SIMBAD references

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, α 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, τ_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/τ_e: we show that our empirical time scale τ_e resembles the theoretical convective turnover time for 0.4≲M/M_☉≲1.2, but it also has the same functional dependence on B-V as L_bol^–1/2 in the color range 0.5≲B-V≲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.