ROSAT all-sky survey of W Ursae Majoris stars and the problem of supersaturation

From ROSAT all-sky survey (RASS) data we obtained X-ray fluxes for 57 W UMa type contact systems. In our sample we detected three stars which are the shortest period main sequence binaries ever found as X-ray sources. For sta rs with (B - V)(0) < 0.6 the normalized X-ray flux decreases with a decreas ing color index but for (B - V)(0) > 0.6 a plateau is reached, similar to t he saturation level observed for single, rapidly rotating stars. The X-ray flux of W UMa stars is about 4-5 times weaker than that of the fastest rota ting single stars. Because early type, low activity variables have longer p eriods, an apparent period-activity relation is seen among our stars, while cool stars with (B - V)(0) > 0.6 and rotation periods between 0.23 and 0.4 5 days do not show any such relation. The lower X-ray emission of the singl e, ultra fast rotators (UFRs) and W UMa stars is interpreted as the result of a decreased coronal filling factor. The physical mechanisms responsible for the decreased surface coverage differs for UFRs and W UMa systems. For UFRs we propose strong polar updrafts within a convection zone, driven by n onuniform heating from below. The updrafts should be accompanied by large s cale poleward flows near the bottom of the convective layer and equatorward flows in the surface layers. The flows drag dynamo generated fields toward the poles and create a field-free equatorial region with a width depending on the stellar rotation rate. For W UMa stars we propose that a large scal e horizontal flow embracing both stars will prevent the magnetic field from producing long-lived structures filled with hot X-ray emitting plasma. The decreased activity of the fastest rotating UFRs increases the angular mome ntum loss time scale of stars in a supersaturated state. Thus the existence of a period cutoff and a limiting mass of W UMa stars can be naturally exp lained.