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 lamb da 6708 Angstrom line strong in absorption and hence classified as wea k-line T Tauri stars (WTTS) in a previous study. The stars are found t o be widely spread throughout the entire studied area of approximate t o 450 squared degrees. We discuss the broadband UBVRIKC JHKL and narro w-band uvby-beta photometry as well as the spectroscopy of these stars . From the broad-band photometry and spectroscopy we derive the stella r parameters assuming that all stars are located at 460 pc and are phy sically associated with the Orion star forming region (SFR). By compar ison with theoretical pre-main sequence (PMS) evolutionary tracks, all stars can be classified as WTTS with masses raging from 0.8M(circle d ot) to about 3.4M(circle dot) 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 (lam bda 6708 Angstrom) line strength tend to concentrate toward the molecu lar clouds. From the analysis of the uvby-beta photometric data we fin d that part of a subsample of the RASS lithium stars are foreground yo ung stars not associated with Orion. We conclude that the sample of li thium RASS stars in Orion is an admixture of different populations of stars located at different distances, namely: true Orion WTTS and a po pulation of foreground and yet young stars. The latter could be associ ated with the Gould Belt or may be pleiades-age stars.