Stellar rotation in the Hyades and Praesepe: Gyrochronology and braking time-scale

We present the results of photometric surveys for stellar rotation in the Hyades and in Praesepe, using data obtained as part of the SuperWASP exoplanetary transit-search programme. We determined accurate rotation periods for more than 120 sources whose cluster membership was confirmed by common proper motion and colour–magnitude fits to the clusters’ isochrones. This allowed us to determine the effect of magnetic braking on a wide range of spectral types for expected ages of ∼600 Myr for the Hyades and Praesepe. Both clusters show a tight and nearly linear relation between J−Ks colour and rotation period in the F, G and K spectral range. This confirms that loss of angular momentum was significant enough that stars with strongly different initial rotation rates have converged to the same rotation period for a given mass, by the ages of Hyades and Praesepe. In the case of the Hyades, our colour–period sequence extends well into the M dwarf regime and shows a steep increase in the scatter of the colour–period relation, with identification of numerous rapid rotators from ∼0.5 M⊙ down to the lowest masses probed by our survey (∼0.25 M⊙). This provides crucial constraints on the rotational braking time-scales and further clears the way to use gyrochronology as an accurate age measurement tool for main-sequence stars.