5. Young stellar groups and OB associations

De Bruijne, in collaboration with Hoogerwerf and de Zeeuw, continued his work on secular parallaxes for the members of nearby young stellar groups. Secular parallaxes are determined using the ``known'' space motion of a moving group and the observed proper motions of its members. The relative accuracy of the secular parallaxes is of the same order as that of the proper motions, and can thus be larger than the relative accuracy of individual Hipparcos trigonometric parallaxes.

The secular parallaxes for stars in the nearby Scorpius OB2 association are $\sim$2 times more precise than the Hipparcos trigonometric parallaxes. The enhanced parallax precision significantly narrows the locus of stars in the colour-absolute magnitude diagram, and allows to resolve the parallax distribution, and thus the spatial structure, of the different subgroups. Secular parallaxes for members of the Hyades cluster are $\sim$3 times more precise than the Hipparcos parallaxes. The ACT (the Astrographic and Tycho Reference Catalogue) and TRC (Tycho Reference Catalogue) proper motions of faint candidate members not observed by Hipparcos allow the derivation of high-precision secular parallaxes, which provides the unique opportunity to locate these stars on the Hertzsprung-Russell diagram. The new parallaxes provide the sharpest view of the three-dimensional structure of the Hyades obtained ever. The colour-absolute magnitude diagram shows a very well-defined main sequence, which provides the first unambiguous observational evidence of Böhm-Vitense's prediction that the onset of surface convection in stars significantly affects their broad-band colours. The precision with which the new parallaxes constrain the location of individual members of the cluster on the theoretical Hertzsprung-Russell diagram is now limited by (systematic) uncertainties related to the transformations from observed colours and absolute magnitudes to effective temperatures and luminosities.

Two new major astrometric catalogues (the TRC and ACT) were used by Hoogerwerf to extend the membership lists of the nearby OB associations as determined by de Zeeuw et al. (1999). These catalogues are complete to V = 10.5 mag, three magnitudes fainter than the Hipparcos Catalogue. The study resulted in candidate membership lists for the nearest associations (Sco OB2 and Per OB3) with a field-star contamination less than 50%. These lists thus form an excellent starting point for follow-up observations. Furthermore, Hoogerwerf in collaboration with Blaauw also investigated the quality of the proper motions in the TRC and ACT catalogues. They find that in general the ACT and TRC proper motions are free of systematic errors except for a fraction of the ACT stars. The proper-motion errors of the latter are underestimated by 30-40%.

Hoogerwerf, de Bruijne, and de Zeeuw continued the study of the nearest OB runaway stars, started by den Hollander last year. The orbits of runaways, present in the Hipparcos Catalogue, were traced back in time to determine their parent groups and their runaway ages. This procedure is possible only because of the milli-arcsecond accuracy proper motions of Hipparcos. They confirmed the proposition by Blaauw & Morgan that the runaways AE Aur and $\mu$ Col originated from the same physical event. These runaways were created by a dynamical encounter in the Trapezium cluster, $\sim$2.5 Myr ago, which also involved the presently massive, highly eccentric binary $\iota$ Ori. Furthermore, retracing the orbit of the runaway $\zeta$ Oph and the orbits of some of the local pulsars Hoogerwerf et al. found that PSR J1932+1059 and $\zeta$ Oph were once two components of the same binary system in the Upper Scorpius association. The two component became separated when the primary exploded as a supernova and the binary became unbound. These two examples prove that both scenario's for the production of runaway stars, the binary supernova scenario and the dynamical ejection scenario, operate.