New low-mass pre-main sequence spectroscopic binaries in Orion

We report the results of a high-resolution spectroscopic monitoring campaign on low-mass pre-main sequence spectroscopic binaries, discovered recently in the general direction of the Orion star-forming region, based on ROSAT all-sky survey X-ray observations. Also included in the present study are two binaries recognized in the course of optical follow-up observations of X-ray sources in a selected sky strip crossing the Orion SFR perpendicular to the galactic plane. Orbital elements for the six double-lined spectroscopic binaries are derived from the analysis of the radial velocities of the components. The orbital periods span from 3 to 47 days. In addition, through a matching of the binary composite spectrum with synthetic binary spectra, we estimate spectral types and luminosity ratios for the components and derive lithium abundances for individual binary components. Using the estimated stellar parameters combined with kinematical information and lithium abundance determinations, we examine the evolutionary status of the objects. We then use the minimum masses derived from the solution of the spectroscopic orbits for the systems of confirmed PMS nature to make comparisons with current theoretical pre-main sequence evolutionary tracks, and attempt to set constraints on some of the most frequently used models.