THE ASTRONOMICAL JOURNAL, 117:354È399, 1999 January ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. A HIPPARCOS CENSUS OF THE NEARBY OB ASSOCIATIONS1 P. T. DE ZEEUW,R.HOOGERWERF, AND J. H. J. DE BRUIJNE Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands A. G. A. BROWN Instituto deAstronom• aMe U.N.A.M., Apartado Postal 877, Ensenada, 22800 Baja California, xico AND A. BLAAUW Kapteyn Instituut, Postbus 800, 9700 AV Groningen, The Netherlands; and Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands Received 1998 June 26; accepted 1998 September 14 ABSTRACT 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,ÏÏ 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 mag), with spectral types earlier than DB5. The Hipparcos measurements allow a major improvement in this situation. Moving groups are identiÐed in the Hipparcos Catalog by combining de BruijneÏs refurbished convergent point method with the ““ Spaghetti method ÏÏ of Hoogerwerf & Aguilar. Astrometric members are listed for 12 young stellar groups, out to a distance of D650 pc. These are the three subgroups Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2, a Persei (Per OB3), CasÈTau, Lac OB1, Cep OB2, and a new group in Cepheus, designated as Cep OB6. The selection procedure corrects the list of previously known astrom- etric and photometric B- and A-type members in these groups and identiÐes many new members, includ- ing a signiÐcant number of F stars, as well as evolved stars, e.g., the Wolf-Rayet stars c2 Vel (WR 11) in Vel OB2 and EZ CMa (WR 6) in Col 121, and the classical Cepheid d Cep in Cep OB6. Membership probabilities are given for all selected stars. Monte Carlo simulations are used to estimate the expected number of interloper Ðeld stars. In the nearest associations, notably in Sco OB2, the later-type members include T Tauri objects and other stars in the Ðnal preÈmain-sequence phase. This provides a Ðrm 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 Ðelds 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 D500 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 Ðeld 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 sometimes caused by the absence of O stars, but in other cases a previously known open cluster turns out to be (part of) an extended OB association. The number of unbound young stellar groups in the solar neighborhood may be signiÐcantly larger than thought previously. Key words: stars: early-type È stars: formation È stars: kinematics È stars: statistics È stars: preÈmain-sequence È open clusters and associations: general È open clusters and associations: individual (Gould Belt) 1. INTRODUCTION hence not uniformly among the stellar population of the GalaxyÈbut instead are concentrated in loose groups. This Ever since spectral classiÐcations for the bright stars inspired research on their individual properties, and on became available, and in particular with the publication of their motions and space distribution. Studies of the stellar Cannon & PickeringÏs monumental Henry Draper Catalog content, the internal velocity distribution, and the common of stellar spectra in 1918È1924, it has been evident that O motion of the group members with respect to the ambient and B stars are not distributed randomly on the skyÈand stellar population naturally relied on the capability of iden- tifying the stars belonging to the group, their ““ members.ÏÏ ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ Among extensive early investigations along these lines in 1 Based on data from the Hipparcos astrometry satellite. the wake of work on ““ moving groups ÏÏ and compact stellar 354 HIPPARCOS AND NEARBY OB ASSOCIATIONS 355 clusters (summarized in Eddington 1914), we note Mathieu 1986; Tian et al. 1996), and so they form coherent KapteynÏs (1914, 1918a, 1918b, 1918c) work on ““ the bright- structures in velocity space. The common space motion er Galactic helium stars ÏÏÈi.e., the B starsÈand the work relative to the Sun is perceived as a convergence of the by Rasmuson (1921, 1927). Among the early investigations proper motions of the members toward a single point on the of the space distribution we note in particular the com- sky (e.g., Blaauw 1946; Bertiau 1958). This can be used to prehensive work by Pannekoek (1929), carried out at the establish membership based on measurements of proper time of the breakthrough of the modern concept of the motions. Whereas many such astrometric membership Galaxy as an isolated, rotating system. PannekoekÏs sum- studies have been carried out for open clusters (e.g., van marizing Table 15 lists 37 groups of B stars, among which Leeuwen 1985, 1994; van Altena et al. 1993; Robichon et al. several, called by him ““ Lacerta,ÏÏ ““ Cep I,ÏÏ ““ Cep II,ÏÏ 1997), there are few such studies for nearby OB associations ““ f Pers,ÏÏ ““ Lupus,ÏÏ ““ Scorpio,ÏÏ and others, may be con- because these generally cover tens to hundreds of square sidered as ““ forerunners ÏÏ of the objects of modern research. degrees on the sky. Ground-based proper motion studies His diagram of the groups of B stars, projected on the therefore almost invariably have been conÐned to modest Galactic plane (loc. cit. p. 65) is a remarkable foreshadow of samples of bright stars(V [ 6 mag) in fundamental cata- some of the Ðgures presented in this paper. In the early logs, or to small areas covered by a single photographic 1950s the work of W. W. Morgan and collaborators provid- plate. Photometric studies extended membership to later ed the Ðrst identiÐcation of the Galactic spiral structure in spectral types (e.g., Warren & Hesser 1977a, 1977b, 1978) the distribution of stellar ““ aggregates ÏÏ (Morgan, Sharpless, but are less reliable due to, e.g., undetected duplicity, or the & Osterbrock 1952a, 1952b; Morgan, Whitford, & Code distance spread within an association. As a result, member- 1953). Herschel (1847) and Gould (1874) had already ship for many associations has previously been determined noticed that the brightest stars are not distributed sym- unambiguously only for spectral types earlier than B5 (e.g., metrically with respect to the plane of the Milky Way, but Blaauw 1964a, 1991). Although this covers the important seemed to form a belt that is inclined D18¡ to it. This upper main-sequence turno† region, it is not known what became known as the Gould Belt and was subsequently the lower mass limit is of the stars that belong to the associ- found to be associated with a signiÐcant amount of inter- ation, so our knowledge of these young stellar groups has stellar material (Lindblad 1967; Sandqvist, Tomboulides, & remained rather limited. Lindblad 1988;Po ppel 1997). The Hipparcos Catalog (ESA 1997) contains accurate Ambartsumian (1947) introduced the term ““ association ÏÏ positions, proper motions, and trigonometric parallaxes, for the groups of OB stars; he pointed out that their stellar which are all tied to the same global reference system ~3 mass density is usually less than 0.1M_ pc . Bok (1934) (ICRS: see ESA 1997, Vol. 1, ° 1.2.2). This makes these had already shown that such low-density stellar groups are measurements ideally suited for the identiÐcation of unstable against Galactic tidal forces, so that OB associ- astrometric members of the nearby OB associations, with ations must be young (Ambartsumian 1949), a conclusion greater reliability, and to much fainter magnitudes than supported later by the ages derived from color-magnitude accessible previously. Accordingly, the SPECTER consor- diagrams. This is in harmony with the fact that these groups tium was formed in Leiden in 1982, which successfully pro- are usually located in or near star-forming regions, and posed the observation by Hipparcos of candidate members hence are prime sites for the study of star formation pro- of nearby OB associations. An extensive program of cesses and of the interaction of early-type stars with the ground-based observations was carried out in anticipation interstellar medium (see e.g., Blaauw 1964a, 1991 for of the release of the Hipparcos data (for a summary, see de reviews). Ruprecht (1966) compiled a list of OB associ- Zeeuw, Brown, & Verschueren 1994 and our ° 2.3). Here we ations, with Ðeld boundaries, bright members, and distance. present the results of our census of the nearby associations He introduced a consistent nomenclature, approved by the based on the Hipparcos measurements.