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 α-parameter increases with magnetic field strength, contrary to the conventional idea of α-quenching. The data also suggest a possibly discontinuous dependence of the ratio of cycle to rotation frequency, ω cyc /Ω, as a function of Rossby number, Ro (or equivalently, activity or age). Stars evolve with ω cyc /Ω ∝ t −0.35 (or Ro -0.7 ), until age t ≈ 2-3 Gyr (roughly at the Vaughan-Preston gap), where a sharp transition occurs, in which ω cyc /Ω increases by a factor of ≈ 6. Thereafter, evolution with ω cyc /Ω ∝ t −0.35 continues. The age at which transition occurs may be mass dependent, with K stars making the transition first.