Magnetic braking and dynamo evolution of $\beta$ Hydri

TL;DR

This study investigates the magnetic braking and dynamo evolution of the G-type subgiant star $eta$ Hydri, revealing its temporary magnetic activity resurgence and implications for stellar angular momentum loss models.

Contribution

It provides observational evidence supporting the 'born-again' dynamo hypothesis and shows that subgiants can temporarily re-establish large-scale magnetic fields.

Findings

$eta$ Hydri exhibits renewed magnetic activity and large-scale magnetic fields.

The star's wind braking torque is stronger than expected for weakened magnetic braking.

Results support the possibility of temporary dynamo reactivation in subgiants.

Abstract

The evolution of magnetic braking and dynamo processes in subgiant stars is essential for understanding how these stars lose angular momentum. We investigate the magnetic braking and dynamo evolution of $\beta$ Hydri, a G-type subgiant, to test the hypothesis of weakened magnetic braking and the potential rejuvenation of large-scale magnetic fields. We analyze spectropolarimetric observations from HARPS (HARPSpol; polarimetric mode of High Accuracy Radial velocity Planet Searcher), and combine them with archival X-ray data and asteroseismic properties from TESS (Transiting Exoplanet Survey Satellite) to estimate the current wind braking torque of $\beta$ Hydri. Despite experiencing weakened magnetic braking during the second half of its main-sequence lifetime, our results indicate that $\beta$ Hydri 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.