A BCool survey of stellar magnetic cycles

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 Avenue Edouard Belin, F-31400 Toulouse, France
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany
⁴ Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD 4350, Australia
⁵ Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, F-34095, Montpellier, France
⁶ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere 61602, Estonia
⁷ Science Division, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁸ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ Dep. de Física, Univ. Federal do Rio Grande do Norte-UFRN, Natal, RN 59078-970, Brazil

^⋆ Corresponding author; bellotti@strw.leidenuniv.nl

Received: 26 September 2024
Accepted: 10 December 2024

Abstract

Context. 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 the subject of research. Observing the magnetic fields of Sun-like stars can bring useful insights to contextualise the solar dynamo.

Aims. With the long-term spectropolarimetric monitoring of stars, the BCool survey studies the evolution of surface magnetic fields to understand how dynamo-generated processes are influenced by key ingredients, such as mass and rotation. Here, we focus on six Sun-like stars with masses between 1.02 and 1.06 M_⊙ and with rotation periods of 3.5–21 d (or 0.3–1.8 in Rossby numbers), a practical sample with which to study magnetic cycles across distinct activity levels.

Methods. We analysed high-resolution spectropolarimetric data collected with ESPaDOnS, Narval, and Neo-Narval between 2007 and 2024 within the BCool programme. We measured longitudinal magnetic field from least-squares deconvolution line profiles and we inspected its long-term behaviour with both a Lomb-Scargle periodogram and a Gaussian process. We then applied Zeeman-Doppler imaging to reconstruct the large-scale magnetic field geometry at the stellar surface for different epochs.

Results. Two of our slow rotators, namely HD 9986 and HD 56124 (P_rot ∼ 20 d), exhibit repeating polarity reversals in the radial or toroidal field component on shorter timescales than the Sun (5–6 yr). HD 73350 (P_rot ∼ 12 d) has one polarity reversal in the toroidal component and HD 76151 (P_rot = 17 d) may have short-term evolution (2.5 yr) modulated by the long-term (16 yr) chromospheric cycle. Our two fast rotators, HD 166435 and HD 175726 (P_rot = 3 − 5 d), manifest complex magnetic fields without an evident cyclic evolution.

Conclusions. Our findings indicate the potential dependence of the magnetic cycles’ nature on the stellar rotation period. For the two stars with likely cycles, the polarity reversal timescale seems to decrease with a decreasing rotation period or Rossby number. These results represent important observational constraints for dynamo models of solar-like stars.