Cycle period, differential rotation, and meridional flow for early M dwarf stars^⋆

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
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² Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary

Received: 6 April 2018
Accepted: 2 December 2018

Abstract

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 (P_rot <  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 u^m 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.