the Plato Mission Conference 2021
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