Retarded Stellar Dynamo in Tidally Deformed M Dwarfs

TL;DR

This study investigates how stellar deformation affects magnetic cycle periods in M dwarfs, revealing an anti-correlation between cycle and rotational periods in tidally distorted stars, supported by numerical simulations.

Contribution

It uncovers a new anti-correlation between stellar cycle and rotational periods in tidally deformed M dwarfs, highlighting the impact of non-spherical geometry on stellar dynamo behavior.

Findings

Anti-correlation between $P_{cyc}$ and $P_{rot}$ in tidally distorted M dwarfs.

Weakened thermal convection in highly distorted stars.

Reduced differential rotation in deformed stars.

Abstract

Current studies of stellar dynamos primarily focus on spherical stars, leaving their behavior in distorted stars largely unexplored. We utilize stars of varying distortions to examine the relation between stellar cycle periods ($P_{\rm cyc}$) and rotational periods ($P_{\rm rot}$), which are closely linked to dynamo processes. By analyzing a sample of tidally distorted M dwarfs in cataclysmic variables, we identify an anti-correlation between $P_{\rm cyc}$ and $P_{\rm rot}$, in contrast to the lack of such a relation in single M dwarfs. This means that stars with greater deformation have longer cycle periods, suggesting variations in dynamo behavior under non-spherical geometries. Our numerical simulations further reveal that, the thermal convection weakens in highly distorted stars, and subsequently, the differential rotation is also reduced. These effects may lengthen the conversion timescale between poloidal and toroidal magnetic fields, potentially explaining the newly discovered $P_{\rm cyc}$-$P_{\rm rot}$ relation in cataclysmic variables.