What sets the magnetic field strength and cycle period in solar-type stars?
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Universidade Federal de Minas Gerais
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Artigo de periódico
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Two fundamental properties of stellar magnetic fields have been determined by observations for solar-like stars
with different Rossby numbers (Ro), namely, the magnetic field strength and the magnetic cycle period. The field
strength exhibits two regimes: (1) for fast rotation, it is independent of Ro, and (2) for slow rotation, it decays with
Ro following a power law. For the magnetic cycle period, two regimes of activity, the active and inactive branches,
have also been identified. For both of them, the longer the rotation period, the longer the activity cycle. Using
global dynamo simulations of solar-like stars with Rossby numbers between ∼0.4 and ∼2, this paper explores the
relevance of rotational shear layers in determining these observational properties. Our results, consistent with
nonlinear a W2 dynamos, show that the total magnetic field strength is independent of the rotation period. Yet at
surface levels, the origin of the magnetic field is determined by Ro. While for Ro 1, it is generated in the
convection zone, for Ro 1, strong toroidal fields are generated at the tachocline and rapidly emerge toward the
surface. In agreement with the observations, the magnetic cycle period increases with the rotational period.
However, a bifurcation is observed for ~Ro 1, separating a regime where oscillatory dynamos operate mainly in the convection zone from the regime where the tachocline has a predominant role. In the latter, the cycles are believed to result from the periodic energy exchange between the dynamo and the magneto-shear instabilities developing in the tachocline and the radiative interior.
Abstract
Assunto
Estrelas, Campos magnéticos
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Dynamo, Stars, Magnetic field
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Curso
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https://iopscience.iop.org/article/10.3847/1538-4357/ab224a