An activity-rotation relationship in F-M dwarfs from MG II H and K flux.

Abstract

Mg II h and k fluxes are given for late-type dwarfs whose rotation periods are known. A good correlation between an activity parameter R_hk (represented by the ratio of Mg II h and k luminosity to bolometric luminosity) versus the Rossby-number is found for main-sequence stars F to late M. Both single and binary stars are found to follow the same relation. A saturation of R_hk occurs at rotation periods of approximately 4 day, it is suggested that stars whose rotation period is close to or shorter than this value are completely covered by active plage regions, although an alternative explanation may be saturation of the dynamo itself.