A [ITAL]Hipparcos[/ITAL] Census of the Nearby OB Associations

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', 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...

Keplerobservations of A–F pre-main-sequence stars in Upper Scorpius: discovery of six new δ Scuti and one γ Doradus stars

“…One can argue that their membership to the USco association should guarantee their youth, considering that not only Pecaut, Mamajek, & Bubar (2012), but also previous studies report an age of 5-10 Myrs (see, e.g. de Zeeuw et al 1999;Luhman & Mamajek 2012, and references therein) for this association. However, as a check, we now proceed to provide further evidence on this assumption.…”

“…This is the case, for example, of NGC 2264, a well-studied cluster where 9 PMS pulsators have been observed from space (Zwintz et al 2014). The Upper Scorpius association (USco) is perhaps of even greater interest due to the large number of known A-F members (de Zeeuw et al 1999;Pecaut, Mamajek, & Bubar 2012) and the availability of accurate individual distances based on Hipparcos parallaxes. Moreover, stellar parameters have been estimated for most of the member stars.…”

“…USco is part of the larger Scorpius-Centaurus (ScoCen) association, a young (∼5-10 Myr) and relatively close (∼120-150 pc) region of recent star formation (de Bruijne 1999;de Zeeuw et al 1999) that includes the Upper Centaurus-Lupus (UCL) and Lower Centaurus-Crux (LCC). USco is the youngest of them and contains many intermediate-mass members, making it an ideal target to investigate the whole extension of the PMS instability strip.…”

The dynamics of an extremely young cluster probed for the first time

“…The typical differential velocity between two stars can reach 3 km s −1 , i.e., 5 mas yr −1 at a distance of 125 pc. The systematic proper motion of the ρ Ophiuchi IR cluster can be estimated from the proper motion of the associated, nearby Upper Scorpius OB association recently measured by de Zeeuw et al (1999) with Hipparcos. They found a global proper motion of 25.8 ± 0.1 mas yr −1 .…”

“…These extremely young clusters (1-2 Myr) appear to be the precursors of the visible open clusters and/or of dispersed Galactic stellar streams. While OB associations have recently been surveyed by Hipparcos for detailed proper-motion studies (de Zeeuw et al 1999), no IR clusters have yet been studied in this sense.…”