Magnetic strucutre in cool stars. IV. CA II and K fluxes from evolved stars.

Quantitative measurements of the Ca II H and K flux of 335 evolved stars are presented and discussed. The results show that there is a large spread in the fluxes from stars with (B-V) less than 0.95 while the Ca II H and K flux of single stars with (B-V) greater than 0.95 correlates with color with little spread. Short-period binaries show a relatively high Ca II H and K flux indicating that high fluxes result from rapid rotation independent of spectral type. The data are consistent with the hypothesis that the emission depends on dynamo action in the convective envelope, the dynamo efficiency decreasing with decreasing rotation rate. The evolution of the emission is discussed as a function of stellar mass. It is shown that stars which leave the main sequence with relatively low or high rotational velocities show relatively low or high emission values, respectively. The flux lasts up to higher (B-V) values for progressively higher masses.