A SYSTEMATIC SEARCH FOR YOUNG BINARIES IN TAURUS

We surveyed all those young stars in the Taurus star forming region fo r duplicity which are contained in the Herbig-Bell catalogue and are b righter than K = 9.5 mag, in total a sample of 104 systems. The range in separation observed is 0.13'' - 13'',with the lower limit given by the diffraction limit of the telescope and the upper limit by confusio n due to the surface density of background stars. Within this range we found 39 systems to be double (of which 24 are new detections), 3 tri ples and 2 quadruples. The resulting degree of multiplicity of 44/104 = 42 +/- 6% is higher than found for main sequence solar type field st ars for the same interval of separation (Duquennoy & Mayor 1991) by a factor of 1.9 +/- 0.3. We suggest two possible lines of interpretation : If the distribution with separation is similar for young stars and f or main sequence stars, this result is compatible with the assumption that all young stars are born double. On the other hand, if one assume s that the total degree of duplicity is about the same in both the sam ple of main sequence stars and in the sample of pre-main sequence star s, then the distribution with separation must be quite different for t hese two samples. In either case we have evidence for evolution of the binary properties before the systems reach the main sequence. However , these conclusions hold only if our census in Taurus is typical for t he average star forming region. In our sample, multiplicity occurs wit h the same frequency for weak-lined as for classical T Tauri stars.