The relation between stellar magnetic field geometry and chromospheric activity cycles II: The rapid 120 day magnetic cycle of Tau Bootis

TL;DR

This study monitors Tau Bootis's magnetic field and discovers a rapid 240-day magnetic cycle with complex field geometry, including a unique strong toroidal component, providing insights into stellar magnetic activity cycles.

Contribution

First detection of a 240-day magnetic cycle in Tau Bootis, revealing rapid magnetic polarity switches and complex field structures not seen in the Sun.

Findings

Tau Bootis has a 240-day magnetic cycle.

The star's magnetic field is complex with a weak dipolar and strong toroidal components.

Evidence of overlapping activity cycles resembling a butterfly pattern.

Abstract

One of the aims of the BCool programme is to search for cycles in other stars and to understand how similar they are to the Sun. In this paper we aim to monitor the evolution of $\tau$ Boo's large-scale magnetic field using high-cadence observations covering its chromospheric activity maximum. For the first time, we detect a polarity switch that is in phase with $\tau$ Boo's 120 day chromospheric activity maximum and its inferred X-ray activity cycle maximum. This means that $\tau$ Boo has a very fast magnetic cycle of only 240 days. At activity maximum $\tau$ Boo's large-scale field geometry is very similar to the Sun at activity maximum: it is complex and there is a weak dipolar component. In contrast, we also see the emergence of a strong toroidal component which has not been observed on the Sun, and a potentially overlapping butterfly pattern where the next cycle begins before the previous one has finished.