What controls the magnetic geometry of M dwarfs?
Abstract
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- January 2013
- DOI:
- 10.1051/0004-6361/201220317
- arXiv:
- arXiv:1212.0136
- Bibcode:
- 2013A&A...549L...5G
- Keywords:
-
- dynamo;
- magnetohydrodynamics (MHD);
- stars: magnetic field;
- stars: rotation;
- stars: low-mass;
- brown dwarfs;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 4 pages, 4 figures, accepted for publication in A&