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The Structure and the Distance of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy
We present further arguments that the Hipparcos parallaxes for some ofthe clusters and associations represented in the Hipparcos catalogshould be used with caution in the study of the Galactic structure. Ithas already been shown that the discrepancy between the Hipparcos andground-based parallaxes for several clusters including the Pleiades,Coma Ber, and NGC 6231 can be resolved by recomputing the Hipparcosastrometric solutions with an improved algorithm diminishing correlatederrors in the attitude parameters. Here we present new parallaxesobtained with this algorithm for another group of stars with discrepantdata-the galactic cluster Cr 121. The original Hipparcos parallaxes ledde Zeeuw et al. to conclude that Cr 121 and the surrounding associationof OB stars form a relatively compact and coherent moving group at adistance of ~=550-600 pc. Our corrected parallaxes reveal a differentspatial distribution of young stellar populace in this area. Both thecluster Cr 121 and the extended OB association are considerably moredistant (750-1000 pc), and the latter has a large depth probablyextending beyond 1 kpc. Therefore, not only are the recalculatedparallaxes in complete agreement with the photometric uvbyβparallaxes, but the structure of the field they reveal is no longer indiscrepancy with that found by the photometric method.
| The Role of Polycyclic Aromatic Hydrocarbons in Ultraviolet Extinction. I. Probing Small Molecular Polycyclic Aromatic Hydrocarbons
We have obtained new Hubble Space Telescope/Space Telescope ImagingSpectrograph spectra to search for structure in the ultravioletinterstellar extinction curve, with particular emphasis on a search forabsorption features produced by polycyclic aromatic hydrocarbons (PAHs).The presence of these molecules in the interstellar medium has beenpostulated to explain the infrared emission features seen in the 3-13μm spectra of numerous sources. Ultraviolet (UV) spectra are uniquelycapable of identifying specific PAH molecules. We obtained highsignal-to-noise ratio UV spectra of stars that are significantly morereddened than those observed in previous studies. These data put limitson the role of small (30-50 carbon atoms) PAHs in UV extinction and callfor further observations to probe the role of larger PAHs. PAHs are ofimportance because of their ubiquity and high abundance inferred fromthe infrared data, and also because they may link the molecular and dustphases of the interstellar medium. A presence or absence of UVabsorption bands due to PAHs could be a definitive test of thishypothesis. We should be able to detect a 20 Å wide feature downto a 3 σ limit of ~0.02 AV. No such absorption featuresare seen other than the well-known 2175 Å bump.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| The region of Collinder 121
The distribution of bright B-type stars in a field with a radius of5° centred at the Galactic open cluster Cr 121 is studied utilizingStrömgren and Hβ photometry. All PPM stars earlier thanspectral type A0 are used, revealing a loose nearby structure at adistance of 660-730pc, and a compact more distant group, which appearsto be a genuine cluster: Cr 121. Based on similar coordinates, distancesand positions on the colour-magnitude (CM) and Hertzsprung-Russell (HR)diagrams, 11 photometric cluster members are selected at a mean distanceof 1085(+/-41 standard error) pc. The results are discussed in the lightof both classical and Hipparcos points of view.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Secondary standard stars for UVBY beta CCD photometry
Accurate standard uvby indices are presented for 73 southern B, A, F andG stars in the $V$ magnitude range 8.2 to 10.9. They cover all threetransformation regions of the $uvby$ system (Olsen \cite[1983]{Olsen1})well. Standard $\beta$ indices are included for the 55 B, A, and F starsin the sample. Our results provide a useful set of secondary standardsfor uvby beta CCD photometry with southern hemisphere 1-2 m classtelescopes. A critical comparison with published photometry, in generalbased on fewer observations, is given. Based on observations made at theEuropean Southern Observatory, La Silla, Chile. Tables 3, 4, 5 are alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The HI distribution in the environment of the WR star HD 50896
The neutral matter distribution from the interstellar medium (ISM)located in the vicinity of the galactic Wolf-Rayet (WR) star HD 50896has been examined by means of HI 21-cm line observations obtained withboth low [half-power beamwidth (HPBW)~=34 arcmin] and intermediate(HPBW~=9 arcmin) angular resolution. The most interesting discovery is ahuge ovoidal HI minimum spanning the velocity range +1.5 to +10.0 kms^-1, created, very likely, by the joint action of the progenitor of HD50896 and the WR itself. Inside this cavity, two minima are clearlydiscernible. The WR star is offset with respect to either thegeometrical centre of the main HI void or the inner HI minima. Aphysical link between S308, the ring nebula associated with HD 50896,and one of the HI minima is suggested by our data. A kinematicaldistance of ~1 kpc for HD 50896 is derived based on the radial velocityof the HI cavity. The dual HI minimum geometry observed inside the mainHI cavity, a feature also seen in the HI distribution of the ISM locatedclose to other galactic WR stars, may be a consequence of theinteraction process itself. The ovoidal shape of the main cavity cannotbe explained within the framework of the standard interstellar bubbletheory. Elongated bubbles may result when the large-scale galacticmagnetic field is taken into account.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| The evolution of low ionization QSO absorption systems
A CCD spectroscopic survey of Mg II and Mg I absorption lines in a C IVselected sample of 35 metal systems, observed toward 12 QSOs between z =1.1 and z = 2.1, is reported. Six Mg I and 10 Mg II absorption lines aredetected in 33 C IV clouds, and some peculiarities are noted. Thecomplex or asymmetric line profiles reveal that multiple components arepresent and that variations in the C IV/Mg II and Mg II/Mg I ratio mayoccur often from one component to another with a velocity separationless than the velocity resolution of about 100 km/s. Comparison with theMg II absorption-line statistics at z-bar = 0.5 implies that, at z-bar =1.6, there are more strong absorbers per unit z with W(2796) greaterthan 0.6 A and fewer weak absorbers. The evolutionary parameter gammaappears to depend on W(min): its value drops as W(min) is reduced and isnegative at W(min) = 0.15 A. This suggests a different redshiftevolution for the population of weak and strong Mg II absorbers.
