Recent measurements of rotation periods in the benchmark open clusters Praesepe (670 Myr), NGC 6811 (1 Gyr), NGC 752 (1.4 Gyr), Ruprecht 147 (2.7 Gyr), and M67 (4 Gyr) demonstrate that, after converging onto a tight sequence of slowly rotating stars in mass-period space, stars temporarily stop spinning down. These data also show that the duration of this epoch of stalled spin-down increases toward lower masses, where 0.5 solar mass stars remain stalled for at least 1.3 Gyr. This process tends to flatten out the slowly rotating sequences over time, where by 2.7 Gyr (the age of Ruprecht 147), the stars with effective temperatures within 3700-6000 K all rotate at approximately the same rate (22 ± 2 days). Empirically tuning a core-envelope coupling model with open cluster data can account for most of the apparent stalling effect. However, alternative explanations, e.g., a temporary reduction in the magnetic braking torque, cannot yet be ruled out.
The attached figure shows rotation data for a selection of benchmarks, together with a model based on the rotation sequence for Praesepe, projected in time using the Skumanich law (Prot = t1/2). Clearly, this formula is no longer viable.