Multiplicity of X-Ray Selected T Tauri Stars in Chamaeleon

We report on a multiplicity survey of a sample of X-ray selected young stars in the Chamaeleon association. We used speckle-interferometry and direct imaging to find companions in the separation range 0.13" to 6". After correction for chance alignment with background stars, we find a multiplicity (number of binaries or multiples divided by number of systems) of (14.0±4.3)% and a companion star frequency (number of companions divided by number of systems) of (14.7±5.1)%. Compared to solar-type main-sequence stars, the companion star frequency is lower by a factor of 0.61±0.27. This is remarkably different from the high multiplicity found in the Taurus-Auriga star-forming region and for T Tauri stars in Chamaeleon known before ROSAT. We find only a few binaries with projected separations of more than 70 AU, also in contrast to the results for stars known before ROSAT. This indicates that the X-ray selected stars belong to a different population than the stars known before ROSAT, a hypothesis further supported by their Hipparcos distances and proper motions.