Cycle period, differential rotation, and meridional flow for early M dwarf stars⋆
Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
2 Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary
Accepted: 2 December 2018
Recent observations suggest the existence of two characteristic cycle times for early-type M stars dependent on the rotation period. They are of order one year for fast rotators (Prot < 1 day) and of order four years for slower rotators. Additionally, the equator-to-pole differences of the rotation rates with δΩ up to 0.03 rad d−1 are known from Kepler data for the fast-rotating stars. These values are well-reproduced by the theory of large-scale flows in rotating convection zones on the basis of the Λ effect. The resulting amplitudes um of the bottom value of the meridional circulation allows for the calculation of the travel time from pole to equator at the base of the convection zone of early-type M stars. These travel times strongly increase with rotation period and they always exceed the observed cycle periods. Therefore, the operation of an advection-dominated dynamo in early M dwarfs, where the travel time must always be shorter than the cycle period, is not confirmed by our model nor the data.
Key words: stars: late-type / stars: magnetic field / stars: activity / magnetohydrodynamics (MHD) / turbulence
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