paper

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

arXiv:2010.02272 · doi:10.3847/1538-4357/abbf58

Abstract

Recent measurements of rotation periods () 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 massperiod 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 mission and the Palomar Transient Factory to measure 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 data for the approximately coeval cluster NGC 6819 (30 stars with M), our new measurements more than double the number of 2.5 Gyr benchmark rotators and extend this sample down to 0.55 M. 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 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 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 coreenvelope 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.

51 pages and 21 figures. Machine-readable tables for Ruprecht 147 and the Benchmark Clusters catalogs are included in arxiv source

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