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Description
Stellar rotation was proposed as a potential age diagnostic that is precise, simple, and applicable to a broad range of low-mass stars (<=1M_{sun}_). Unfortunately, rotation period (P_rot_) measurements of low-mass members of open clusters have undermined the idea that stars spin down with a common age dependence (i.e., P_rot{propto}age^0.5^): K dwarfs appear to spin down more slowly than F and G dwarfs. Agueros+ (2018, J/ApJ/862/33) interpreted data for the ~1.4Gyr-old cluster NGC 752 differently, proposing that after having converged onto a slow-rotating sequence in their first 600-700Myr (by the age of Praesepe), K dwarf P_rot_ stall on that sequence for an extended period of time. We use data from Gaia DR2 to identify likely single-star members of the ~1Gyr-old cluster NGC 6811 with Kepler light curves. We measure P_rot_ for 171 members, more than doubling the sample relative to the existing catalog and extending the mass limit from ~0.8 to ~0.6M_{sun}_. We then apply a gyrochronology formula calibrated with Praesepe and the Sun to 27 single G dwarfs in NGC 6811 to derive a precise gyrochronological age for the cluster of 1.04+/-0.07Gyr. However, when our new low-mass rotators are included, NGC 6811's color-P_rot_ sequence deviates away from the naive 1Gyr projection down to T_eff_~4295K (K5V, 0.7M_{sun}), where it clearly overlaps with Praesepe's. Combining these data with P_rot_ for other clusters, we conclude that the assumption that mass and age are separable dependencies is invalid. Furthermore, the cluster data show definitively that stars experience a temporary epoch of reduced braking efficiency where P_rot_ stall, and that the duration of this epoch lasts longer for lower-mass stars.
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