2003AJ....126.1996M

Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSAT surveys, a sample of 100 stars most luminous in X-rays within or around a distance of 50 pc is culled. The smallest X-ray luminosity in the sample, in units of 10²⁹ ergs/s, is L_X=9.8; the strongest source in the solar neighborhood is II Peg, a RS CVn star, at L_X=175.8. With respect to the origin of X-ray emission, the sample is divided into partly overlapping classes of pre-main-sequence, post-T Tauri, and very young ZAMS objects (type XY), RS CVn-type binary stars (type RS), other active short-period binaries, including binary BY Dra-type objects (type XO), apparently single or long-period binary active evolved stars (type XG), contact binaries of WU UMa kind (type WU), apparently single or long-period binary variable stars of BY Dra kind (type BY), and objects of unknown nature (type X?). Chromospherically active, short-period binaries (RS and XO) make up 40% of the brightest X-ray emitters, followed by young stars (XY) at 30% and unknown sources (X?) at 15%. The fraction of spectroscopically single evolved X-ray emitters of spectral classes IV and III is quite large (10%). The sources identified as RS CVn-type stars (RS, 23 objects) are considerably stronger in X-ray than the XY-objects and the other active binaries (XO and WU, 20 objects). Seven objects have L_X>100, all RS except one XY, viz., BO Mic. Only five (22%) RS objects have L_X<25, while only three (10%) XY stars have L_X>25. Formally, the limit of L_X=25 could serve as a statistical criterion to differentiate RS and XY stars. However, the other short-period binaries (including eclipsing stars of Algol and β Lyr type) have a distribution of L_Xvery similar to the XY objects. The contact binaries (WU) appear to be much weaker in X-rays than their detached counterparts of RS type, but the sample of the former is too small (three objects) to reach a firm conclusion. Sources matched with giants (either single or in binaries) are found to be significantly harder, with only 7% of hardness ratios below 0, than subgiants (66% of HR1<0) and dwarfs (59% of HR1<0). Almost all objects in the sample are binary or multiple stars; the fraction of components (FC), defined as the total number of components in all binary and multiple systems divided by the sum of the total number of components and single stars, is at least 0.90. The FC for the XY objects reaches 0.81, and for the unknown type 0.89. About 70% of RS objects have also visual or astrometric companions, which makes them hierarchical multiple systems. The RS objects (mostly old, evolved stars) and the XY stars have quite different kinematics. While the RS objects move at considerable velocities in apparently random directions with respect to the local standard of rest, the young stars have smaller and orderly velocities and tend to comprise expanding mini-associations such as the β Pic and the Tucana groups. The majority of the young X-ray active stars belong to the Pleiades stream with the mean heliocentric velocity (U,V,W)=(-9.6,-21.8,-7.7) km/s.