The evolving magnetic topology of τ Boötis

Abstract

We present six epochs of spectropolarimetric observations of the hot-Jupiter-hosting star τ Boötis that extend the exceptional previous multiyear data set of its large-scale magnetic field. Our results confirm that the large-scale magnetic field of τ Boötis varies cyclicly, with the observation of two further magnetic reversals; between 2013 December and 2014 May and between 2015 January and March. 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, τ Boötis and this long-term monitoring campaign presents a unique opportunity for studying stellar magnetic cycles.