What controls the magnetic geometry of M dwarfs?

被引:71
作者
Gastine, T. [1 ]
Morin, J. [2 ]
Duarte, L. [1 ]
Reiners, A. [2 ]
Christensen, U. R. [1 ]
Wicht, J. [1 ]
机构
[1] Max Planck Inst Sonnensyst Forsch, D-37191 Katlenburg Lindau, Germany
[2] Univ Gottingen, Inst Astrophys, D-37077 Gottingen, Germany
关键词
dynamo; magnetohydrodynamics (MHD); stars: magnetic field; stars: rotation; stars: low-mass; brown dwarfs; ROTATING SPHERICAL-SHELLS; CONVECTION-DRIVEN DYNAMOS; LOW-MASS STARS; ACTIVE STARS; FIELD; TOPOLOGIES; PLANETS; SIMULATIONS; EQUATIONS; DIPOLAR;
D O I
10.1051/0004-6361/201220317
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
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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页数:4
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