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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.
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This content will become publicly available on September 15, 2026
Zeeman Doppler Imaging of τ Ceti: The Weakest Magnetic Field Detected in a Sun-like Star
Abstract For nearly a decade, observations have shown that many older Sun-like stars spin faster than predicted, a phenomenon known as weakened magnetic braking (WMB). The leading hypothesis for WMB is a weakening of the large-scale dipole field, which leads to a less efficient angular momentum loss. To test this hypothesis on a star known to be in the WMB regime, we present the first Zeeman Doppler imaging (ZDI) map of the Sun-like starτCeti, reconstructed using spectropolarimetric data from the Canada–France–Hawai‘i Telescope. Our ZDI analysis reveals a remarkably simple, stable, and weak (〈B〉 = 0.17 G) magnetic field, characterized by a predominantly dipolar (∼92% magnetic energy contained inl= 1 modes), and highly axisymmetric (∼88% magnetic energy contained inm < l/2 modes) morphology. We infer a dipole field strength ofBdip = 0.31 G, nearly an order of magnitude weaker than standard braking model predictions, providing direct confirmation of the weakened large-scale dipole predicted by the WMB hypothesis. This work establishes a new benchmark for ZDI, demonstrating that even extremely quiet stars in the WMB regime are accessible to this technique.
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- Award ID(s):
- 2205919
- PAR ID:
- 10636070
- Publisher / Repository:
- The Astrophysical Journal Letters
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 991
- Issue:
- 1
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L13
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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