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

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.

A New Asteroseismic Kepler Benchmark Constrains the Onset of Weakened Magnetic Braking in Mature Sun-like Stars

“…Furthermore, results from K2 and TESS (Metcalfe et al 2020;Hatt et al 2023) will continue to contribute to our understanding of the connections between stellar age, activity, and rotation. Lastly, as discussed in Saunders et al (2024), we emphasize there is uncertainty on ≈Gyr scales regarding the exact point at which solar-like stars begin weakened braking. This is primarily driven by the lack of calibrating sources around the regime where weakened magnetic braking models diverge from the standard.…”

In This Day and Age: An Empirical Gyrochronology Relation for Partially and Fully Convective Single Field Stars

“…We also included new wide binary pairs from El-Badry & Rix (2018) where new rotation periods are measured from ZTF (Lu et al 2022;this work). We then excluded stars with Rossby number > 1.866 as those stars are likely going through weakened magnetic braking (e.g., van Saders et al 2016;Saunders et al 2024). We also excluded stars with M G < 4.2 mag as they lay outside of the model training parameter space.…”

“…There exists an obvious metallicity trend for stars with [Fe/H] < 0.0 dex, in which the ages can deviate up to ∼2 Gyr as metallicity goes down to ∼−0.5 dex. Age prediction significantly worsens for stars with Rossby number >Ro crit , which is ∼1.866 according toSaunders et al (2024).…”

Kepler main-sequence solar-like stars: surface rotation and magnetic-activity evolution

“…Given their asteroseismic ages, some of these stars spin faster than expected if their spin-down was consistent with the Skumanich law (Angus et al, 2015;van Saders et al, 2016;Hall et al, 2021). This observation led to the formulation of the weakened magnetic braking (WMB), which would take place around the middle of the main-sequence lifetime, at a given critical Ro (e.g., Metcalfe et al, 2016;van Saders et al, 2016;Metcalfe and van Saders, 2017;Saunders et al, 2023). Despite the observational support for WMB, its physical cause(s) remains unclear.…”