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