Simulations of dynamo action in slowly rotating M dwarfs: Dependence on dimensionless parameters

Aims. The aim of this study is to explore the magnetic and flow properties of fully convective M dwarfs as a function of rotation period P-rot and magnetic Reynolds Re-M and Prandlt numbers Pr-M.Methods. We performed three-dimensional simulations of fully convective stars using a star-in-a-box set-up. This set-up allows global dynamo simulations in a sphere embedded in a Cartesian cube. The equations of non-ideal magnetohydrodynamics were solved with the PENCIL CODE. We used the stellar parameters of an M5 dwarf with 0.21 M-circle dot at three rotation rates corresponding to rotation periods (P-rot) of 43, 61, and 90 days, and varied the magnetic Prandtl number in the range from 0.1 to 10.
Results. We found systematic differences in the behaviour of the large-scale magnetic field as functions of rotation and Pr-M. For the simulations with P-rot = 43 days and Pr-M <= 2, we found cyclic large-scale magnetic fields. For Pr-M > 2, the cycles vanish and the field shows irregular reversals. In the simulations with P-rot = 61 days for Pr-M <= 2, the cycles are less clear and the reversal are less periodic. In the higher Pr-M cases, the axisymmetric mean field shows irregular variations. For the slowest rotation case with P-rot = 90 days, the field has an important dipolar component for Pr-M <= 5. For the highest Pr-M the large-scale magnetic field is predominantly irregular at mid-latitudes, with quasi-stationary fields near the poles. For the simulations with cycles, the cycle period length slightly increases with increasing Re-M.