Time evolution of the magnetic activity cycle period

We propose a new interpretation of the relationships between the dynamo cycle period (P-cyc) as observed in Ca II H and K, the rotational period (P-rot), the activity level, and other stellar properties. Viewed within this framework, the data suggest that the dynamo alpha-parameter increases with magnetic field strength, contrary to the conventional idea of alpha-quenching. The data also suggest a possibly discontinuous dependence of the ratio of cycle to rotation frequency, omega(cyc)/Omega, as a function of Rossby number, Ro (or equivalently, activity or age). Stars evolve with omega(cyc)/Omega proportional to t(-0.35) (or RO-0.7), Until age t approximate to 2-3 Gyr (roughly at the Vaughan-Preston gap), where a sharp transition occurs, in which omega(cyc)/Omega increases by a factor of approximate to 6. Thereafter, evolution with omega(cyc)/Omega proportional to t(-0.35) continues. The age at which transition occurs may be mass dependent, with K stars making the transition first.