Time Evolution of the Magnetic Activity Cycle Period

Abstract

We propose a new interpretation of the relationships between the dynamo cycle period (Pcyc,) as observed in Ca II H and K, the rotational period (Prott), the activity level, and other stellar properties. Viewed within this framework, the data suggest that the dynamo ?-parameter increases with magnetic field strength, contrary to the conventional idea of ?-quenching. The data also suggest a possibly discontinuous dependence of the ratio of cycle to rotation frequency, ?cyc/?, as a function of Rossby number, Ro (or equivalently, activity or age). Stars evolve with ?cyc/? ? 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 ?cyc/? increases by a factor of ?6. Thereafter, evolution with ?cyc/? ? t-0.35 continues. The age at which transition occurs may be mass dependent, with K stars making the transition first. ? 1998. The American Asironomical Socieiy. All rights reserved.