2019ApJ...879...49C

Stellar rotation was proposed as a potential age diagnostic that is precise, simple, and applicable to a broad range of low-mass stars (≤ 1 M_☉). 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∝ \sqrtage): K dwarfs appear to spin down more slowly than F and G dwarfs. Agueros et al. interpreted data for the ≃1.4 Gyr-old cluster NGC 752 differently, proposing that after having converged onto a slow-rotating sequence in their first 600-700 Myr (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 ≃1 Gyr-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 ≃0.6 M_☉. 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 ± 0.07 Gyr. However, when our new low-mass rotators are included, NGC 6811's color-P_rot sequence deviates away from the naive 1 Gyr projection down to T_eff~4295 K (K5V, 0.7 M_☉), 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.