1996A&A...307..121B

We present first results of a survey for companions among X-ray selected pre-main sequence stars, most of them being weak-line T Tauri stars (WTTS). These T Tauri stars have been identified in the course of optical follow-up observations of sources from the ROSAT All Sky Survey associated with star forming regions. The areas surveyed include the T associations of Chamaeleon and Lupus as well as Upper Scorpius, the latter being part of the Scorpius Centaurus OB association (Sco OB 2). Using SUSI at the NTT under subarcsec seeing conditions we observed 195 T Tauri stars through a 1µm ("Z") filter and identified companions to 31 of them (among these 12 subarcsec binaries). Based on statistical arguments we conclude that almost all of them are indeed physical (i.e. gravitationally bound) binary or multiple systems. For 10 systems located in Upper Scorpius and Lupus, we additionally obtained spatially resolved near-infrared photometry in the J, H, and K bands with the MPIA 2.2m telescope at ESO, La Silla. The near-infrared colours of the secondaries are consistent with those of dwarfs and are clearly distinct from those of late type giant stars. Based on astrometric measurements of some binaries we show that the components of these binaries are common proper motion pairs, very likely in a gravitationally bound orbit around each other. We find that the overall binary frequency among T Tauri stars in a range of separations between 120 and 1800 AU is in agreement with the binary frequency observed among main sequence stars in the solar neighbourhood. However, we note that within individual regions the spatial distribution of binaries - within a distinct range of separation - is non-uniform. In particular, in Upper Scorpius, WTTS in the vicinity of early type stars seem to be almost devoid of multiple systems, whereas in another area in Upper Scorpius half of all WTTS have a companion in a range of separation between 0.7" and 3.0." Furthermore, we find no preponderance of systems with large brightness differences between primary and companion stars (median {DELTA}Z=1.0mag ...1.5).mag We conclude that binarity is established very early in stellar evolution, that the orbital parameters of wide binaries (a≥120AU) remain virtually unchanged during their pre-main sequence evolution, and that these wide binaries were formed either through collisional fragmentation or fragmentation of rotating filaments.