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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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TESS Asteroseismology of β Hydri: A Subgiant with a Born-again Dynamo
Abstract The solar-type subgiantβHyi has long been studied as an old analog of the Sun. Although the rotation period has never been measured directly, it was estimated to be near 27 days. As a Southern Hemisphere target, it was not monitored by long-term stellar activity surveys, but archival International Ultraviolet Explorer data revealed a 12 yr activity cycle. Previous ground-based asteroseismology suggested that the star is slightly more massive and substantially larger and older than the Sun, so the similarity of both the rotation rate and the activity cycle period to solar values is perplexing. We use two months of precise time-series photometry from the Transiting Exoplanet Survey Satellite to detect solar-like oscillations inβHyi and determine the fundamental stellar properties from asteroseismic modeling. We also obtain a direct measurement of the rotation period, which was previously estimated from an ultraviolet activity–rotation relation. We then use rotational evolution modeling to predict the rotation period expected from either standard spin-down or weakened magnetic braking (WMB). We conclude that the rotation period ofβHyi is consistent with WMB and that changes in stellar structure on the subgiant branch can reinvigorate the large-scale dynamo and briefly sustain magnetic activity cycles. Our results support the existence of a “born-again” dynamo in evolved subgiants—previously suggested to explain the cycle in 94 Aqr Aa—which can best be understood within the WMB scenario.
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- Award ID(s):
- 2205888
- PAR ID:
- 10613349
- Publisher / Repository:
- The Astrophysical Journal
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 974
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 31
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/ad6dd6
