MULTIPLICITY AMONG T-TAURI STARS IN OB-ASSOCIATIONS AND T-ASSOCIATIONS - IMPLICATIONS FOR BINARY STAR-FORMATION

We present first results of a survey for companions among X-ray select ed 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 opt ical follow-up observations of sources from the ROSAT All Sky Survey a ssociated with star forming regions. The areas surveyed include the T associations of Chamaeleon and Lupus as well as Upper Scorpius, the la tter being part of the Scorpius Centaurus OB association (Sco OB 2). U sing SUSI at the NTT under subarcsec seeing conditions we observed 195 T Tauri stars through a 1 mu m (''Z'') filter and identified companio ns to 31 of them (among these 12 subarcsec binaries). Based on statist ical arguments we conclude that almost all of them are indeed physical (i.e. gravitationally bound) binary or multiple systems. For 10 syste ms located in Upper Scorpius and Lupus, we additionally obtained spati ally resolved near-infrared photometry in the J, H, and K bands with t he MPIA 2.2m telescope at ESO, La Silla. The near-infrared colours of the secondaries are consistent with those of dwarfs and are clearly di stinct from those of late type giant stars. Based on astrometric measu rements of some binaries we show that the components of these binaries are common proper motion pairs, very likely in a gravitationally boun d orbit around each other. We find that the overall binary frequency a mong T Tauri stars in a range of separations between 120 and 1800 AU i s in agreement with the binary frequency observed among main sequence stars in the solar neighbourhood. However, we note that within individ ual regions the spatial distribution of binaries within a distinct ran ge of separation - is non-uniform. In particular, in Upper Scorpius, W TTS in the vicinity of early type stars seem to be almost devoid of mu ltiple 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 brightne ss differences between primary and companion stars (median Delta Z = 1 .(m)0...1.(m)5). We conclude that binarity is established very early i n stellar evolution, that the orbital parameters of wide binaries (a g reater than or equal to 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.