CORONAL X-RAY-EMISSION AND ROTATION OF COOL MAIN-SEQUENCE STARS

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-ban d photometry) determined rotation periods, using X-ray data from the R OSAT all-sky survey. Our sample contains both field stars in the solar neighbourhood and members of the Pleiades and Hyades open clusters. F ield stars and members of the two young open clusters follow the same rotation-activity relation, i.e., we find no intrinsic dependence of c oronal activity on age. Assuming a power law relationship between coro nal X-ray emission and stellar rotation, we estimate a power law index close to unity. With a high level of confidence (alpha = 0.99), we fi nd a qualitative change in behaviour around Rossby number values Ro ap proximate to 1. For Ro > 1, coronal activity drops more rapidly with i ncreasing Rossby number as for Ro < 1. Assuming an exponential relatio n between the L(X)/L(bol) ratios and Rossby number, Ro approximate to 1/3 is the characteristic Rossby number for a drop of X-ray activity.