Chromospheric variations in main-sequence stars.

An analysis is given of fluxes measured in 1-A bands centered on the Ca II H and K lines for a sample of 91 main-sequence stars, extending from about F5 to M2, over time intervals of nine to 11 years. The results are discussed separately for a set of 18 stars with minimal chromospheric fluxes used as standards, the program stars of spectral types F7 to G3, the program stars of types G5 and later, as well as the sun. K/H ratios are determined and compared for the different groups of stars. It is tentatively concluded that: (1) no stellar chromospheres are likely to be constant in time; (2) short-term fluctuations tend to increase in size with average flux; (3) cyclical variations occur with periods ranging from about 7 years to probably at least twice as long; (4) the stellar cycles observed in H and K flux should be regarded as evidence for analogs of the solar cycle; and (5) the incidence of complete or probable partial cycles increases toward later spectral types.