Spectroscopic binaries in a sample of ROSAT X-ray sources south of the taurus molecular clouds

We report the results of our radial velocity monitoring of spectroscopic binary systems in a sample of X-ray sources from the ROSAT All Sky Survey south of the Taurus-Auriga star- forming region. The original sample of similar to120 sources by Neuhauser et al. was selected on the basis of their X-ray properties and the visual magnitude of the nearest optical counterpart, in such a way as to promote the inclusion of young objects. Roughly 20% of those sources have previously been confirmed to be very young. We focus here on the subset of the original sample that shows variable radial velocities ( 43 objects), a few of which have also been flagged previously as being young. New spectroscopic orbits are presented for 42 of those systems. Two of the binaries, RX J0528.9+1046 and RX J0529.3+1210, are indeed weak-lined T Tauri stars likely to be associated with the lambda Ori region. Most of the other binaries are active objects of the RS Canum Venaticorum type, including several W Ursae Majoris and Algol systems. We detect a strong excess of short-period binaries compared with the field and an unusually large fraction of double-lined systems. This, along with the overall high frequency of binaries out of the original sample of similar to120 sources, can be understood as a selection effect, since all these properties tend to favor the inclusion of the objects in a flux-limited X-ray survey such as this by making them brighter in X-rays. A short description of the physical properties of each binary is provided, and a comparison with evolutionary tracks is made using the stellar density as a distance-independent measure of evolution. We rely for this on our new determinations of the effective temperature and projected rotational velocities of all visible components of the binaries. A number of the systems merit follow-up observations, including at least four confirmed or probable eclipsing binaries. One of these, RX J0239.1-1028, consists of a pair of detached K dwarfs and may provide for a potentially important test of stellar evolution models once the absolute dimensions of the components are determined