CORONAL X-RAY-EMISSION OF COOL STARS IN RELATION TO CHROMOSPHERIC ACTIVITY AND MAGNETIC CYCLES

We study the relationship between the coronal X-ray emission of single , main-sequence F-K stars and the characteristics of their magnetic cy cles. 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 pro,gram. Accordi ng to their CaII H+K long-term variability characteristics, we divide the stars into three groups: non-variable, regular variable and irregu lar (chaotic) variable stars. We show that the regular and the irregul ar stars differ mainly in their Rossby-numbers (Ro): regular stars hav e almost always Ro < 1 whereas the irregular group is characterized by Ro > 1, further, the X-ray surface flux distributions differ signific antly between these three groups. We discuss to what extent stars exhi biting constant Ca II fluxes can be considered ''Maunder minimum'' sta rs, and demonstrate - in a statistical sense - that cyclic chromospher ic 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 th e constant stars on one hand and the irregular stars on the other hand . Performing regression analysis in the form of a power law, the coeff icient kappa is derived to be kappa approximate to 1 for constant and regular stars whereas kappa approximate to 2 for the more active irreg ular stars. We discuss our findings in the context of a transition fro m a nonlinear to a linear dynamo regime when going from irregular to r egular stars.