To appear in Astron. Nachr. 334, issue 1-2 (2013)
Proceedings of the 17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, 2012, Barcelona, Spain.

Sven Wedemeyer (Institute of Theoretical Astrophysics, Postboks 1029 Blindern, 0315 Oslo, Norway / Center of Mathematics for Applications University of Oslo, Postboks 1053 Blindern, N-0316 Oslo, Norway),
Hans-Günter Ludwig (ZAH, Landessternwarte Königstuhl, D-69117 Heidelberg, Germany),
Oskar Steiner (Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, D-79104 Freiburg, Germany)

Three-dimensional magnetohydrodynamic simulations of M-dwarf chromospheres

We present first results from three-dimensional radiation magnetohydrodynamic simulations of M-type dwarf stars with CO5BOLD. The local models include the top of the convection zone, the photosphere, and the chromosphere. The results are illustrated for models with an effective temperature of 3240 K and a gravitational acceleration of log g = 4.5, which represent analogues of AD Leo. The models have different initial magnetic field strengths and field topologies. This first generation of models demonstrates that the atmospheres of M-dwarfs are highly dynamic and intermittent. Magnetic fields and propagating shock waves produce a complicated fine-structure, which is clearly visible in synthetic intensity maps in the core of the Ca II K spectral line and also at millimeter wavelengths. The dynamic small-scale pattern cannot be described by means of one-dimensional models, which has important implications for the construction of semi-empirical model atmospheres and thus for the interpretation of observations in general. Detailed three-dimensional numerical simulations are valuable in this respect. Furthermore, such models facilitate the analysis of small-scale processes, which cannot be observed on stars but nevertheless might be essential for understanding M-dwarf atmospheres and their activity. An example are so-called "magnetic tornadoes", which have recently been found on the Sun and are presented here in M-dwarf models for the first time.

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