SIMBAD references

We analyse the coronal X-ray emission of single main sequence stars of spectral type F through M with photometrically (CaII H+K or broad-band photometry) determined rotation periods, using X-ray data from the ROSAT all-sky survey. Our sample contains both field stars in the solar neighbourhood and members of the Pleiades and Hyades open clusters. Field stars and members of the two young open clusters follow the same rotation-activity relation, i.e., we find no intrinsic dependence of coronal activity on age. Assuming a power law relationship between coronal X-ray emission and stellar rotation, we estimate a power law index close to unity. With a high level of confidence (α=0.99), we find a qualitative change in behaviour around Rossby number values Ro≃1. For Ro>1, coronal activity drops more rapidly with increasing Rossby number as for Ro<1. Assuming an exponential relation between the L_X/L_bol ratios and Rossby number, Ro≃1/3 is the characteristic Rossby number for a drop of X-ray activity.