Antisolar Differential Rotation May Have Revived Magnetic Braking in the Subgiant 31 Aquilae

Abstract Recent observations have shown that sufficiently slow rotation disrupts the organization of large-scale magnetic field in older main-sequence stars, leading to weakened magnetic braking (WMB) and a collapse in the efficiency of the global stellar dynamo. Recent simulations predict a shift from solar-like to antisolar differential rotation (DR) at slower rotation rates, which typically do not occur on the main sequence due to WMB. However, physical expansion on the subgiant branch can eventually slow the stellar rotation beyond this threshold, yielding a noncycling large-scale field that revives magnetic braking. We combine asteroseismology from the Transiting Exoplanet Survey Satellite (TESS) with spectropolarimetry from the Large Binocular Telescope (LBT) to test these predictions in the old metal-rich subgiant 31 Aql. The LBT observations reveal a strong large-scale magnetic field in this star, and archival measurements of its chromospheric emission over 50 yr confirm that it is noncycling, as predicted. The star exhibits a variety of rotation periods during different observing seasons, consistent with DR but with no means of distinguishing between solar-like and antisolar patterns. We incorporate the TESS observations to estimate the current wind-braking torque of 31 Aql, demonstrating that it supports revived magnetic braking in this old subgiant. We also use rotational evolution modeling to place a preliminary constraint on the stellar Rossby number for the transition to antisolar DR. Future refinements in both asteroseismic observations and rotational modeling may yield improvements to this initial analysis.

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Publication Details

Journal
The Astronomical Journal
Published
2026-04-14
DOI
https://doi.org/10.3847/1538-3881/ae5484
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
0.00

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article

Antisolar Differential Rotation May Have Revived Magnetic Braking in the Subgiant 31 Aquilae

Ricky Egeland, P. Petit, J. J. Drake, I. Ilyin et al.
The Astronomical Journal
Stellar, planetary, and galactic studies
article

Antisolar Differential Rotation May Have Revived Magnetic Braking in the Subgiant 31 Aquilae

Ricky Egeland, P. Petit, J. J. Drake, I. Ilyin, T. R. Bedding, O. Kochukhov, Jennifer L. van Saders, K. G. Strassmeier, T. R. Ayres, Sylvain Breton, Marc H. Pinsonneault, Derek L. Buzasi, Steven H. Saar, Sarbani Basu, K G Stassun, Amalie Stokholm, Travis S. Metcalfe, J. M. Joel Ong, Rafael A. García
article en

Abstract

Abstract Recent observations have shown that sufficiently slow rotation disrupts the organization of large-scale magnetic field in older main-sequence stars, leading to weakened magnetic braking (WMB) and a collapse in the efficiency of the global stellar dynamo. Recent simulations predict a shift from solar-like to antisolar differential rotation (DR) at slower rotation rates, which typically do not occur on the main sequence due to WMB. However, physical expansion on the subgiant branch can eventually slow the stellar rotation beyond this threshold, yielding a noncycling large-scale field that revives magnetic braking. We combine asteroseismology from the Transiting Exoplanet Survey Satellite (TESS) with spectropolarimetry from the Large Binocular Telescope (LBT) to test these predictions in the old metal-rich subgiant 31 Aql. The LBT observations reveal a strong large-scale magnetic field in this star, and archival measurements of its chromospheric emission over 50 yr confirm that it is noncycling, as predicted. The star exhibits a variety of rotation periods during different observing seasons, consistent with DR but with no means of distinguishing between solar-like and antisolar patterns. We incorporate the TESS observations to estimate the current wind-braking torque of 31 Aql, demonstrating that it supports revived magnetic braking in this old subgiant. We also use rotational evolution modeling to place a preliminary constraint on the stellar Rossby number for the transition to antisolar DR. Future refinements in both asteroseismic observations and rotational modeling may yield improvements to this initial analysis.

The Astronomical JournalVol. 171(5)
Uppsala University (SE), Lockheed Martin (United States) (US), Centre National de la Recherche Scientifique (FR), The University of Sydney (AU), University of Hawaii System (US), Université Fédérale de Toulouse Midi-Pyrénées (FR), University of Colorado Boulder (US), Vanderbilt University (US), Université Paris Cité (FR), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Université Paris-Saclay (FR), Centre National d'Études Spatiales (FR), Leibniz Institute for Astrophysics Potsdam (DE), Yale University (US), University of Chicago (US), Community Connections (US), Astrophysique, Instrumentation et Modélisation (FR), Laboratory for Atmospheric and Space Physics (US), Osservatorio Astrofisico di Catania (IT), Center for Astrophysics Harvard & Smithsonian (US), CEA Paris-Saclay (FR), The Ohio State University (US), University of Birmingham (GB)
National Aeronautics and Space Administration
Openalex Percentile: Top 20%
Stellar, planetary, and galactic studies
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