Revisiting Hipparcos data for pre-main sequence stars

We cross-correlate the Herbig & Bell and Hipparcos Catalogues in order to e xtract the results for young stellar objects (YSOs). We compare the distanc es of individual young stars and the distance of their presumably associate d molecular clouds, taking into account post-Hipparcos distances to the rel evant associations and using Hipparcos intermediate astrometric data to der ive new parallaxes of the pre-main sequence stars based on their grouping. We confirm that YSOs are located in their associated clouds, as anticipated by a large body of work, and discuss reasons which make the individual par allaxes of some YSOs doubtful. We find in particular that the distance of T aurus YSOs as a group is entirely consistent with the molecular cloud dista nce, although Hipparcos distances of some faint Taurus-Auriga stars must be viewed with caution. We then improve some of the solutions for the binary and multiple pre-main sequence stars. In particular, we confirm three new a strometric young binaries discovered by Hipparcos: RY Tau, UX Ori, and IX O ph.