Dynamics of magnetic flux tubes in close binary stars II. Nonlinear evolution and surface distributions

TL;DR

This study models the nonlinear evolution of magnetic flux tubes in close binary stars, showing that even small tidal effects can cause flux emergence to cluster at preferred longitudes, influencing starspot distribution.

Contribution

It introduces a numerical model incorporating tidal forces and stellar deformation to explain non-uniform starspot distributions in close binary systems.

Findings

Tidal effects lead to flux tube clustering at preferred longitudes.

Flux emergence patterns depend on initial flux tube parameters.

Resonant tidal interactions influence surface magnetic activity.

Abstract

Observations of magnetically active close binaries with orbital periods of a few days reveal the existence of starspots at preferred longitudes (with respect to the direction of the companion star). We numerically investigate the non-linear dynamics and evolution of magnetic flux tubes in the convection zoneof a fast-rotating component of a close binary system and explore whether the tidal effects are able to generate non-uniformities in the surface distribution of erupting flux tubes. Assuming a synchronised system with a rotation period of two days and consisting of two solar-type components, both the tidal force and the deviation of the stellar structure from spherical shape are considered in lowest-order perturbation theory. The magnetic field is initially stored in the form of toroidal magnetic flux rings within the stably stratified overshoot region beneath the convection zone. Once the field has grown sufficiently strong, instabilities initiate the formation of rising flux loops, which rise through the convection zone and emerge at the stellar surface. We find that although the magnitude of tidal effects is rather small, they nevertheless lead to the formation of clusters of flux tube eruptions at preferred longitudes on opposite sides of the star, which result from the cumulative and resonant character of the action of tidal effects on rising flux tubes. The longitude distribution of the clusters depends on the initial parameters of flux tubes in the overshoot region like magnetic field strength and latitude, implying that there is no globally unique preferred longitude along a fixed direction.