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Despite a growing sample of precisely measured stellar rotation periods andages, the strength of magnetic braking and the degree of departure fromstandard (Skumanich-like) spindown have remained persistent questions,particularly for stars more evolved than the Sun. Rotation periods can bemeasured for stars older than the Sun by leveraging asteroseismology, enablingmodels to be tested against a larger sample of old field stars. Becauseasteroseismic measurements of rotation do not depend on starspot modulation,they avoid potential biases introduced by the need for a stellar dynamo todrive starspot production. Using a neural network trained on a grid of stellarevolution models and a hierarchical model-fitting approach, we constrain theonset of weakened magnetic braking. We find that a sample of stars withasteroseismically-measured rotation periods and ages is consistent with modelsthat depart from standard spindown prior to reaching the evolutionary stage ofthe Sun. We test our approach using neural networks trained on model gridsproduced by separate stellar evolution codes with differing physicalassumptions and find that the choices of grid physics can influence theinferred properties of the braking law. We identify the normalized criticalRossby number ${\rm Ro}_{\rm crit}/{\rm Ro}_\odot = 0.91\pm0.03$ as thethreshold for the departure from standard rotational evolution. This suggeststhat weakened magnetic braking poses challenges to gyrochronology for roughlyhalf of the main sequence lifetime of sun-like stars.

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