A. R. G. Santos, T. S. Metcalfe, O. Kochukhov, T. R. Ayres, R. Gafeira, T. L. Campante
Abstract
The evolution of magnetic braking and dynamo processes in subgiant stars plays a crucial role in shaping their angular momentum loss and long-term magnetic activity. We investigate the magnetic evolution of β Hydri, a G-type subgiant and old solar analog, using spectropolarimetric observations from HARPSpol, archival X-ray data, and asteroseismic constraints from TESS. Our results indicate that β Hydri, despite undergoing weakened magnetic braking during the latter half of its main-sequence evolution, has re-established large-scale magnetic fields—supporting the “born-again” dynamo hypothesis. The estimated wind braking torque is significantly stronger than expected for stars in the weakened braking regime, implying that subgiants with deepening convective envelopes may temporarily recover large-scale dynamo action. These findings provide constraints for stellar rotation models and improve our understanding of the interplay between magnetic fields and cycles, and angular momentum evolution in old solar-type stars.
TASC9/KASC16 9th TESS/16th Kepler Asteroseismic Science Consortium Workshop
, Number 17
2025 October
