A BCool survey of stellar magnetic cycles

TL;DR

This survey investigates magnetic cycles in Sun-like stars, revealing how their magnetic field reversals relate to stellar rotation, providing key observational data to inform dynamo theories.

Contribution

It presents the first detailed analysis of magnetic cycle behavior in a sample of Sun-like stars, linking cycle characteristics to stellar rotation and magnetic field geometry.

Findings

Stars with longer rotation periods show clear magnetic polarity reversals.

Shorter rotation period stars exhibit complex, non-cyclic magnetic fields.

Polarity reversal timescales tend to decrease with faster stellar rotation.

Abstract

The magnetic cycle on the Sun consists of two consecutive 11-yr sunspot cycles and exhibits a polarity reversal around sunspot maximum. Although solar dynamo theories have progressively become more sophisticated, the details as to how the dynamo sustains magnetic fields are still subject of research. Observing the magnetic fields of Sun-like stars are useful to contextualise the solar dynamo. The BCool survey studies the evolution of surface magnetic fields to understand how dynamo-generated processes are influenced by key ingredients, like mass and rotation. Here, we focus on six Sun-like stars with mass between 1.02 and 1.06 MSun and with 3.5-21 d rotation period. We analysed high-resolution spectropolarimetric data collected with ESPaDOnS, Narval and Neo-Narval. We measured the longitudinal magnetic field from least-squares deconvolution line profiles and inspected its long-term behaviour with a Lomb-Scargle periodogram and a Gaussian process. We applied Zeeman-Doppler imaging to reconstruct the large-scale magnetic field geometry at the stellar surface for different epochs. Two stars, namely HD 9986 and HD 56124 (Prot ~ 20 d) exhibit repeating polarity reversals of the radial or toroidal field component on time scales of 5 to 6 yr. HD 73350 (Prot = 12 d) has one polarity reversal of the toroidal component and HD 76151 (Prot=17 d) may have short-term evolution (2.5 yr) modulated by the long-term (16 yr) chromospheric cycle. HD 166435 and HD 175726 (Prot =3-5 d), manifest complex magnetic fields without cyclic evolution. Our findings indicate the potential dependence of the magnetic cycles nature with stellar rotation period. For the two stars with likely cycles, the polarity reversal time scale seems to decrease with decreasing rotation period or Rossby number. These results represent important observational constraints for dynamo models of solar-like stars.