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When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147
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. Thes...
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Published in: | arXiv.org 2020-10 |
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Main Authors: | , , , , , , , , , , , , , , , , , , , |
Format: | Article |
Language: | English |
Subjects: | |
Online Access: | Get full text |
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Summary: | 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. |
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ISSN: | 2331-8422 |
DOI: | 10.48550/arxiv.2010.02272 |