Simulations of fully convective M dwarfs: dynamo action with varying magnetic Prandtl numbers

TL;DR

This study uses 3D MHD simulations to investigate how varying magnetic Prandtl numbers influence dynamo behavior and magnetic field cycles in fully convective M dwarf stars with a 43-day rotation period.

Contribution

It demonstrates the sensitivity of dynamo solutions to magnetic Prandtl numbers and identifies the transition from cyclic to irregular magnetic behavior at higher Prandtl values.

Findings

Periodic magnetic cycles occur at Pr_M ≤ 2.

Cycles disappear and irregular solutions emerge at higher Pr_M.

Dynamo behavior in fully convective stars resembles that in partially convective stars.

Abstract

M dwarfs are low-mass main-sequence stars, the most numerous type of stars in the solar neighbourhood, which are known to have significant magnetic activity. The aim of this work is to explore the dynamo solutions and magnetic fields of fully convective M dwarfs with varying magnetic Prandtl numbers ${\rm Pr_M}$, and a rotation period (${\rm P_{rot}}$) of 43 days. ${\rm Pr_M}$ is known to play an important role in the dynamo action; dynamos for low-$\rm Pr_M$ and large-$\rm Pr_M$ have very different properties. We performed three-dimensional magnetohydrodynamical (MHD) numerical simulations with the ``star-in-a-box'' model using stellar parameters for an M5 dwarf with $0.21 M_{\odot}$. We found that the dynamo solutions are sensitive to ${\rm Pr_M}$. The simulations at this rotation period present periodic cycles of the large-scale magnetic field up to ${\rm Pr_M} \leq 2$; for higher values the cycles disappear and irregular solutions to arise. Our results are consistent with previous studies and suggest that the dynamos operating in fully convective stars behave similarly as those in partially convective stars.