Rotational velocities of low-mass stars in the Pleiades and Hyades

We have obtained high-resolution spectra of 89 M dwarf members of the Pleia des and Hyades and have derived radial velocities, Ha equivalent widths, an d spectroscopic rotational velocities for these stars. Typical masses of th e newly observed Pleiades and Hyades stars are similar to 0.4 M. and simila r to 0.2 M., respectively. We combine our new observations with previously published data to explore the rotational evolution of young stars with M le ss than or equal to 0.4 M.. The average rotation rate in the Hyades (age 60 0 Myr) is about 0.4 times that in the Pleiades (110 Myr), and the mean equi valent widths of H alpha are also lower. As found in previous studies, the correlation between rotation and chromospheric activity is identical in bot h clusters, implying that the lower activity in the Hyades is a result of t he lower rotation rates. We show that a simple scaling of the Pleiades rota tional distribution for M less than or equal to 0.4 M., corrected for the e ffects of structural evolution, matches that of the Hyades if the average a ngular momentum loss from the Pleiades to the Hyades age is factor of appro ximate to 6. This suggests that the distribution of initial angular momenta and disk-locking lifetimes for the lowest mass stars was similar in both c lusters. We argue that this result provides further evidence for a saturati on of the angular momentum loss rate at high rotational velocities.