Monitoring the large-scale magnetic field of AD Leo with SPIRou, ESPaDOnS, and Narval

Towards a magnetic polarity reversal?^★

¹ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Edouard Belin, 31400 Toulouse, France
e-mail: stefano.bellotti@irap.omp.eu
² Science Division, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
³ Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, 34095, Montpellier, France
⁴ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere, 61602 Tartumaa, Estonia
⁵ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁷ Laboratório Nacional de Astrofísica, Rua Estados Unidos 154, 37504-364 Itajubá, MG, Brazil
⁸ Institut d’Astrophysique de Paris, CNRS, UMR 7095, Sorbonne Université, 98 bis bd Arago, 75014 Paris, France
⁹ Universidade Federal de Minas Gerais, Belo Horizonte, MG 31270-901, Brazil
¹⁰ Université de Montréal, Département de Physique, IREX, Montréal, QC H3C 3J7, Canada
¹¹ Aix-Marseille Univ, CNRS, CNES, LAM, Marseille, France
¹² Observatoire de Genève, Université de Genève, Chemin Pegasi, 51, 1290 Sauverny, Switzerland
¹³ Observatoire de Haute-Provence, 1912 Route de l’Observatoire, 04870 St Michel l’Observatoire, France
¹⁴ Department of Physics & Space Science, Royal Military College of Canada, PO Box 17000 Station Forces, Kingston, ON K7K 0C6, Canada
¹⁵ Department of Physics & Astronomy, McMaster University, 1280 Main St West, Hamilton, ON L8S 4L8, Canada

Received: 8 May 2023
Accepted: 30 June 2023

Abstract

Context. One clear manifestation of dynamo action on the Sun is the 22-yr magnetic cycle, exhibiting a polarity reversal and a periodic conversion between poloidal and toroidal fields. For M dwarfs, several authors claim evidence of activity cycles from photometry and analyses of spectroscopic indices, but no clear polarity reversal has been identified from spectropolarimetric observations. These stars are excellent laboratories to investigate dynamo-powered magnetic fields under different stellar interior conditions, that is partly or fully convective.

Aims. Our aim is to monitor the evolution of the large-scale field of AD Leo, which has shown hints of a secular evolution from past dedicated spectropolarimetric campaigns. This is of central interest to inform distinct dynamo theories, contextualise the evolution of the solar magnetic field, and explain the variety of magnetic field geometries observed in the past.

Methods. We analysed near-infrared spectropolarimetric observations of the active M dwarf AD Leo taken with SPIRou between 2019 and 2020 and archival optical data collected with ESPaDOnS and Narval between 2006 and 2019. We searched for long-term variability in the longitudinal field, the width of unpolarised Stokes profiles, the unsigned magnetic flux derived from Zeeman broadening, and the geometry of the large-scale magnetic field using both Zeeman-Doppler imaging and principal component analysis.

Results. We found evidence of a long-term evolution of the magnetic field, featuring a decrease in axisymmetry (from 99% to 60%). This is accompanied by a weakening of the longitudinal field (−300 to −50 G) and a correlated increase in the unsigned magnetic flux (2.8–3.6 kG). Likewise, the width of the mean profile computed with selected near-infrared lines manifests a long-term evolution corresponding to field strength changes over the full time series, but does not exhibit modulation with the stellar rotation of AD Leo in individual epochs.

Conclusions. The large-scale magnetic field of AD Leo manifested first hints of a polarity reversal in late 2020 in the form of a substantially increased dipole obliquity, while the topology remained predominantly poloidal and dipolar for 14 yr. This suggests that low-mass M dwarfs with a dipole-dominated magnetic field can undergo magnetic cycles.