Revisiting Hipparcos data for pre-main sequence stars

We cross-correlate the Herbig & Bell and Hipparcos Catalogues in order to extract the results for young stellar objects (YSOs). We compare the distances of individual young stars and the distance of their presumably associated molecular clouds, taking into account post-Hipparcos distances to the relevant associations and using Hipparcos intermediate astrometric data to derive 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 parallaxes of some YSOs doubtful. We find in particular that the distance of Taurus YSOs as a group is entirely consistent with the molecular cloud distance, 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 astrometric young binaries discovered by Hipparcos: RY Tau, UX Ori, and IX Oph.