A Temporary Epoch of Stalled Spin-down for Low-mass Stars: Insights from NGC 6811 with Gaia and Kepler

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-circle dot). 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 proportional to root age): K dwarfs appear to spin down more slowly than F and G dwarfs. Agiieros et al. interpreted data for the approximate to 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 approximate to 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 approximate to 0.8 to approximate to 0.6 M-circle dot. 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 approximate to 4295 K (K5V, 0.7 M-circle dot), 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.