Magnetic structure in cool stars. XII - Chromospheric activity and rotation of giants and dwarfs

The relation between the Ca II H and K line-core flux density and rotation rate is studied for 313 F- to M-type stars ranging in luminosity class from II-III to V. The activity-rotation relation for Ca II is compared with similar relations for Mg II and soft X-rays. Dwarfs and the majority of the giants studied follow the same color-dependent relation between the Ca II H and K line-core excess flux density and the rotation period P. The activity-rotation relations for the Mg II h and k excess flux density and the soft X-rays flux density are compatible with the color-dependent relations for Ca II. For rapidly rotating stars, excess flux density decreases slowly with increasing P, whereas for longer P the excess flux density decreases much faster with P. The color-dependent shape of this relation helps explain the existence of the Vaughan-Preston gap and the absence of this gap beyond B - V = 0.9.