Letter to the Editor

Magnetic braking and dynamo evolution of β Hydri

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, PT4150-762 Porto, Portugal
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, PT4169-007 Porto, Portugal
³ Center for Solar-Stellar Connections, White Dwarf Research Corporation, 9020 Brumm Trail, Golden, CO 80403, USA
⁴ Department of Physics and Astronomy, Uppsala University, Box 516 SE-75120 Uppsala, Sweden
⁵ Center for Astrophysics and Space Astronomy, 389 UCB, University of Colorado, Boulder CO 80309, USA
⁶ Geophysical and Astronomical Observatory, Faculty of Science and Technology, University of Coimbra, Rua do Observatório s/n, PT3040-004 Coimbra, Portugal
⁷ Instituto de Astrofísica e Ciências do Espaço, Department of Physics, University of Coimbra, Rua Larga, PT3040-004 Coimbra, Portugal

^⋆ Corresponding author: Angela.Santos@astro.up.pt

Received: 24 March 2025
Accepted: 9 May 2025

Abstract

The evolution of magnetic braking and dynamo processes in subgiant stars is essential for understanding how these stars lose angular momentum. In this work, we investigate the magnetic braking and dynamo evolution of the G-type subgiant β Hyi to test the hypothesis of weakened magnetic braking and the potential rejuvenation of large-scale magnetic fields. We analyzed spectropolarimetric observations from the polarimetric mode of High Accuracy Radial velocity Planet Searcher (HARPSpol) and combined them with archival X-ray data and asteroseismic properties from Transiting Exoplanet Survey Satellite (TESS) to estimate the current wind-braking torque of β Hyi. Despite experiencing weakened magnetic braking during the second half of its main-sequence lifetime, our results indicate that β Hyi has regained significant magnetic activity and a large-scale magnetic field. This observation aligns with the “born-again” dynamo hypothesis. Furthermore, our estimated wind braking torque is considerably stronger than what would be expected for a star in the weakened magnetic braking regime. This suggests that subgiants with extended convective zones can temporarily re-establish large-scale dynamo action. These results provide critical constraints on stellar rotation models and improve our understanding of the interplay between magnetic field structure, stellar activity cycles, and angular momentum evolution in old solar-type stars.