On the relation between activity and rotation in main‐sequence stars

A modified index of the level of stellar chromospheric activity, A ' Call , is proposed. It is calculated from the measured emission flux density in cores of Ca II H and K lines; however, unlike the other known indices, it is free from the influence of the colour of a star. Application of the new index leads to amodified activityrotation relationship in lower main‐sequence stars. It is shown that in the coordinate system log A' Call , log Ro ( Ro being Rossby number) F‐M stars obey a common relationship corresponding to the parameter α = 1.6. The obtained curve shows no evidence for activity saturation for stars with small Rossby numbers, but in slowly rotating stars with thin convection zones the activity level tends to a constant value.