Multiplicity of X-ray selected T Tauri stars in the Scorpius-Centaurus OB association

We report the results of a search for binarity among young stars, performed in the Scorpius-Centaurus OB association on a sample of 118 X-ray selected T Tauri stars. We use speckle interferometry and direct-imaging observations to find companions in the separation range 0.13 – 6. After corrections to account for confusion with background stars and for the bias induced by the X-ray selection, we find a multiplicity (number of binaries or multiples divided by number of systems) of (32.6± 6.1)%, and a number of companions per system of (35.2± 6.3)%. This is higher by a factor of 1.59± 0.34 compared to main-sequence stars, but slightly lower than in a sample in the Taurus-Auriga star-forming region that was selected and studied similary. In Scorpius-Centaurus, we find fewer binaries with nearly equal brightness than in Taurus-Auriga. There are significant differences between the period distributions in the two subgroups Upper Scorpius A and B: The peak of the distribution of stars in US-A is at about 10 days, while that of stars in US-B is around 10 days. We compared our results with the optical multiplicity survey of Brandner et al. (1996), whose sample contains 49 stars that were also observed by us, and find no infrared companions. The flux ratio distributions of close and wide binaries in our sample show no significant difference.