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

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 Epsilon Eridani. Our results show that 61 Cygni A follows the Solar dynamo with a regular cycle at all wavelengths, while Epsilon 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 Epsilon Eridani, we show that the two chromospheric cycle periods, of ~3 and ~13 years, correspond to two superimposed magnetic cycles.

8 pages, 5 figures: Accepted by A&A

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• The CARMENES search for exoplanets around M dwarfs. Variability on long timescales as seen in chromospheric indicators
• Dynamo modelling for cycle variability and occurrence of grand minima in Sun-like stars: Rotation rate dependence
• Solar and stellar activity cycles -- no synchronization with exoplanets
• A BCool survey of stellar magnetic cycles
• Evolution of solar wind sources and coronal rotation driven by the cyclic variation of the Sun's large-scale magnetic field
• A multi-wavelength view of the multiple activity cycles of $\epsilon$~Eridani
• Far beyond the Sun: III. The magnetic cycle of $\mathbf{\iota }$ Horologii