The Evolving Magnetic Topology of $\tau$ Bo\"otis

TL;DR

This study presents multi-year spectropolarimetric observations of $ au$ Bo"otis, revealing its cyclic magnetic field reversals, solar-like evolution, and a potential rapid magnetic activity cycle, providing insights into stellar magnetic dynamics.

Contribution

It extends previous magnetic field data of $ au$ Bo"otis with new observations, confirming magnetic reversals and linking magnetic and chromospheric activity cycles.

Findings

Magnetic field of $ au$ Bo"otis$ reverses cyclically.

Magnetic evolution resembles solar-type behavior.

Chromospheric activity cycle correlates with magnetic cycle.

Abstract

We present six epochs of spectropolarimetric observations of the hot-Jupiter-hosting star $\tau$ Bo\"otis that extend the exceptional previous multi-year data set of its large-scale magnetic field. Our results confirm that the large-scale magnetic field of $\tau$ Bo\"otis varies cyclicly, with the observation of two further magnetic reversals; between December 2013 and May 2014 and between January and March 2015. We also show that the field evolves in a broadly solar-type manner in contrast to other F-type stars. We further present new results which indicate that the chromospheric activity cycle and the magnetic activity cycles are related, which would indicate a very rapid magnetic cycle. As an exemplar of long-term magnetic field evolution, $\tau$ Bo\"otis and this longterm monitoring campaign presents a unique opportunity for studying stellar magnetic cycles.