LITHIUM, X-RAY ACTIVITY AND ROTATION IN AN X-RAY SELECTED SAMPLE OF SOLAR-TYPE STARS

We present an analysis of the connection between X-ray activity level, photospheric abundance of lithium and surface rotation in late type a ctive main sequence stars (G and K), using the ratio between optical a nd X-ray luminosity f(X)/f(v) as a uniform activity indicator. We perf orm this analysis for a sample of X-ray selected sources from Einstein -based surveys compared to stars from the Pleiades open cluster and to a sample of active binary stars. We show that these parameters show d ifferent degrees of statistical correlation in the three samples. In p articular, the Pleiades sample shows a significant correlation between all three quantities, while in the X-ray selected sample lithium and rotation are significantly correlated with each other but neither is c orrelated with the activity level. No correlation is evident for the t hree quantities studied in the active binary sample. We show how the b ehavior of the X-ray selected sample can be used to discriminate among different hypotheses about the nature of the so-called ''yellow star excess'' observed in X-ray flux-limited surveys, showing that this is composed by a population of young, near ZAMS stars with characteristic s similar to the Pleiades.