A confined dynamo: magnetic activity of the K-dwarf component in the pre-cataclysmic binary system V471 Tauri

TL;DR

This study investigates the magnetic activity of the red dwarf in the V471 Tauri binary system, revealing a permanent active longitude, weak differential rotation, and the influence of the white dwarf companion on the star's magnetic phenomena.

Contribution

It provides detailed multi-wavelength observations and analysis of the red dwarf's magnetic activity, highlighting the effects of tidal forces and revealing an activity cycle in a pre-cataclysmic binary system.

Findings

Presence of a permanent active longitude facing the white dwarf

Weak differential rotation with a shear coefficient of 0.0026

Evidence of a 12.7-year activity cycle

Abstract

We scrutinize the red dwarf component in the eclipsing binary system V471 Tau in order to unravel relations between different activity layers from the stellar surface through the chromosphere up to the corona. We aim at studying how the magnetic dynamo in the late-type component is affected by the close white dwarf companion. We use space photometry, high resolution spectroscopy and X-ray observations from different space instruments to explore the main characteristics of magnetic activity. From K2 photomery we find that 5-10 per cent of the apparent surface of the red dwarf is covered by cool starspots. From seasonal photometric period changes we estimate a weak differential rotation. From the flare activity we derive a cumulative flare frequency diagram which suggests that frequent flaring could have a significant role in heating the corona. Using high resolution spectroscopy we reconstruct four Doppler images for different epochs which reveal an active longitude, that is, a permanent dominant spot facing the white dwarf. From short term changes in the consecutive Doppler images we derive a weak solar-type surface differential rotation with 0.0026 shear coefficient, similar to that provided by photometry. The long-term evolution of X-ray luminosity reveals a possible activity cycle length of 12.7 ys, traces of which were discovered also in the H$\alpha$ spectra. We conclude that the magnetic activity of the red dwarf component in V471 Tau is strongly influenced by the close white dwarf companion. We confirm the presence of a permanent dominant spot (active longitude) on the red dwarf facing the white dwarf. The weak differential rotation of the red dwarf is very likely the result of tidal confinement by the companion. We find that the periodic appearance of the inter-binary H$\alpha$ emission from the vicinity of the inner Lagrangian point is correlated with the activity cycle.