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Recent measurements of rotation periods ($P_\text{rot}$) in the benchmarkopen clusters Praesepe (670 Myr), NGC 6811 (1 Gyr), and NGC 752 (1.4 Gyr)demonstrate that, after converging onto a tight sequence of slowly rotatingstars in mass$-$period space, stars temporarily stop spinning down. These dataalso show that the duration of this epoch of stalled spin-down increases towardlower masses. To determine when stalled stars resume spinning down, we use datafrom the $K2$ mission and the Palomar Transient Factory to measure$P_\text{rot}$ for 58 dwarf members of the 2.7-Gyr-old cluster Ruprecht 147, 39of which satisfy our criteria designed to remove short-period ornear-equal-mass binaries. Combined with the $Kepler$ $P_\text{rot}$ data forthe approximately coeval cluster NGC 6819 (30 stars with $M_\star > 0.85$M$_\odot$), our new measurements more than double the number of $\approx$2.5Gyr benchmark rotators and extend this sample down to $\approx$0.55 M$_\odot$.The slowly rotating sequence for this joint sample appears relatively flat (22$\pm$ 2 days) compared to sequences for younger clusters. This sequence alsointersects the $Kepler$ intermediate period gap, demonstrating that this gapwas not created by a lull in star formation. We calculate the time at whichstars resume spinning down, and find that 0.55 M$_\odot$ stars remain stalledfor at least 1.3 Gyr. To accurately age-date low-mass stars in the field,gyrochronology formulae must be modified to account for this stallingtimescale. Empirically tuning a core$-$envelope coupling model with opencluster data can account for most of the apparent stalling effect. However,alternative explanations, e.g., a temporary reduction in the magnetic brakingtorque, cannot yet be ruled out.

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