VISUAL BINARIES AMONG PRE-MAIN-SEQUENCE STARS

In an effort to determine statistical properties of visual pre-main se quence (PMS) binaries, we have carried out an extensive systematic CCD imaging survey of young low-mass stars in nearby southern dark clouds , all at about 150 pc distance. The observations were made with the ES O New Technology Telescope at a wavelength of 0.9 mum under sub-arcsec ond seeing conditions. Among the 238 young stars systematically observ ed, we detect 37 binaries (22 new discoveries) and 1 triple system (Sz 30, also new) in the range of separations between 1'' and 12''. Furth er binaries were identified outside these limits, including 6 sub-arcs ec binaries. The lower limit of 1'' was imposed to ensure completeness of our sample, which we have tested through numerical simulations. We show that the upper limit is small enough that we have essentially no contamination by fore- and background stars. Thus, we find a frequenc y of 16% for PMS binaries with projected separations between about 150 AU and 1800 AU. The distribution of separations is a steeply rising f unction towards smaller separations. We compare these observations wit h the known main sequence (MS) distributions of G and K dwarfs in the same interval of separations, and find an apparent excess of PMS binar ies. Under the assumption that total PMS and MS binary frequencies sho uld be the same, the excess may imply that binary orbits undergo secul ar evolution towards the main sequence. The histogram of flux-ratios a t 0.9 mum shows a gradual increase towards unequal components. We find very few binaries with components of nearly equal brightness. We disc uss the difficulties involved in translating this flux-ratio distribut ion for PMS binaries into a distribution of mass-ratios. There is no s ignificant dependence of flux-ratio on the separation of the component s. We briefly discuss implications of our results for the star formati on efficiency and the initial mass function. Our data suggest that in T associations binary star formation appears to be the rule, while the formation of single stars is probably the exception. Our distance-lim ited systematic survey sample is actually a sub-sample of our total su rvey sample which comprised most known PMS objects accessible from the southern hemisphere except Orion objects. We have in the same way sea rched for multiplicity in this extended sample. As a result, we have a dditionally discovered another 18 new PMS binaries and have reobserved another 32 already known PMS binaries. Therefore, in total we here pr ovide information (component separations, brightness-ratios, and posit ion angles) on 87 PMS binaries and 1 triple, approximately half of whi ch are newly discovered.