1999A&A...351.1003G

We present the result of the cross-correlation of the ROSAT All-Sky Survey with the Tycho and Hipparcos catalogs. The constructed RASS-Tycho (RasTyc) and RASS-Hipparcos (RasHip) samples respectively consist of 13875 and 6200 matches and represent the largest and most comprehensive samples of stellar X-ray sources constructed so far. The X-ray horizon allows to probe distances up to about 200pc for F-G RasTyc-RasHip stars younger than 100Myr but only to 80pc or less for older ones. The magnitude limit of the optical catalogs determine the horizon for K-M RasTyc-RasHip stars which are sampled only within about 50pc (or less) of the Sun whatever their ages are. We compare the Hipparcos and RasHip HR-diagrams and discuss the differences. X-ray selection strengthens the Zero Age Main Sequence but evolved stars are detected as well. We compute detection rate, mean Fx/Fopt and X-ray luminosity with an unprecedented color bin resolution for on (between the Zero and Terminal Age Main Sequence i.e. class V) and off (above the Terminal Age Main Sequence i.e. class III) main sequence regions. Once corrected for Fx/Fopt bias, the detection rate is remarkably constant for G-M on main sequence stars but reveals a peak of detection for F-type stars. Detection rate in the A-type stars region is compatible with those computed for F-M stars, as expected if a late type companion is responsible for the X-ray emission. High mass stars evolving along the post-main sequence evolutionary tracks are clearly detected in the main sequence turnoff and blue part of the ``clump" while no significant detection arises on the cool side. Theoretical considerations naturally explain these observations. We address the question of the presence of very young stars in the solar neighborhood and derive an upper limit on the number of ``possible" isolated pre-main sequence stars in the RasTyc-RasHip samples. Finally we discuss briefly the pending questions for which the RasTyc and RasHip samples are likely to give new insight.