Magnetic structure in cool stars. IV - Rotation and CA II H and K emission of main-sequence stars

The measured Ca II H and K emission indices of stars are converted to surface fluxes (F_H + F_K). This conversion largely eliminates the dependence on spectral type in the relation between the uncorrected Ca II H and K emission and the rotational velocity of a star which has been found by Vaughan et al. This relation holds for single main-sequence stars as well as for short-period binaries indicating that the enhanced emission in short-period binaries is a result of rapid rotation enforced by tidal coupling. A plot of surface fluxes against υ sin i values suggests a color-dependent discontinuity in the relation. This discontinuity may explain the two branches in (F_H + F_K) for (B-V) < 1.00 among main-sequence stars. From the relation between (F_H + F_K) and rotational velocity it is deduced that the average rotational velocity of late-type main-sequence Hyades decreases with decreasing effective temperature.