Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri
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.
Full text
Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri Ângela R. G. Santos 📧 [email protected] 0000-0001-7195-6542 Travis S. Metcalfe, Oleg Kochukhov, Thomas R. Ayres, Ricardo Gafeira, Tiago L. Campante TASC9/KASC16 Workshop Vienna Woods, Austria — 8 July 2025 © NASA/SDO
Rotation and Activity Evolution Ângela R. G. Santos – Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri 2 TASC9/KASC16 adapted from Mathur et al. 2025 Full single MS Kepler ●Rotation rate and magnetic activity generally decrease as stars age (e.g. Skumanich 1972; Barnes 2003; Lorenzo-Oliveira et al 2018) ●But these properties’ evolution is not smooth, it is complex with multiple regimes (e.g. McQuillan et al. 2014; van Saders et al. 2016, Metcalfe et al. 2016, 2020, 2022, 2024; Mathur et al. 2023, 2025; Santos et al. 2025a,b)
Rotation and Activity Evolution Ângela R. G. Santos – Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri 3 TASC9/KASC16 adapted from Mathur et al. 2025 Full single MS Kepler ●Rotation rate and magnetic activity generally decrease as stars age (e.g. Skumanich 1972; Barnes 2003; Lorenzo-Oliveira et al 2018) ●But these properties’ evolution is not smooth, it is complex with multiple regimes (e.g. McQuillan et al. 2014; van Saders et al. 2016, Metcalfe et al. 2016, 2020, 2022, 2024; Mathur et al. 2023, 2025; Santos et al. 2025a,b) ●Subgiant phase: ○Stars show flat activity ○Exceptions: 1) Stars originated above the Kraft break (Kraft 1967) ; 2) Stars slightly more massive than the Sun (94 Aqr & 𝛽 Hydri)
Sun’s Elder Twin: 𝞫 Hydri (HD 2151) Ângela R. G. Santos – Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri 4 TASC9/KASC16 Metcalfe et al. 2007 (see also Dravins et al. 1993) (1) (2) (2) (3) (3) (3) ●Subgiant star considered as an old Sun (e.g. Dravins et al. 1993, 1998) ●Slightly more massive than the Sun (e.g. Kjeldsen et al. 2008; Brandão et al. 2011; Metcalfe et al. 2024) ●Magnetic activity level comparable to the Sun’s (Dravins et al. 1993, Metcalfe et al. 2007, Ayres 2025) ●Long-term magnetic cycle (Pcyc~12 yr) (Metcalfe et al. 2007) (1) North et al. 2007; (2) Brunt et al. 2010; (3) Metcalfe et al. 2024
Sun’s Elder Twin: 𝞫 Hydri (HD 2151) Ângela R. G. Santos – Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri 5 TASC9/KASC16 ●The current rotation rate cannot be explained by standard evolution models ●Weakened magnetic braking, at midlife, is required to reproduce the observations (Metcalfe et al. 2024) ●Consistent also with the observation for 94 Aqr (Metcalfe et al. 2020) Metcalfe et al. 2024 standard spin-down model 𝛽 Hydri weakened magnetic braking model Rocrit
Sun’s Elder Twin: 𝞫 Hydri (HD 2151) Ângela R. G. Santos – Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri 6 TASC9/KASC16 ●The current rotation rate cannot be explained by standard evolution models ●Weakened magnetic braking, at midlife, is required to reproduce the observations (Metcalfe et al. 2024) ●Consistent also with the observation for 94 Aqr (Metcalfe et al. 2020) Born-again dynamo Metcalfe et al. 2024 standard spin-down model 𝛽 Hydri weakened magnetic braking model Rocrit
Ângela R. G. Santos – Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri 7 TASC9/KASC16 Is the born-again dynamo phase is accompanied by the return of the large-scale magnetic field and the ensuing higher rate of angular momentum loss? Magnetic field in 𝞫 Hydri
Ângela R. G. Santos – Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri 8 TASC9/KASC16 Is the born-again dynamo phase is accompanied by the return of the large-scale magnetic field and the ensuing higher rate of angular momentum loss? ●We obtained spectropolarimetry with HARPSPol for 𝛽 Hydri (ESO 113.26SZ.001) ●49 spectra treated with REDUCE (Piskunov & Valenti 2002; Rusomarov et al. 2013) ●~4800 spectral lines are combined to compute the least-squares deconvolved (LSD) Stokes I and V profiles (following Donati et al. 1997; Kochukhov et al. 2010) Magnetic field in 𝞫 Hydri
Santos et al. 2025b Stokes V profile for 𝛽 Hyi Best fit models: axisymmetric field inclined field ●Significant detection of large-scale magnetic field in 𝛽 Hydri ●Mean longitudinal field: ❬Bz❭ = -0.298 ± 0086 G ●Best fit (INVERSLSD; Kochukhov et al. 2014): oblique magnetic field of Bd = -0.64 G (𝛽=87.3º) Ângela R. G. Santos – Magnetic Evolution Beyond the Main Sequence: a Born-again Dynamo in β Hydri 9 TASC9/KASC16 Magnetic field in 𝞫 Hydri Bd v sini Bz Ri Prot (Metcalfe et al. 2024) Brunt et al. 2010