The stellar content of soft X-ray surveys - II. Cross-correlation of the ROSAT All-Sky Survey with the Tycho and Hipparcos catalogs

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 13 875 and 62 00 matches and represent the largest and most comprehensive samples of stel lar X-ray sources constructed so far. The X-ray horizon allows to probe dis tances up to about 200 pc for F - G RasTyc - RasHip stars younger than 100 Myr but only to 80 pc or less for older ones. The magnitude limit of the op tical catalogs determine the horizon for K - hi RasTyc - RasHip stars which are sampled only within about 50 pc (or less) of the Sun whatever their ag es are. We compare the Hipparcos and RasHip HR-diagrams and discuss the dif ferences. X-ray selection strengthens the Zero Age Main Sequence but evolve d 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 Te rminal Age Main Sequence i.e. class III) main sequence regions. Once correc ted 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. Detec tion 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-ra y emission. High mass stars evolving along the post-main sequence evolution ary tracks are clearly detected in the main sequence turnoff and blue part of the "clump" while no significant detection arises on the cool side. Theo retical 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 sequen ce stars in the RasTyc-RasHip samples. Finally we discuss briefly the pendi ng questions for which the RasTyc and RasHip samples are likely to give new insight.