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

Abstract Stellar rotation was proposed as a potential age diagnostic that is precise, simple, and applicable to a broad range of low-mass stars ( 1 ). Unfortunately, rotation period ( ) measurements of low-mass members of open clusters have undermined the idea that stars spin down with a common age dependence (i.e., ): K dwarfs appear to spin down more slowly than F and G dwarfs. Agüeros 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 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 for 171 members, more than doubling the sample relative to the existing catalog and extending the mass limit from to ≈0.6 . 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– sequence deviates away from the naive 1 Gyr projection down to K (K5V, 0.7 ), where it clearly overlaps with Praesepe’s. Combining these data with 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 stall, and that the duration of this epoch lasts longer for lower-mass stars.

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