1996A&A...305..284H

We study the relationship between the coronal X-ray emission of single, main-sequence F-K stars and the characteristics of their magnetic cycles. We use X-ray data primarily from the ROSAT all-sky survey (RASS) as well as data acquired by us in the ROSAT pointed program, and the published data of the Mt. Wilson CaII H+K monitoring program. According to their CaII H+K long-term variability characteristics, we divide the stars into three groups: non-variable, regular variable and irregular (chaotic) variable stars. We show that the regular and the irregular stars differ mainly in their Rossby-numbers (Ro): regular stars have almost always Ro<1 whereas the irregular group is characterized by Ro>1; further, the X-ray surface flux distributions differ significantly between these three groups. We discuss to what extent stars exhibiting constant Ca II fluxes can be considered "Maunder minimum" stars, and demonstrate - in a statistical sense - that cyclic chromospheric activity also implies cyclic coronal activity. From a reanalysis of the flux-flux relation between the calcium excess flux density ({DELTA}F_Ca) and F_X, we find different relations between the regular and the constant stars on one hand and the irregular stars on the other hand. Performing regression analysis in the form of a power law, the coefficient κ is derived to be κ≃1 for constant and regular stars whereas κ≃2 for the more active irregular stars. We discuss our findings in the context of a transition from a nonlinear to a linear dynamo regime when going from irregular to regular stars.