The crucial role of surface magnetic fields for stellar dynamos: ϵ Eridani, 61 Cygni A, and the Sun^⋆

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-weg 3, 37077 Göttingen, Germany
e-mail: jeffers@mps.mpg.de
² Centre for Astrophysics, University of Southern Queensland, Toowoomba, Queensland 4350, Australia
³ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁴ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere, 61602 Tartumaa, Estonia
⁵ SUPA, University of St Andrews, School of Physics and Astronomy, North Haugh, St Andrews KY16 9SS, UK
⁶ Laboratoire Univers et Particules de Montpellier, Centre national de la recherche scientifique (CNRS), Université de Montpellier, Place Eugène Bataillon, 34095 Montpellier, France
⁷ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, 14 Av. Edouard Belin, 31400 Toulouse, France
⁸ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁹ University of Exeter, Department of Physics & Astronomy, Stocker Road, Exeter, Devon EX4 4QL, UK
¹⁰ Trinity College Dublin, Dublin 2, Ireland
¹¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 10 September 2021
Accepted: 14 December 2021

Abstract

Cool main-sequence stars, such as the Sun, have magnetic fields which are generated by an internal dynamo mechanism. In the Sun, the dynamo mechanism produces a balance between the amounts of magnetic flux generated and lost over the Sun’s 11-year activity cycle and it is visible in the Sun’s different atmospheric layers using multi-wavelength observations. We used the same observational diagnostics, spanning several decades, to probe the emergence of magnetic flux on the two close by, active- and low-mass K dwarfs: 61 Cygni A and ϵ Eridani. Our results show that 61 Cygni A follows the Solar dynamo with a regular cycle at all wavelengths, while ϵ Eridani represents a more extreme level of the Solar dynamo, while also showing strong Solar-like characteristics. For the first time we show magnetic butterfly diagrams for stars other than the Sun. For the two K stars and the Sun, the rate at which the toroidal field is generated from surface poloidal field is similar to the rate at which toroidal flux is lost through flux emergence. This suggests that the surface field plays a crucial role in the dynamos of all three stars. Finally, for ϵ Eridani, we show that the two chromospheric cycle periods, of ∼3 and ∼13 years, correspond to two superimposed magnetic cycles.