CHROMOSPHERIC VARIATIONS IN MAIN-SEQUENCE STARS .2.

The fluxes in passbands 0.1 nm wide and centered on the Ca II H and K emission cores have been monitored in 111 stars of spectral type F2-M2 on or near the main sequence in a continuation of an observing progra m started by O. C. Wilson. Most of the measurements began in 1966, wit h observations scheduled monthly until 1980, when observations were sc heduled several times per week. The records, with a long-term precisio n of about 1.5%, display fluctuations that can be identified with vari ations on timescales similar to the 11 yr cycle of solar activity as w ell as axial rotation, and the growth and decay of emitting regions. W e present the records of chromospheric emission and general conclusion s about variations in surface magnetic activity on timescales greater than 1 yr but less than a few decades. The results for stars of spectr al type G0-K5 V indicate a pattern of change in rotation and chromosph eric activity on an evolutionary timescale, in which (1) young stars e xhibit high average levels of activity, rapid rotation rates, no Maund er minimum phase and rarely display a smooth, cyclic variation; (2) st ars of intermediate age (approximately 1-2 Gyr for 1 M.) have moderate levels of activity and rotation rates, and occasional smooth cycles; and (3) stars as old as the Sun and older have slower rotation rates, lower activity levels, and smooth cycles with occasional Maunder minim um-phases.