| The ultraviolet spectrum and interstellar environment of HD 50896
Data from 92 high-resolution IUE 1150-3274-A spectra are compiled intables and graphs and analyzed in detail to characterize HD 50896 andits interstellar environment. Findings reported include unexceptionaldepletions in the low-velocity H I system, strong low-velocity lines(attributed to excitation in an H II region associated with HD 50896 atdistance 2-3 kpc), an absorption system blueshifted by about 30 km/sfrom the low-velocity gas (attributed to ring nebula S308), andhigh-velocity absorption systems with evidence of grain destruction(attributed to an old SNR).
| High-velocity interstellar gas in the line of sight to the Wolf-Rayet star HD 50896
The large shell of interstellar gas (IG) discovered toward HD 50896 byHeckathorn and Fesen (1984) is characterized on the basis ofhigh-dispersion IUE SWP and LWR spectra of 19 objects located within 4deg of HD 50896 (but outside the optical ring nebula S308) at distances0.6-2.9 kpc (compared to 1.5 kpc for HD 50896). The IG is found to havetwo components (at velocities -80 and -125 km/s), diameter 90 pc orgreater, and distance 1.0 + or - 0.2 kpc, demonstrating that it is notrelated to HD 50896 and suggesting that it is a highly evolved supernovaremnant associated with cluster Cr 121.
| The galactic reddening law - The evidence from uvby-beta photometry of B stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985A&A...142..189T&db_key=AST
| High-velocity interstellar gas in the line-of-sight to HD 50896
Using high-dispersion IUE spectra, a large interstellar shell structurewas discovered in the line-of-sight to the Wolf-Rayet star HD 50896.Blue-shifted interstellar absorption lines indicative of high-velocitygas are present in the spectra of four B stars located up to 2 deg awayfrom HD 50896 and at a distance of 1000 to 1400 parsecs, suggesting alinear diameter for the structure of at least 40 parsecs. Thesehigh-velocity components, present only in the low ionization lines andexhibiting nearly cosmic abundances, can be interpreted as a heretoforeunknown and extremely old supernova remnant. The existence of such asupernova remnant potentially associated with HD 50896, a runawayWolf-Rayet star believed to have a compact companion, implies that HD50896 may be a binary in its second Wolf-Rayet phase of evolution.
| Photometric search for Ap-stars in open cluster. IV - NGC 2287, CR 121, NGC 2422 and supplementary measurements in NGC 1662 and NGC 2516
Additional results are presented concerning the search for Ap-stars inopen clusters with the photoelectric peculiarity index Delta-a (Maitzen,1976). NGC 2287-56, NGC 2422-P3, and NGC 1662-4 are found to beCP2-stars as members in their respective clusters. The findings indicatethat HD 51088, which is also peculiar according to its Delta-a value,may be a member of one of the two very young star concentrationspreviously called Cr 121. It is determined that there is only a smallprobability for HD 49333 to be a member of NGC 2287, and that HD 49333is very likely to be a CP4 ( = He weak) star with a Delta-a significantfor peculiarity. It was necessary to reject 10 other stars with reportedindications of peculiarity as CP2 stars since their Delta-a values werenormal. It is concluded that the frequency of peculiar objects is ratherlow in the actual sample considering the number of CP2 detections.
| The region of NGC 2287 and CR 121
Intermediate band and H-beta observations of 135 stars in the regions ofthe clusters NGC 2287 and Cr 121 are discussed, and a luminositycalibration of photometric parameters for late G- to early K-type brightgiants and supergiants is introduced. Results indicate that NGC 2287 isat a distance of 740 pc, very little reddened, 100-million years old,and contains three or four G8-K2 bright giants and supergiants and ablue straggler. Cr 121 is 1.17 kpc distant, very little reddened, 1.5million years old and an extension of CMa OB1. Cr 121 contains a nearerconcentration of stars at the same distance as NGC 2287, and theassociation appears to be the same age as CMa OB1, although presequencestars may exist. Intermingling of stars in CMa OB2 and NGC 2287 is alsoconsidered possible, and a test of the calibration of two methods ofluminosity determination of early A-type stars using photometricparameters shows them to be entirely consistent.
| H-beta photometry of southern early-type stars
H-beta photoelectric photometry is presented for 209 southern hemisphereearly-type stars from the HD catalog with galactic latitudes /b/ greaterthan 6 deg. Four-color photometry exists for all these stars and MKtypes for most of them. Absolute magnitudes have been estimated for allbut the emission-line stars and distances from the sun and the galacticplane determined.
| Four colour photometry of southern early-type stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978MNRAS.182..629K&db_key=AST
| Collinder 121: a Young Southern Open Cluster Similar to H and χ Persei
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1967ApJ...149..107F&db_key=AST
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Observation and Astrometry data
| Constellation: | Canis Major |
| Right ascension: | 06h54m42.58s |
| Declination: | -24°55'28.8" |
| Apparent magnitude: | 9.121 |
| Proper motion RA: | -2 |
| Proper motion Dec: | 1.8 |
| B-T magnitude: | 8.938 |
| V-T magnitude: | 9.106 |
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