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Interstellar absorption lines toward Cep OB4
An analysis of optical echelle spectra towards nine stars in the Cep OB4association is presented. Interstellar absorption lines which arise inthe (1, 0) and (2, 0) bands of the CN A 2Π - X2Σ+ red system towards BD+66o1661, BD+66o 1674, and BD+66o 1675 are used toinfer accurate CN column densities N(CN). A comparison with earliermeasurements in the CN violet system allows to infer a CN Doppler bparameter of b(CN) = 1.2{-}2.4 km s-1. Molecular carbonabsorption lines which arise in the (1, 0), (2, 0) and (3, 0) bands ofthe C2 A1Πu -X1Σg+ Phillips system is used toinfer gaskinetic temperatures of 35±10 K and densities of n = 700± 200 cm-3 towards BD+66o 1661 andBD+66o 1675, and a temperature of 60±10 K and adensity of 800 ± 400 cm-3 towards BD+66o1674. The R(1) line of the (0, 0) band of the CH A2Δ -X2Π system is detected towards 6 stars. A tightcorrelation exists between N(C2) and N(CH). N(CN) increaseswith N(CH) and with N(C2). Interstellar CH+ ismarginally detected towards four stars. Inferred CH+ columndensities are significantly lower than towards other lines of sight withsimilar reddening. The velocity structure towards Cep OB4 and thechemical abundances suggest that CN, C2, CH, andCH+ are formed in quiescent material. A previous suggestionthat the molecules form in a photon-dominated region close to the starsis not supported by the observations.Based on observations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-PlanckInstitut für Astronomie and the Instituto de Astrofisica deAndalucia (CSIC).
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, 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 eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| 21-cm observations of the CEP IV star-formation region
The Cep IV star-formation region has been mapped at 21 cm. The resultingH I distribution indicates that the available gas forms a broken ringcoincident with the optical nebulosity. Comparison of this feature withother kinematic data for both stars and gas suggests that sequentialstar formation is an ongoing process in this region.
| A Study of the Cepheus IV Association
Abstract image available at:http://adsabs.harvard.edu/abs/1968ApJS...16..275M
| The Cepheus IV Association.
Abstract image available at:http://adsabs.harvard.edu/abs/1965ApJ...141..660W
| A New O - B Association with an Unusual Reddening Effect.
Abstract image available at:http://adsabs.harvard.edu/abs/1959ApJ...130..482B
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