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

¹ Departamento de Astronomía, Facultad de Ciencias Físicas y Matemáticas, Universidad de Concepción, Av. Esteban Iturra s/n Barrio Universitario, Casilla 160-C, Chile
e-mail: ortizrodcarolina@gmail.com
² Leibniz-Institut für Sonnenphysik (KIS), Schöneckstr. 6, 79104 Freiburg, Germany
³ Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Nordita, KTH Royal Institute of Technology and Stockholm University, 10691 Stockholm, Sweden
⁵ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 2 August 2022
Accepted: 2 August 2023

Abstract

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_⊙ 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.