A comprehensive census of the stellar content of the OB associations within 1 kpc from the Sun is presented, based on Hipparcos positions, proper motions, and parallaxes. It is a key part of a long-term project to study the formation, structure, and evolution of nearby young stellar groups and related star-forming regions.OB associations are unbound 'moving groups', which can be detected kinematically because of their small internal velocity dispersion. The nearby associations have a large extent on the sky, which traditionally has limited astrometric membership determination to bright stars (V ∼ < 6 m ), with spectral types earlier than ∼B5. The Hipparcos measurements allow a major improvement in this situation. Moving groups are identified in the Hipparcos Catalogue by combining de Bruijne's refurbished convergent point method with the 'Spaghetti method' of Hoogerwerf & Aguilar. Astrometric members are listed for 12 young stellar groups, out to a distance of ∼650 pc. These are the 3 subgroups Upper Scorpius, Upper Centaurus Lupus and Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2, α Persei (Per OB3), Cas-Tau, Lac OB1, Cep OB2, and a new group in Cepheus, designated as Cep OB6. The selection procedure corrects the list of previously known astrometric and photometric B-and A-type members in these groups, and identifies many new members, including a significant number of F stars, as well as evolved stars, e.g., the Wolf-Rayet stars γ 2 Vel (WR11) in Vel OB2 and EZ CMa (WR6) in Col 121, and the classical Cepheid δ Cep in Cep OB6. Membership probabilities are given for all selected stars. Monte Carlo simulations are used to estimate the expected number of interloper field stars. In the nearest associations, notably in Sco OB2, the later-type members include T Tauri objects and other stars in the final pre-main sequence phase. This provides a firm link between the classical high-mass stellar content and ongoing low-mass star formation. Detailed studies of these 12 groups, and their relation to the surrounding interstellar medium, will be presented elsewhere.Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive. OB associations do exist in many of these regions, but they are either at distances beyond ∼500 pc where the Hipparcos parallaxes are of limited use, or they have unfavorable kinematics, so that the group proper motion does not distinguish it from the field stars in the Galactic disk.The mean distances of the well-established groups are systematically smaller than the pre-Hipparcos photometric estimates. While part of this may be caused by the improved membership lists, a recalibration of the upper main sequence in the Hertzsprung-Russell diagram may be called for. The mean motions display a systematic pattern, which is discussed in relation to the Gould Belt.Six of the 12 detected moving groups do not appear in the classical list of nearby OB associations. This is sometim...

Abstract. We use milli-arcsecond accuracy astrometry (proper motions and parallaxes) from Hipparcos and from radio observations to retrace the orbits of 56 runaway stars and nine compact objects with distances less than 700 pc, to identify the parent stellar group. It is possible to deduce the specific formation scenario with near certainty for two cases. (i) We find that the runaway star ζ Ophiuchi and the pulsar PSR J1932+1059 originated about 1 Myr ago in a supernova explosion in a binary in the Upper Scorpius subgroup of the Sco OB2 association. The pulsar received a kick velocity of ∼350 km s −1 in this event, which dissociated the binary, and gave ζ Oph its large space velocity. (ii) Blaauw & Morgan and Gies & Bolton already postulated a common origin for the runaway-pair AE Aur and µ Col, possibly involving the massive highly-eccentric binary ι Ori, based on their equal and opposite velocities. We demonstrate that these three objects indeed occupied a very small volume ∼2.5 Myr ago, and show that they were ejected from the nascent Trapezium cluster. We identify the parent group for two more pulsars: both likely originate in the ∼50 Myr old association Per OB3, which contains the open cluster α Persei. At least 21 of the 56 runaway stars in our sample can be linked to the nearby associations and young open clusters. These include the classical runaways 53 Arietis (Ori OB1), ξ Persei (Per OB2), and λ Cephei (Cep OB3), and fifteen new identifications, amongst which a pair of stars running away in opposite directions from the region containing the λ Ori cluster. Other currently nearby runaways and pulsars originated beyond 700 pc, where our knowledge of the parent groups is very incomplete.

