Rotational Studies of Late-Type Stars. VII. M34 (NGC 1039) and the Evolution of Angular Momentum and Activity in Young Solar-Type Stars

We analyze Keck Hires observations of rotation in F, G, and K dwarf members of the open cluster M34 (NGC 1039), which is 250 Myr old, and we compare them to the Pleiades, Hyades, and NGC 6475. The upper bound to rotation seen in M34 is about a factor of two lower than for the 100 Myr-old Pleiades, but most M34 stars are well below this upper bound, and it is the overall convergence in rotation rates that is most striking. A few K dwarfs in M34 are still rapid rotators, suggesting that they have undergone core-envelope decoupling, followed by replenishment of surface angular momentum from an internal reservoir. Our comparison of rotation in these clusters indicates that the time scale for the coupling of the envelope to the core must be close to 100 Myr if decoupling does, in fact, occur.