Talk

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

Jason Curtis, Columbia University

Stellar rotation was proposed as a potential age diagnostic that is precise, simple, and applicable to a broad range of low-mass stars (<1 solar masses). 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., square-root of age): K dwarfs appear to spin down more slowly than F and G dwarfs. Agüeros et al. (2018) 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 rotation periods 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 rotation periods for 170 members, more than doubling the sample relative to the existing catalog and extending the mass limit from 0.8 to 0.6 solar masses. Standard gyrochronology relations applied to the G dwarfs show it to be 1 Gyr in age, consistent with the isochrone solution. However, when our new low-mass rotators are included, NGC 6811's color-period sequence deviates away from the naive 1 Gyr projection down to 4300 K (K5V, 0.7 solar masses), where it clearly overlaps with Praesepe's. Combining these data with rotation data 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 rotation periods stall, and that the duration of this epoch lasts longer for lower-mass stars. We will suggest one method for reformulating the empirical relations using periods from older clusters (e.g., NGC 6819, Ruprecht 147, and M67) to tune the stalling timescale, and we are eager to see how these new data can be used to recalibrate theoretical angular momentum evolution models (e.g., core-envelope coupling timescales), so that both classes of models accurately describe the discrete phases of stellar spin-down.