When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147

Recent measurements of rotation periods (P-rot) in the benchmark open clusters Praesepe (670Myr), NGC.6811 (1Gyr), and NGC.752 (1.4Gyr) demonstrate that, after converging onto a tight sequence of slowly rotating stars in mass-period space, stars temporarily stop spinning down. These data also show that the duration of this epoch of stalled spin-down increases toward lower masses. To determine when stalled stars resume spinning down, we use data from the K2 mission and the Palomar Transient Factory to measure P-rot for 58 dwarf members of the 2.7 Gyr old cluster Ruprecht.147, 39 of which satisfy our criteria designed to remove short-period or near-equal-mass binaries. Combined with the Kepler P-rot data for the approximately coeval cluster NGC.6819 (30 stars with M-star > 0.85 M-circle dot), our new measurements more than double the number of approximate to 2.5.Gyr benchmark rotators and extend this sample down to approximate to 0.55.M-circle dot. The slowly rotating sequence for this joint sample appears relatively flat (22 +/- 2 days) compared to sequences for younger clusters. This sequence also intersects the Kepler intermediate-period gap, demonstrating that this gap was not created by a lull in star formation. We calculate the time at which stars resume spinning down and find that 0.55 M-circle dot stars remain stalled for at least 1.3.Gyr. To accurately age-date low-mass stars in the field, gyrochronology formulae must be modified to account for this stalling timescale. Empirically tuning a core-envelope coupling model with open cluster data can account for most of the apparent stalling effect. However, alternative explanations, e.g., a temporary reduction in the magnetic braking torque, cannot yet be ruled out.