ϵ Eridani: an active K dwarf and a planet hosting star?

The variability of its large-scale magnetic field topology^⋆

¹ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: Jeffers@astro.physik.uni-goettingen.de
² Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, Toulouse, France
³ Institut de Recherche en Astrophysique et Planétologie, 14 avenue Edouard Belin, 31400 Toulouse, France
⁴ Computational Engineering and Science Research Centre, University of Southern Queensland, 4350 Toowoomba, Australia
⁵ LUPM, UMR 5299, CNRS & Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France

Received: 27 February 2014
Accepted: 1 July 2014

Abstract

The young K-dwarf ϵ Eridani is an example of a young active planet hosting star that has shown over long-term monitoring of its chromospheric emission to exhibit cyclic magnetic activity. In this paper, we investigate how ϵ Eridani’s large-scale magnetic field geometry evolves over the timescale of its S-index cycle using spectropolarimetric observations and the technique of Zeeman-Doppler imaging. Our observations comprise six epochs secured over a time period of nearly seven years, with each almost yearly observational epoch showing a dramatic change in the large-scale magnetic field topology, with no stable regions. The poloidal field varies from strongly dipolar to mono-polar and the toroidal field is non-existent to begin with and then emerges to dominate the magnetic field energy before disappearing and re-emerging again. A potential cycle is detected in the poloidal field, but further observations are needed to confirm this.