Journal
ASTRONOMY & ASTROPHYSICS
Volume 549, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201220317
Keywords
dynamo; magnetohydrodynamics (MHD); stars: magnetic field; stars: rotation; stars: low-mass; brown dwarfs
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Funding
- Special Priority Program 1488 (PlanetMag) of the German Science Foundation
- Alexander von Humboldt foundation
- DFG [RE 1664/9-1]
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Context. Observations of rapidly rotating M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. In dynamo models, the relative importance of inertia in the force balance, which is quantified by the local Rossby number, is known to have a strong impact on the magnetic field geometry. Aims. We aim to assess the relevance of the local Rossby number in controlling the large-scale magnetic field geometry of M dwarfs. Methods. We have explored the similarities between anelastic dynamo models in spherical shells and observations of active M-dwarfs, focusing on field geometries derived from spectropolarimetric studies. To do so, we constructed observation-based quantities aimed to reflect the diagnostic parameters employed in numerical models. Results. The transition between dipole-dominated and multipolar large-scale fields in early to mid M dwarfs is tentatively attributed to a Rossby number threshold. We interpret late M dwarfs magnetism to result from a dynamo bistability occurring at low Rossby number. By analogy with numerical models, we expect different amplitudes of differential rotation on the two dynamo branches.
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