Stellar Rotation and Structure of the α Persei Complex: When Does Gyrochronology Start to Work?

On the pre-main sequence, the rotation rates of Sun-like stars are dictated by the interplay between the protostellar disk and the star's contraction. At ages exceeding 100 Myr, magnetic spindown erases the initial stellar spin rate and enables rotation-based age dating (gyrochronology). The exact time at which the transition between these two regimes occurs depends on stellar mass, and has been challenging to empirically resolve due to a lack of viable calibration clusters. The α Persei open cluster (t ≈ 80 Myr, d ≈ 170 pc) may provide the needed calibrator, but recent analyses of the Gaia data have provided wildly varying views of its age and spatial extent. As such, we analyze a combination of TESS, Gaia, and LAMOST data to calibrate gyrochronology at the age of α Per and to uncover the cluster's true morphology. By assembling a list of rotationally confirmed α Per members, we provide strong evidence that α Per is part of a larger complex of similarly aged stars. Through kinematic back-integration, we show that the most diffuse components of α Per were five times closer together 50 Myr ago. Finally, we use our stellar rotation periods to derive a relative gyrochronology age for α Per of 67% ± 12% the age of the Pleiades, which yields 86 ± 16 Myr given current knowledge. We show that by this age, stars more massive than ≈0.8 M⊙ have converged to form a well-defined slow sequence.