The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs

We present the results of a new study on the relationship between coronal X-ray emission and stellar rotation in late-type main-sequence stars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0, including 110 field stars and 149 members of the Pleiades, Hyades, alpha Persei, IC 2602 and IC 2391 open clusters. All the stars have been observed with ROSAT, and most of them have photometrically-measured rotation periods available. Our results confirm that two emission regimes exist, one in which the rotation period is a good predictor of the total X-ray luminosity, and the other in which a constant saturated X-ray to bolometric luminosity ratio is attained; we present a quantitative estimate of the critical rotation periods below which stars of different masses (or spectral types) enter the saturated regime. In this work we have also empirically derived a characteristic time scale, tau(e), which we have used to investigate the relationship between the X-ray emission level and an X-ray-based Rossby number R-e = P-rot/tau(e): we show that our empirical time scale tau(e) resembles the theoretical convective turnover time for 0.4 less than or similar toM/M-. less than or similar to 1.2, but it also has the same functional dependence on B-V as L-bol(-1/2) in the color range 0.5 less than or similar to B-V less than or similar to 1.5. Our results imply that - for non-saturated coronae - the L-x - P-rot relation is equivalent to the L-x/L-bol vs. R-e relation.