Far beyond the Sun

III. The magnetic cycle of ι Horologii

¹ Leibniz Institute for Astrophysics Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
² European Space Agency, ESTEC, Keplerlan 1, 2201 AZ Noordwijk, The Netherlands
³ School of Earth and Space Sciences, Peking University, Beijing 100781, China
⁴ Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Straße 24/25, 14476 Potsdam-Golm, Germany
⁵ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁶ CNRS-IRAP, 14, avenue Edouard Belin, F-31400 Toulouse, France
⁷ Eberhard Karls Universität, Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany
⁸ Centro de Astrobiología (CSIC-INTA), ESAC Campus, Camino Bajo del Castillo, E-28692 Villanueva de la Cañada, Madrid, Spain

^⋆ Corresponding author: julian.alvarado-gomez@aip.de

Received: 30 April 2025
Accepted: 29 September 2025

Abstract

We present a comprehensive investigation of the magnetic cycle of the young, active solar analog ι Horologii based on intensive spectropolarimetric monitoring with HARPSpol. Over a nearly three-year campaign, the technique of Zeeman-Doppler imaging was used to reconstruct 18 maps of the large-scale surface magnetic field of the star. These maps trace the evolution of the magnetic field morphology over approximately 139 stellar rotations. Our analysis uncovers a pronounced temporal evolution, including multiple polarity reversals and changes in the field strength and geometry. We examined the evolution of the poloidal and toroidal field components, finding that the toroidal component varies strongly in concert with the chromospheric activity. Furthermore, for the first time, we reconstructed stellar magnetic butterfly diagrams, which we used to trace the migration of large-scale magnetic features across the stellar surface, determining a magnetic polarity reversal timescale of roughly 100 rotations (∼773 d). In addition, by tracking the field-weighted latitudinal positions, we obtained the first estimates of the large-scale flow properties on a star other than the Sun, identifying possible poleward and equatorward drift speeds for different field polarities. These results provide critical insights into the dynamo processes operating in young solar-type stars and enable a direct comparison with the solar magnetic cycle.