How to age-date your star using rotation: a user’s guide to gyrochronology

Thousands of planets have been discovered via transits detected by NASA’s Kepler/K2 and TESS missions. These same light curves can be used to measure rotation periods for the planet-hosting stars. Since low-mass stars spin down over time via magnetic braking, it has become popular to infer ages for these planet hosts using gyrochronology models available in the literature. However, recent surveys of rotation in older open clusters (e.g., NGC 6811, NGC 752, Ruprecht 147) reveal that stars temporarily stop spinning down, and that the duration of this pause increases toward lower stellar masses. Ignoring this phenomenon, as most models do, can lead to huge age biases accompanied by severely overestimated precisions. We introduce an empirical, cluster-based procedure for age-dating low-mass K dwarfs. We also explain some limitations to gyrochronology stemming from a lack of very low-mass stars with periods in old clusters, as well as an intrinsic increase in age uncertainty that arises during the temporary pause.