Abstract. Hipparcos parallaxes fix distances to individual stars in the Hyades cluster with an accuracy of ∼6 percent. We use the Hipparcos proper motions, which have a larger relative precision than the trigonometric parallaxes, to derive ∼3 times more precise distance estimates, by assuming that all members share the same space motion. An investigation of the available kinematic data confirms that the Hyades velocity field does not contain significant structure in the form of rotation and/or shear, but is fully consistent with a common space motion plus a (one-dimensional) internal velocity dispersion of ∼0.30 km s −1 . The improved parallaxes as a set are statistically consistent with the Hipparcos parallaxes. The maximum expected systematic error in the proper motion-based parallaxes for stars in the outer regions of the cluster (i.e., beyond ∼2 tidal radii ∼20 pc) is < ∼ 0.30 mas. The new parallaxes confirm that the Hipparcos measurements are correlated on small angular scales, consistent with the limits specified in the Hipparcos Catalogue, though with significantly smaller "amplitudes" than claimed by Narayanan & Gould. We use the Tycho-2 long time-baseline astrometric catalogue to derive a set of independent proper motion-based parallaxes for the Hipparcos members. The new parallaxes provide a uniquely sharp view of the three-dimensional structure of the Hyades. The colour-absolute magnitude diagram of the cluster based on the new parallaxes shows a well-defined main sequence with two "gaps"/"turn-offs". These features provide the first direct observational support of Böhm-Vitense's prediction that (the onset of) surface convection in stars significantly affects their (B − V ) colours. We present and discuss the theoretical Hertzsprung-Russell diagram (log L versus log T eff ) for an objectively defined set of 88 high-fidelity members of the cluster as well as the δ Scuti star θ 2 Tau, the giants δ 1 , θ 1 , , and γ Tau, and the white dwarfs V471 Tau and HD 27483 (all of which are also members). The precision with which the new parallaxes place individual Hyades in the Hertzsprung-Russell diagram is limited by (systematic) uncertainties related to the transformations from observed colours and absolute magnitudes to effective temperatures and luminosities. The new parallaxes provide stringent constraints on the calibration of such transformations when combined with detailed theoretical stellar evolutionary modelling, tailored to the chemical composition and age of the Hyades, over the large stellar mass range of the cluster probed by Hipparcos.

Milli-arcsecond astrometry provided by Hipparcos and by radio observations makes it possible to retrace the orbits of some of the nearest runaway stars and pulsars to determine their site of origin. The orbits of the runaways AE Aurigae and µ Columbae and of the eccentric binary ι Orionis intersect each other ∼2.5 Myr ago in the nascent Trapezium cluster, confirming that these runaways were formed in a binary-binary encounter. The path of the runaway star ζ Ophiuchi intersects that of the nearby pulsar PSR J1932+1059, ∼1 Myr ago, in the young stellar group Upper Scorpius. We propose that this neutron star is the remnant of a supernova that occurred in a binary system which also contained ζ Oph, and deduce that the pulsar received a kick velocity of ∼ 350 km s −1 in the explosion. These two cases provide the first specific kinematic evidence that both mechanisms proposed for the production of runaway stars, the dynamical ejection scenario and the binary-supernova scenario, operate in nature.

Abstract. Based on its Hipparcos proper motion, we propose that the high-mass X-ray binary HD153919/4U1700-37 originates in the OB association Sco OB1. At a distance of 1.9 kpc the space velocity of 4U1700-37 with respect to Sco OB1 is 75 km s −1 . This runaway velocity indicates that the progenitor of the compact X-ray source lost about 7 M during the (assumed symmetric) supernova explosion. The system's kinematical age is about 2 ± 0.5 million years which marks the date of the supernova explosion forming the compact object. The present age of Sco OB1 is < ∼ 8 Myr; its suggested core, NGC 6231, seems to be somewhat younger (∼ 5 Myr). If HD153919/4U1700-37 was born as a member of Sco OB1, this implies that the initially most massive star in the system terminated its evolution within < ∼ 6 million years, corresponding to an initial mass > ∼ 30 M . With these parameters the evolution of the binary system can be constrained.