K2 ROTATION PERIODS FOR LOW-MASS HYADS AND THE IMPLICATIONS FOR GYROCHRONOLOGY

As the closest open cluster to the Sun, the Hyades is an important benchmark for many stellar properties, but its members are also scattered widely over the sky. Previous studies of stellar rotation in the Hyades relied on targeted observations of single stars or data from shallower all-sky variability surveys. The re-purposed Kepler mission, K2, is the first opportunity to measure rotation periods (P-rot) for many Hyads simultaneously while also being sensitive to fully convective M dwarf members. We analyze K2 data for 65 Hyads and present P-rot values for 48. Thirty-seven of these are new measurements, including the first P-rot measurements for fully convective Hyads. For 9 of the 11 stars with P-rot in the literature and this work, the measurements are consistent; we attribute the two discrepant cases to spot evolution. Nearly all stars with masses less than or similar to 0.3 M-circle dot are rapidly rotating, indicating a change in rotation properties at the boundary to full convection. When confirmed and candidate binaries are removed from the mass-period plane, only three rapid rotators with masses greater than or similar to 0.3 M-circle dot remain. This is in contrast to previous results showing that the single-valued mass-period sequence for approximate to 600 Myr old stars ends at approximate to 0.65 M-circle dot when binaries are included. We also find that models of rotational evolution predict faster rotation than is actually observed at approximate to 600 Myr for stars less than or similar to 0.9M(circle dot). The dearth of single rapid rotators more massive than approximate to 0.3M(circle dot) indicates that magnetic braking is more efficient than previously thought, and that age-rotation studies must account for multiplicity.