Magnetic field topology of the cool, active, short-period binary system σ² Coronae Borealis^★

¹ Department Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
e-mail: oleg.kochukhov@physics.uu.se
² Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁴ LUPM, Université de Montpellier, CNRS, Place Eugène Bataillon, 34095, France
⁵ Department of Physics and Space Science, Royal Military College of Canada, PO Box 17000, Station “Forces”, Kingston, Ontario K7K 7B4, Canada

Received: 3 August 2017
Accepted: 29 January 2018

Abstract

Aims. The goal of this work is to study the cool, active binary star σ² CrB, focussing on its magnetic field. The two F9–G0 components of this system are tidally locked and in a close orbit, increasing the chance of interaction between their magnetospheres.

Methods. We used Stokes IV data from the twin spectropolarimeters Narval at the TBL and ESPaDOnS at the CFHT. The least-squares deconvolution multi-line technique was used to increase the signal-to-noise ratio of the data. We then applied a new binary Zeeman–Doppler imaging code to reconstruct simultaneously the magnetic topology and brightness distribution of both components of σ² CrB. This analysis was carried out for two observational epochs in 2014 and 2017.

Results. A previously unconfirmed magnetic field of the primary star has been securely detected. At the same time, the polarisation signatures of the secondary appear to have a systematically larger amplitude than that of the primary. This corresponds to a stronger magnetic field, for which the magnetic energy of the secondary exceeds that of the primary by a factor of 3.3–5.7. While the magnetic energy is similar for the secondary star in the two epochs, the magnetic energy is about twice as high in 2017 for the primary. The magnetic field topology of the two stars in the earlier epoch (2014) is very different. The fractions of energy in the dipole and quadrupole components of the secondary are similar and thereafter decrease with increasing harmonic angular degree ℓ. At the same time, for the primary the fraction of energy in the dipole component is low and the maximum energy contribution comes from ℓ = 4. However, in the 2017 epoch both stars have similar field topologies and a systematically decreasing energy with increasing ℓ. In the earlier epoch, the magnetic field at the visible pole appears to be of opposite polarity for the primary and secondary, suggesting linked magnetospheres. The apparent rotational periods of both σ² CrB components are longer than the orbital period, which we interpret as an evidence of a solar-like differential rotation.

Conclusions. Despite their nearly identical fundamental parameters, the components of σ² CrB system exhibit different magnetic field properties. This indicates that the magnetic dynamo process is a very sensitive function of stellar parameters.