Volume 549, January 2013
|Number of page(s)||4|
|Published online||21 December 2012|
What controls the magnetic geometry of M dwarfs?
1 Max Planck Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg Lindau, Germany
2 Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund Platz, 37077 Göttingen, Germany
Received: 31 August 2012
Accepted: 1 December 2012
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.
Key words: dynamo / magnetohydrodynamics (MHD) / stars: magnetic field / stars: rotation / stars: low-mass / brown dwarfs
© ESO, 2012
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