Letter to the Editor

Direct evidence of a full dipole flip during the magnetic cycle of a sun-like star

¹ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: sudeshna.boro.saikia@univie.ac.at
² University of Goettingen, Institute for Astrophysics, Friedrich Hund Platz 1, 37077 Goettingen, Germany
³ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
⁴ CNRS, Institut de Recherche en Astrophysique et Planetologie, 14, avenue Edouard Belin, 31400 Toulouse, France
⁵ University of Exeter, Department of Physics & Astronomy, Stocker Road, Devon, Exeter, EX4 4QL UK
⁶ University of Southern Queensland, Centre for Astrophysics, Toowoomba, QLD, 4350 Australia
⁷ School of Physics, Trinity College Dublin, University of Dublin, Dublin-2, Ireland
⁸ LUPM-UMR 5299, CNRS & Université Montpellier, place Eugène Bataillon, 34095 Montpellier Cedex 05, France

Received: 30 September 2018
Accepted: 22 November 2018

Abstract

Context. The behaviour of the large-scale dipolar field, during a star’s magnetic cycle, can provide valuable insight into the stellar dynamo and associated magnetic field manifestations such as stellar winds.

Aims. We investigate the temporal evolution of the dipolar field of the K dwarf 61 Cyg A using spectropolarimetric observations covering nearly one magnetic cycle equivalent to two chromospheric activity cycles.

Methods. The large-scale magnetic field geometry is reconstructed using Zeeman Doppler imaging, a tomographic inversion technique. Additionally, the chromospheric activity is also monitored.

Results. The observations provide an unprecedented sampling of the large-scale field over a single magnetic cycle of a star other than the Sun. Our results show that 61 Cyg A has a dominant dipolar geometry except at chromospheric activity maximum. The dipole axis migrates from the southern to the northern hemisphere during the magnetic cycle. It is located at higher latitudes at chromospheric activity cycle minimum and at middle latitudes during cycle maximum. The dipole is strongest at activity cycle minimum and much weaker at activity cycle maximum.

Conclusions. The behaviour of the large-scale dipolar field during the magnetic cycle resembles the solar magnetic cycle. Our results are further confirmation that 61 Cyg A indeed has a large-scale magnetic geometry that is comparable to the Sun’s, despite being a slightly older and cooler K dwarf.