SIMBAD references

We present an analysis of the connection between X-ray activity level, photospheric abundance of lithium and surface rotation in late type active main sequence stars (G and K), using the ratio between optical and X-ray luminosity f_X/f_v as a uniform activity indicator. We perform 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 different degrees of statistical correlation in the three samples. In particular, 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 correlated with the activity level. No correlation is evident for the three quantities studied in the active binary sample. We show how the behavior 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 characteristics similar to the Pleiades.