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

When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147
See arXiv version
51 pages and 21 figures. Machine-readable tables for Ruprecht 147 and the Benchmark Clusters catalogs are included in arxiv source

### Abstract

Recent measurements of rotation periods ($$P_\text{rot}$$) in the benchmark open clusters Praesepe (670 Myr), NGC 6811 (1 Gyr), and NGC 752 (1.4 Gyr) 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 $$P_\text{rot}$$ 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 $$Kepler$$ $$P_\text{rot}$$ data for the approximately coeval cluster NGC 6819 (30 stars with $$M_\star > 0.85$$ M$$_\odot$$), our new measurements more than double the number of $$\approx$$2.5 Gyr 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 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$$_\odot$$ 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.