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 multi-year 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 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, τ Boötis and this longterm monitoring campaign presents a unique opportunity for studying stellar magnetic cycles.

Temporal variability of the wind from the star τ Boötis

“…It is the only star to date, other than our Sun, observed to have a magnetic field cycle, with multiple field reversals observed (Catala et al 2007;Donati et al 2008;Fares et al 2009;Fares et al 2013;Mengel et al 2016). These observations reveal that the star has a magnetic cyclic period estimated to be a rapid 740 days .…”

“…The polarization information in the spectra observed at different stellar rotation phases are used to reconstruct the surface magnetic field topologies using Zeeman Doppler Imaging (ZDI, Donati & Landstreet 2009). The reconstruction of the magnetic field maps used in this work is presented in Mengel et al (2016). The radial field topologies are of sole interest for the stellar wind modelling as the other, non-potential field components have been shown to have a negligible effect on the wind solution .…”

“…The radial field topologies are of sole interest for the stellar wind modelling as the other, non-potential field components have been shown to have a negligible effect on the wind solution . Fares et al (2013); however, for consistency the maps presented here are an updated version using the same methodology as used with the other five epochs, the details of which are presented in Mengel et al (2016). Due to the inclination of the star, the area below −40…”

Stellar magnetic activity and exoplanets

“…The Sun, for instance, varies in magnetic field intensity and X-ray emission along its cycle (see left-most and bottom panels of Figure 1, showing two points for the Sun, one during activity minimum and the other during maximum of activity). Other stars present, like our Sun, variations on their surface magnetic fields, some of which are associated to well-defined magnetic cycles [27][28][29][30]. This medium term variability (on the order of years) acts as to increase the spread in rotation-magnetic-activity relations.…”

“…The famous planet-host star τ Boo, has been monitored with ZDI for about a decade [27,28,30,50,52]. This star shows flips in magnetic field polarity every year, making it the star with the shortest full (2 years) magnetic cycle mapped with ZDI.…”