On the nature of the ROSAT X-ray selected weak-line T Tauri stars in Orion

We analyse the nature of the ROSAT all-sky survey (RASS) X-ray sources in the direction of Orion identified with stars showing the Li I lambdambda \:6708 Angstroms line strong in absorption and hence classified as weak-line T Tauri stars (WTTS) in a previous study. The stars are found to be widely spread throughout the entire studied area of ~ 450 squared degrees. We discuss the broad-band UBVRI_KC JHKL and narrow-band uvby-beta photometry as well as the spectroscopy of these stars. From the broad-band photometry and spectroscopy we derive the stellar parameters assuming that all stars are located at 460 pc and are physically associated with the Orion star forming region (SFR). By comparison with theoretical pre-main sequence (PMS) evolutionary tracks, all stars can be classified as WTTS with masses raging from 0.8M_sun to about 3.4M_sun and ages from 2x10(5) yr to 7x10(6) yr. We do not find any correlation between the spatial distribution and age or any other stellar parameter if the above distance for all the stars is assumed. We do find, however, that the stars with higher Li I (lambdambda 6708 Angstroms) line strength tend to concentrate toward the molecular clouds. From the analysis of the uvby-beta photometric data we find that part of a subsample of the RASS lithium stars are foreground young stars not associated with Orion. We conclude that the sample of lithium RASS stars in Orion is an admixture of different populations of stars located at different distances, namely: true Orion WTTS and a population of foreground and yet young stars. The latter could be associated with the Gould Belt or may be pleiades-age stars.