Stellar Cruise Control: Weakened Magnetic Braking Leads to Sustained Rapid Rotation of Old Stars

Nicholas Saunders*, Jennifer L. van Saders, Alexander J. Lyttle, Travis S. Metcalfe, Tanda Li, Guy R. Davies, Oliver J. Hall, Warrick H. Ball, Richard Townsend, Orlagh Creevey, Curt Dodds

*Corresponding author for this work

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Abstract

Despite a growing sample of precisely measured stellar rotation periods and ages, the strength of magnetic braking and the degree of departure from standard (Skumanich-like) spin-down have remained persistent questions, particularly for stars more evolved than the Sun. Rotation periods can be measured for stars older than the Sun by leveraging asteroseismology, enabling models to be tested against a larger sample of old field stars. Because asteroseismic measurements of rotation do not depend on starspot modulation, they avoid potential biases introduced by the need for a stellar dynamo to drive starspot production. Using a neural network trained on a grid of stellar evolution models and a hierarchical model-fitting approach, we constrain the onset of weakened magnetic braking (WMB). We find that a sample of stars with asteroseismically measured rotation periods and ages is consistent with models that depart from standard spin-down prior to reaching the evolutionary stage of the Sun. We test our approach using neural networks trained on model grids produced by separate stellar evolution codes with differing physical assumptions and find that the choices of grid physics can influence the inferred properties of the braking law. We identify the normalized critical Rossby number Rocrit/Ro = 0.91 ± 0.03 as the threshold for the departure from standard rotational evolution. This suggests that WMB poses challenges to gyrochronology for roughly half of the main-sequence lifetime of Sun-like stars.
Original languageEnglish
Article number138
Number of pages15
JournalThe Astrophysical Journal
Volume962
Issue number2
Early online date14 Feb 2024
DOIs
Publication statusPublished - 20 Feb 2024

Bibliographical note

Acknowledgments:
N.S. acknowledges support by the National Science Foundation Graduate Research Fellowship Program under grant Nos. 1842402 and 2236415. J.v.S. and N.S. acknowledge support from the Research Corporation for Science Advancement through Scialog award #39436, in partnership with the Heising-Simons Foundation. J.v.S. also acknowledges support from the National Science Foundation grant AST-2205888. This work has received funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (CartographY GA. 804752). T.S.M. acknowledges support from NASA grant No. 80NSSC22K0475. Computational time at the Texas Advanced Computing Center was provided through XSEDE allocation TG-AST090107. A.J.L. acknowledges the support of the Science and Technology Facilities Council. R.H.D.T. acknowledges support from NASA grant No. 80NSSC20K0515.

Keywords

  • Stellar properties
  • Stellar rotation
  • Stellar magnetic fields
  • Stellar ages
  • Asteroseismology
  • Stellar evolution
  • Solar analogs

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