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) spindown 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. We find that a sample of stars with
asteroseismically-measured rotation periods and ages is consistent with models
that depart from standard spindown 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 weakened magnetic braking poses challenges to gyrochronology for roughly
half of the main sequence lifetime of sun-like stars.Comment: 26 pages, 10 figure