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

S Boro Saikia,J Morin,A Reiners,The Bcool Collaboration ,C P Folsom,T Lueftinger,P Petit,M Guedel,S C Marsden,V See,S V Jeffers,A A Vidotto

doi:10.1051/0004-6361/201834347

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.

Highlights

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
Our results show that 61 Cyg A has a dominant dipolar geometry except at chromospheric activity maximum
In recent years, with the advances made in stellar magnetic field observations, the technique of Zeeman Doppler imaging (ZDI; Semel 1989; Brown et al 1991; Donati & Brown 1997; Piskunov & Kochukhov 2002; Kochukhov & Piskunov 2002) has been able to reveal the diversity in stellar largescale magnetic field geometry

Summary

Introduction

With the advances made in stellar magnetic field observations, the technique of Zeeman Doppler imaging (ZDI; Semel 1989; Brown et al 1991; Donati & Brown 1997; Piskunov & Kochukhov 2002; Kochukhov & Piskunov 2002) has been able to reveal the diversity in stellar largescale magnetic field geometry. Approximately 20 stars have been observed over multi-epochs, the majority of which were observed by the BCool Collaboration (Marsden et al 2014) Of these stars only a few exhibit polarity flips (Petit et al 2009; Morgenthaler et al 2011; Fares et al 2009; Mengel et al 2016; Boro Saikia et al 2016; Jeffers et al 2018), and only 61 Cyg A (Boro Saikia et al 2016) and τ Boo (Jeffers et al 2018) are known to exhibit polarity flips of the large-scale field in phase with the star’s chromospheric activity cycle. The LSD Stokes V and Stokes I profiles are obtained following the same procedure as applied in Paper I

Methods

Results

Discussion and conclusions